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Introduction

In the study of chemistry and physics, such concepts as "atom", "relative atomic and molar masses chemical element". It would seem that nothing new in this area has been discovered for a long time. However, the International Union of Pure and Applied Chemistry (IUPAC) annually updates the values ​​of the atomic masses of chemical elements. Over the past 20 years, the atomic masses of 36 elements have been corrected, with 18 of them having no isotopes.

Taking part in the All-Russian full-time round of the Olympiad in Natural Science, we were offered the following problem: “Suggest a method for determining molar mass substances in a school laboratory. "

This task was purely theoretical and I successfully completed it. So I decided experimentally, in a school laboratory, to calculate the molar mass of a substance.

Target:

Determine experimentally the molar mass of a substance in a school laboratory.

Tasks:

Explore the scientific literature that describes how to calculate the relative atomic and molar mass.

Experimentally determine the molar mass of a substance in a gaseous and solid state using physical methods.

Draw conclusions.

II. Main part

Basic concepts:

Relative atomic mass is the mass of a chemical element expressed in atomic mass units (amu). For 1 amu 1/12 of the mass of the carbon isotope with an atomic weight of 12 is assumed. 1 amu = 1.6605655 · 10 -27 kg.

Relative atomic mass - shows how many times the mass of a given atom of a chemical element is more than 1/12 of the mass of the isotope 12 C.

Isotopes- atoms of one chemical element, having a different number of neutrons, and the same number of protons in the nucleus, therefore, having different relative atomic masses.

Molar mass of a substance - this mass of a substance taken in an amount of 1 mol.

1 mol - this is the amount of a substance that contains the same number of atoms (molecules) as there are in 12 g of carbon.

Specific heat of a substance is a physical quantity that shows how much heat must be communicated to a subject weighing 1 kg in order to change its temperature by 1 0 C.

Heat capacity- this is the product of the specific heat capacity of a substance and its mass.

The history of determining the atomic masses of chemical elements:

After analyzing various sources of literature on the history of determining the relative atomic masses of various chemical elements, I decided to summarize the data in a table, which is quite convenient, since in various sources of literature, information is given vaguely:

Name of scientist, year	Contribution to the study and determination of relative atomic masses	Note
John Dalton	It is clear that it is impossible to weigh the atoms directly. Dalton talked only about "the ratio of the weights of the smallest particles of gaseous and other bodies," that is, about their relative masses. As a unit of mass, Dalton took the mass of a hydrogen atom, and to find the masses of other atoms, he used the percentage compositions of various compounds of hydrogen with other elements found by various researchers. Dalton compiled the world's first table of the relative atomic masses of certain elements.
William Prout	He suggested that all other elements could have arisen from the lightest element - hydrogen by condensation. In this case, the atomic masses of all elements must be multiples of the mass of the hydrogen atom. For a unit of atomic mass, he proposed to choose hydrogen.	Only later- over the next few years, it turned out that Prout's hypothesis actually confirmed was: all the elements were actually formed during the explosion of supernovae from the nuclei of hydrogen atoms - protons, as well as neutrons.
1819 Dulong P.I., A.T. Pti:	Rule of thumb: product of atomic mass and heat capacity- the value is constant. The rule is still used to determine the relative atomic mass of certain substances.	Berzelius corrected some of the atomic masses of metals on the basis of the rule
Stas, Richards	Clarification of the relative atomic mass of some elements.
S. Ca-Nizzaro	Determination of the relative atomic mass of some elements by determining the known relative molecular masses of volatile compounds of elements
Stas, Belgium	He suggested changing the atomic mass unit and choosing the oxygen atom as the new standard. The mass of the oxygen atom was taken equal to 16,000, the unit of measurement became 1/16 of this mass of oxygen.
	Complete refutation of Prout's hypothesis based on the determination of the ratio of the masses of chemical elements in some compounds
D.I. Mendeleev	Determined and corrected on the basis of the periodic table the relative atomic masses of some known and not yet discovered chemical elements.
	The so-called oxygen scale was approved, where the mass of the oxygen atom was taken as the standard.
Theodore William Richards	At the beginning of the 20th century. very accurately determined the atomic masses of 25 chemical elements and corrected mistakes previously made by other chemists.
	A mass spectrograph has been created to determine the relative atomic masses
	For the atomic mass unit (amu), 1/12 of the mass of the carbon isotope 12C (carbon unit) was taken. (1 amu, or 1D (dalton), in SI units is 1.6605710-27 kg.)

Knowing the relative atomic mass of an atom, you can determine the molar mass of a substance: М = Аr · 10 ³ kg / mol

Methods for determining the molecular weights of elements:

Atomic and molecular weights can be determined either by physical or chemical methods. Chemical methods differ in that at one of the stages, not the atoms themselves appear in them, but their combinations.

Physical methods:

1 way. Dylogue and Petit's law

In 1819, Dulong, together with A.T. Petit, established the law of the heat capacity of solids, according to which the product of the specific heat capacities of simple solids by the relative atomic mass of the constituent elements is an approximately constant value (in modern units of measurement equal to approximately Cv Ar = 25.12 J / (G.K)); now this ratio is called the "Dulong-Petit law". The law of specific heat, which for quite a long time remained unnoticed by contemporaries, subsequently served as the basis for a method for an approximate assessment of the atomic masses of heavy elements. It follows from the Dulong and Petit law that dividing 25.12 by the specific heat simple substance, easily determined experimentally, you can find the approximate value of the relative atomic mass of a given element. And knowing the relative atomic mass of an element, you can determine the molar mass of a substance.

М = Мr · 10̵ ³ kg / mol

On initial stage In the development of physics and chemistry, the specific heat of an element was easier to determine than many other parameters, therefore, using this law, approximate values ​​of the RELATIVE ATOMIC MASS were established.

Means, Ar = 25.12 / s

c is the specific heat of the substance

To determine the specific heat of a solid, we will conduct the following experiment:

1. 1. 1. Pour into the calorimeter hot water and determine its mass and initial temperature.

         Let's determine the mass of a solid made from an unknown substance, the relative atomic mass of which we need to determine. We will also determine its initial temperature (its initial temperature is equal to the room temperature of the air, since the body has been in this room for a long time).

         We put it into the calorimeter with hot water solid and determine the temperature established in the calorimeter.

         Having made the necessary calculation, we determine the specific heat of the solid.

Q1 = c1m1 (t-t1), where Q1 is the amount of heat given off by water as a result of heat exchange, c1 is the specific heat capacity of water (tabular value), m1 is the mass of water, t is the final temperature, t 1 is the initial temperature of the water, Q2 = c2m2 (t-t2), where Q2 is the amount of heat received by the solid as a result of heat exchange, c2 is the specific heat capacity of the substance (to be determined), m2 is the mass of the substance, t 2 is the initial temperature of the investigated body, since the heat balance equation has the form: Q1 + Q2 = 0 ,

then c2 = c1m1 (t-t1) / (- m2 (t-t2))

						c, J / (kg 0 K)

Mean relative atomic mass substance turned out

Ar = 26.5 amu

Hence, molar mass a is equal M = 0.0265 kg / mol.

Solid body - aluminum bar

Method 2. Let's calculate the molar mass of air.

Using the equilibrium condition of the system, you can also calculate the molar mass of a substance, for example a gas, for example air.

Fa = F(The force of Archimedes acting on the balloon is balanced by the total force of gravity acting on the shell of the balloon, the gas in the balloon, and the load suspended from the balloon.). Of course, considering that the ball is suspended in the air (it does not rise or fall).

Fa- the force of Archimedes, acting on the ball in the air

Fa = ρvg Vsh

ρv - air density

F1- the force of gravity acting on the shell of the ball and the gas (helium) inside the ball

F1 = mobg + mgelg

F2- the force of gravity acting on the load

F2 = mgr g

We get the formula: ρвg Vш= mob g + mgel g + mgr g (1)

Let's use the Mendeleev-Clapeyron formula to calculate the molar mass of air:

Let us express the molar mass of air:

In equation (3) we substitute equation (2) instead of air density. So, we got the formula for calculating the molar mass of air:

Therefore, to find the molar mass of air, you need to measure:

1) weight of cargo

2) the mass of helium

3) shell mass

4) air temperature

5) air pressure ( Atmosphere pressure)

6) the volume of the ball

R- universal gas constant, R = 8.31 J / (mol K)

Barometer showed atmospheric pressure

equal pa = 96000Pa

Indoor air temperature:

T = 23 + 273 = 297K

We determined the mass of the load and the mass of the shell of the ball using electronic scales:

mgr = 8.02g

ball shell mass:

mob = 3.15g

We determined the volume of the ball in two ways:

a) our ball turned out to be round. Having measured the circumference of the ball in several places, we determined the radius of the ball. And then its volume: V = 4/3 · πR³

L = 2πR, Lav = 85.8cm = 0.858m, therefore R = 0.137m

Vsh = 0.0107m³

b) poured water into a bucket to the very edge, after placing it in a tray to drain the water. We lowered the balloon completely into the water, part of the water poured into the bath under the bucket, measuring the volume of water poured out of the bucket, we determined the volume of the balloon: Vwater = Vsh = 0.011m³

(The ball in the picture was closer to the camera, so it seems larger)

So, for the calculation, we took the average value of the volume of the ball:

Vsh = 0.0109m³

We determine the mass of helium in using the Mendeleev-Clapeyron equation, taking into account that the temperature of helium is equal to the temperature of the air, and the pressure of helium inside the ball is equal to atmospheric.

Molar mass of helium 0.004 kg / mol:

mgel = 0.00169 kg

Substituting all measurement results into formula (4), we obtain the value of the molar mass of air:

M = 0.030 kg / mol

(table value of molar mass

air 0.029 kg / mol)

Output: in the school laboratory you can determine physical methods the relative atomic mass of a chemical element and the molar mass of a substance. Having done this work, I learned a lot about the methods of determining the relative atomic mass. Of course, many methods are not available for a school laboratory, but, nevertheless, even using elementary equipment, I was able to experimentally determine by physical methods the relative atomic mass of a chemical element and the molar mass of a substance. Consequently, I have fulfilled the goal and tasks set in this work.

List of used literature

alhimik.ru

alhimikov.net

https://ru.wikipedia.org/wiki/Molar_mass

G.I.Deryabina, G.V. Kantaria. 2.2 Mole, molar mass. Organic chemistry: web tutorial.

http://kf.info.urfu.ru/glavnaja/

https://ru.wikipedia.org/wiki/Molar_mass h

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Molar mass calculator

Moth

All substances are made up of atoms and molecules. In chemistry, it is important to accurately measure the mass of substances that react and result from it. By definition, a mole is an amount of a substance that contains the same structural elements(atoms, molecules, ions, electrons and other particles or their groups), how many atoms are contained in 12 grams of a carbon isotope with a relative atomic mass of 12. This number is called constant or Avogadro's number and is equal to 6.02214129 (27) × 10²³ mol⁻¹ ...

Avogadro's number N A = 6.02214129 (27) × 10²³ mol⁻¹

In other words, a mole is an amount of a substance equal in mass to the sum of the atomic masses of atoms and molecules of a substance, multiplied by Avogadro's number. The unit of amount of a substance, mol, is one of the seven basic units of the SI system and is denoted by mol. Since the name of the unit and its symbol are the same, it should be noted that the symbol is not declined, unlike the name of the unit, which can be declined according to the usual rules of the Russian language. By definition, one mole of pure carbon-12 is exactly 12 g.

Molar mass

Molar mass - physical property substance, defined as the ratio of the mass of this substance to the amount of substance in moles. In other words, it is the mass of one mole of a substance. In SI, the unit of molar mass is kilogram / mol (kg / mol). However, chemists are accustomed to using a more convenient unit of g / mol.

molar mass = g / mol

Molar mass of elements and compounds

Compounds are substances made up of different atoms that are chemically bonded to each other. For example, the following substances that can be found in the kitchen of any housewife are chemical compounds:

• salt (sodium chloride) NaCl
• sugar (sucrose) C₁₂H₂₂O₁₁
• vinegar (acetic acid solution) CH₃COOH

The molar mass of chemical elements in grams per mole numerically coincides with the mass of the element's atoms, expressed in atomic mass units (or daltons). The molar mass of compounds is equal to the sum of the molar masses of the elements that make up the compound, taking into account the number of atoms in the compound. For example, the molar mass of water (H₂O) is approximately 2 × 2 + 16 = 18 g / mol.

Molecular mass

Molecular weight (formerly called molecular weight) is the mass of a molecule, calculated as the sum of the masses of each atom in a molecule multiplied by the number of atoms in that molecule. Molecular weight is dimensionless physical quantity, numerically equal to the molar mass. That is, the molecular weight differs from the molar weight in dimension. Despite the fact that molecular weight is a dimensionless quantity, it still has a quantity called an atomic mass unit (amu) or dalton (Da), and approximately equal to the mass of one proton or neutron. The atomic mass unit is also numerically equal to 1 g / mol.

Calculating molar mass

The molar mass is calculated as follows:

• determine the atomic masses of elements according to the periodic table;
• determine the number of atoms of each element in the compound formula;
• determine the molar mass by adding the atomic masses of the elements included in the compound, multiplied by their number.

For example, let's calculate the molar mass of acetic acid

It consists of:

• two carbon atoms
• four hydrogen atoms
• two oxygen atoms

• carbon C = 2 × 12.0107 g / mol = 24.0214 g / mol
• hydrogen H = 4 × 1.00794 g / mol = 4.03176 g / mol
• oxygen O = 2 × 15.9994 g / mol = 31.9988 g / mol
• molar mass = 24.0214 + 4.03176 + 31.9988 = 60.05196 g / mol

Our calculator does just that. You can enter the acetic acid formula into it and check what happens.

Do you find it difficult to translate a unit of measurement from one language to another? Colleagues are ready to help you. Post a question to TCTerms and you will receive an answer within a few minutes.

To do this, you must use the periodic table. In the cell of any element, a number is given, most often, with an accuracy of 3-4 decimal places - this is the relative molecular mass (molar mass) of this element. Usually, the molecular weight is rounded up according to the appropriate mathematical rules, with the exception of chlorine - the molecular weight of a chlorine atom is 35.5. Molecular mass complex substance is equal to the sum of the molecular weights of its constituent elements. For example, water is H2O. The molecular weight of hydrogen is 1, oxygen - 16. This means that the molecular weight of water is 2 * 1 + 16 = 18 g / mol.

To determine the molar mass of substances, you must:

• have a table periodic system chemical elements D.I. Mendeleev;
• know the number of atoms of each element, in the formula of the substance in question;
• know the definition of the concepts "molar mass", "mol".

Formula of substance

To describe a substance, it is necessary to know how many atoms and what type one molecule of the substance under consideration contains. For example, the inert gas krypton exists under normal conditions (atmospheric pressure 101325 Pa = 760 mm Hg, temperature 273.15 K = 0 ° C) in the atomic form of Kr. The carbon monoxide molecule consists of two carbon atoms C and an oxygen atom O: CO2. And the coolant of the refrigerator - freon 134 - has a more complex formula: СF3CFH2.

Definitions

Molar mass Mr is the mass of one mole of a substance, measured in g / mol.

Mole is the amount of a substance that contains a certain number of atoms of a given type. It is defined as the number of atoms in 12 g of the carbon isotope C-12 and is equal to Avogadro's constant N = 6.022 * 10 ^ 23 1 / mol.

Calculating molar mass

To determine the molar mass Mr of a substance, it is necessary to find out the atomic mass Ar of each element included in the substance using the table of the periodic system of chemical elements of D.I. Mendeleev, and know the number of atoms of each element.

For example, the molar mass Mr of sodium tetraborate Na2B4O7 * 10 H2O is:

M r (Na2B4O7 * 10 H2O) = 2 * Ar (Na) + 4 * Ar (B) + 7 * Ar (O) + 10 * 2 * Ar (H) + 10 * Ar (O) = 2 * 23 + 4 * 11 + 7 * 16 + 10 * 2 * 1 * 16 = 223 g / mol.

Instructions

If you carefully examine the table of Dmitry Ivanovich Mendeleev, you can see that it looks like a multi-apartment multi-storey building in which there are "residents" - the elements. Each of them has a surname () and a chemical. Moreover, each of the elements lives in its own apartment, and therefore has. This information is presented in all cells of the table.

However, there is one more figure, at first glance completely incomprehensible. Moreover, it is indicated with several values ​​after the decimal point, which is done for greater accuracy. It is this number that you need to pay attention to, because this is the relative atomic mass. Moreover, this is a constant value that does not need to be memorized and can be found from the table. By the way, even on the exam according to D.I. Mendeleev is reference material, available for use, and each is in an individual package - KIM.

Molecular mass, or rather the relative substance is designated by letters (Mr) is the sum of the relative atomic masses (Ar) of the elements that form the molecule. Relative atomic mass is just that mysterious figure that appears in every cell of the table. For calculations, these values ​​must be rounded to the nearest whole number. The only exception is the chlorine atom, which has a relative atomic mass of 35.5. This characteristic has no units of measurement.

Example 1. Find the molecular mass(KOH)
A potassium hydroxide molecule consists of one potassium (K) atom, one oxygen (O) and one hydrogen (H) atom. Therefore, we find:
Mr (KOH) = Ar (K) + Ar (O) + Ar (H)


Hence: Mr (KOH) = 39 + 16 + 1 = 56

Example 2. Find the molecular mass sulfuric acid (H2SO4 ash-two-es-o-four)
The sulfuric acid molecule consists of two hydrogen atoms (H), one sulfur atom (S), and four oxygen atoms (O). Therefore, we find:
Mr (H2SO4) = 2Ar (H) + Ar (S) + 4Ar (O)
According to the table of D.I. Mendeleev, we find the values ​​of the relative atomic masses of the elements:
Ar (K) = 39, Ar (O) = 16, Ar (H) = 1
Hence: Mr (H2SO4) = 2 x 2 + 32 + 4 x 16 = 98

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note

In calculations, multiplication or division is performed first, and only then addition or subtraction

Helpful advice

When determining the relative atomic mass, round off the values ​​that are found in the table D.I. Mendeleev to an integer

Sources:

• how to calculate molecular weight
• Molecular weight definition

To find the molecular mass find the molar mass substances in grams per mole, since these values ​​are numerically equal. Or find mass particles of a molecule in atomic mass units, add their values ​​and get the molecular mass... To find the molecular weight of a gas, you can use the Clapeyron-Mendeleev equation.

You will need

• For calculations, you will need the periodic table, scales, thermometer, pressure gauge.

Instructions

Calculation using the periodic table. Determine the chemical formula of the test substance. In the periodic table, find the chemical elements that make up the molecule. In the corresponding cells, find their atomic mass... If the table contains a fractional number, round it to the nearest whole. If the same element occurs several times in a molecule, multiply it mass by the number of occurrences. Add up all the atoms. The result is a substance.

Calculation of molecular weight when converted from grams. If given the mass of one molecule in grams, multiply it by Avogadro's constant, which is 6.022 10 ^ (23) 1 / mol. The result is the substance in grams per mole. Its numerical value coincides with the molecular weight in atomic mass units.

Calculation of the molecular weight of an arbitrary gas Take a cylinder of known volume, measured in cubic meters, evacuate air from it and weigh it on a balance. Then, pump gas into it, molecular mass which you want to define. Find again mass balloon. The difference between the gas cylinder and the empty cylinder will be equal to the gas mass, spend in grams. Measure the pressure with a manometer (c) and the temperature with a thermometer, transferring it. To do this, add the number 273 to the degrees Celsius obtained as a result of the measurement. To find the molar mass gas, its mass multiply by temperature and 8.31 (universal gas constant). Divide the obtained result by the value of the gas pressure and its volume M = m 8.31 T / (P V). This indicator, expressed in grams per mole, is the numerical molecular weight of a gas, expressed in atomic mass units.

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Sources:

• molecular weight calculation

The relative molecular weight of a substance (or simply - molecular weight) is the ratio of the value of the mass of a given substance to 1/12 of the mass of one carbon atom (C). Find the relative molecular weight mass very easy.

You will need

• Periodic table and table of molecular weights

Instructions

Relative matter is the sum of its atomic masses. To learn atomic mass one or another, just look at the periodic table. It can be found on the cover of any software, or purchased separately from a bookstore. For a pocket version, or an A4 sheet, is quite suitable. Any modern chemistry is equipped with a full-scale wall periodic table.

Having learned the atomic mass element, you can start calculating the molecular weight of the substance. This is most easily illustrated with an example:
It is required to calculate the molecular mass water (H2O). It can be seen from the molecular formula that a water molecule consists of two H atoms and one O atom. Therefore, the calculation of the molecular weight of water can be reduced to the action:
1.008*2 + 16 = 18.016

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note

Atomic mass as a concept appeared in 1803, thanks to the works of the well-known chemist John Dalton at that time. In those days, the mass of any atom was compared to the mass of a hydrogen atom. Further development this concept was received in the works of another chemist, Berzelius, in 1818, when he proposed using an oxygen atom instead of a hydrogen atom. Since 1961, chemists of all countries have taken the mass of 1/16 of an oxygen atom, or the mass of 1/12 of a carbon atom, as a unit of atomic mass. The latter is just indicated in the table of chemical elements of Mendeleev.

Helpful advice

When using the periodic table in the form in which it is presented in most chemistry textbooks and other reference books, one must understand that this table is a shortened version of the original periodic table. In its most complete version, a separate line is dedicated to each chemical element.

The molecular weight of a substance means the total atomic mass of all chemical elements that are part of a given substance. To calculate the molecular mass substances, no special effort is required.

You will need

• Mendeleev table.

Instructions

Now you need to take a closer look at any of the elements in this table. Under the name of any of the elements indicated in the table there is numerical value... It is this and the atomic mass of this element.

Now it is worth examining several examples of calculating the molecular weight, based on the fact that the atomic masses are now known. For example, you can calculate the molecular weight of a substance such as water (H2O). A water molecule contains one oxygen (O) and two hydrogen (H) atoms. Then, having found the atomic masses of hydrogen and oxygen from the periodic table, one can begin to count the molecular mass: 2 * 1.0008 (after all, there are two hydrogen) + 15.999 = 18.0006 amu (atomic mass units).

One more . The next substance, molecular mass which can be calculated, let it be ordinary table salt (NaCl). As seen from the molecular formula, the molecule table salt contains one Na atom and one chlorine Cl atom. In this case, it is considered as follows: 22.99 + 35.453 = 58.443 amu.

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note

I would like to note that the atomic masses of the isotopes various substances differ from the atomic masses in the periodic table. This is due to the fact that the number of neutrons in the nucleus of an atom and inside the isotope of the same substance is different, therefore the atomic masses are also noticeably different. Therefore, isotopes of various elements are usually designated by the letter of the given element, while adding its mass number in the upper left corner. An example of an isotope is deuterium ("heavy hydrogen"), the atomic mass of which is not equal to one, as in an ordinary atom, but to two.

Molar is weight one mole of a substance, that is, such an amount, which contains as many atoms as 12 grams of carbon. In other words, such a quantity is called the number (or constant) of Avogadro, in honor of the Italian scientist who first put forward the hypothesis. According to it, equal volumes of ideal gases (at the same temperatures and pressures) must contain the same number of molecules.

It must be firmly remembered that one mole of any substance is approximately 6.022 * 1023 molecules (either atoms or ions) of this substance. Consequently, any amount of any substance can be represented by elementary calculations in the form of a certain number of moles. And why was the prayer introduced at all? To facilitate calculations. After all, the number of elementary (molecules, atoms, ions) even in the smallest sample of matter is simply colossal! Agree, it is much more convenient to express the amount of substances in moles than in huge ones with endless rows of zeros! weight substance is determined by adding up the molar masses of all the elements included in it, taking into account the indices. For example, it is necessary to determine the molar mass of anhydrous sodium sulfate. First of all, write down its chemical formula: Na2SO4. Perform the calculations: 23 * 2 + 32 + 16 * 4 = 142 grams / mol. This will be the molar weight of this salt. And if you need to determine the molar mass of a simple substance? The rule is absolutely the same. For example, molar weight oxygen O2 = 16 * 2 = 32 grams / mol, molar weight N2 = 14 * 2 = 28 grams / mol, etc. It is even easier to determine the molar mass, the molecule of which consists of one atom. For example, molar weight sodium is 23 / mol, silver - 108 grams / mol, etc. Of course, rounded values ​​are used here to simplify calculations. If the accuracy is greater, it is necessary for the same sodium to consider its relative atomic mass equal not to 23, but to 22.98. It must also be remembered that the value of the molar mass of a substance depends on its quantitative and qualitative composition. Therefore, different substances with the same number of moles have different molar masses.

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Tip 6: How to Determine the Relative Molecular Weight

The relative molecular weight of a substance is a value that shows how many times the mass of one molecule of a given substance is more than 1/12 of the mass of the carbon isotope. In other words, it can simply be called molecular weight. How can you find the relative molecular mass?

You will need

• Mendeleev table.

Instructions

All you need for this is the periodic table and the elementary ability to perform calculations. After all, the relative molecular weight is the sum of the atomic masses of the elements that make up the one you are interested in. Of course, taking into account the indices of each element. The atomic mass of each element is indicated in the periodic table along with another important information and with very high accuracy. Rounded values ​​are fine for this purpose.

Now take the Periodic Table and determine the atomic masses of each element that makes up its composition. There are three such elements:, sulfur,. Atomic mass (H) = 1, atomic mass of sulfur (S) = 32, atomic mass of oxygen (O) = 16. Given the indices, add up: 2 + 32 + 64 = 98. This is the relative molecular mass of sulfuric acid. Note that this is an approximate, rounded result. If, for some reason, accuracy is required, then you will have to take into account that the atomic mass of sulfur is not exactly 32, but 32.06, hydrogen is not exactly 1, but 1.008, etc.

note

If you do not have the periodic table at hand, find out the relative molecular weight of a particular substance with the help of chemistry reference books.

Helpful advice

The mass of a substance in grams, which is numerically equal to its relative molecular weight, is called a mole.

The relative molecular weight of a substance shows how many times a molecule of a given substance is heavier than 1/12 of an atom of pure carbon. It can be found if it is known chemical formula using periodic table elements of Mendeleev. Otherwise, use other methods to find the molecular weight, given that it is numerically equal to the molar mass of a substance, expressed in grams per mole.

You will need

• - periodic table of chemical elements;
• - sealed container;
• - scales;
• - pressure gauge;
• - thermometer.

Instructions

If the substance is known, determine its molecular weight using the periodic table of chemical elements of Mendeleev. To do this, define the elements that are in the formula of the substance. Then, find their relative atomic masses, which are recorded in the table. If the atomic mass in the table is a fractional number, round it to the nearest whole number. If it contains several atoms of a given element, multiply the mass of one atom by their number. Add up the obtained atomic masses and get the relative molecular weight of the substance.

For example, to find the molecular weight of sulfuric H2SO4, find the relative atomic masses of the elements that are included in the formula, respectively, sulfur and oxygen Ar (H) = 1, Ar (S) = 32, Ar (O) = 16. Considering that there are 2 atoms of hydrogen in a molecule, and 4 atoms of oxygen, calculate the molecular weight of the substance Mr (H2SO4) = 2 1 + 32 + 4 ∙ 16 = 98 atomic mass units.

In the event that you know the amount of substance in moles ν and the mass of the substance m, expressed in grams, determine its molar mass for this, divide the mass by the amount of substance M = m / ν. It will be numerically equal to its relative molecular weight.

If you know the number of molecules of a substance N of known mass m, find its molar mass. It will be equal to the molecular weight, finding the ratio of the mass in grams to the number of molecules of the substance in this mass, and multiply the result by Avogadro's constant NA = 6.022 ^ 23 1 / mol (M = m ∙ N / NA).

To find the molecular weight of an unknown gas, find its mass in an airtight known volume. To do this, pump out the gas from it, creating a vacuum there. Weigh it. Then pump the gas back in and find its mass again. The difference between the masses of the empty and injected cylinder will be equal to the mass of the gas. Measure the pressure inside the cylinder using a pressure gauge in Pascals, and in Kelvin. To do this, measure the temperature of the ambient air, it will be equal inside the cylinder in degrees Celsius, to convert it to Kelvin, add 273 to the resulting value.

Determine the molar mass of the gas by finding the product of the temperature T, the mass of the gas m and the universal gas constant R (8.31). Divide the resulting number by the values ​​of pressure P and volume V, measured in m³ (M = m 8.31 T / (P V)). This number will correspond to the molecular weight of the test gas.

Hydrogen is the first element of the periodic table and the most abundant in the Universe, since it is from it that stars are mainly composed. It is a part of a substance vital for biological life - water. Hydrogen, like any other chemical element, has specific characteristics, including molar mass.

Instructions

Remember molar mass? This is the mass of one mole, that is, such an amount in which there are approximately 6.022 * 10 ^ 23 elementary particles of matter (atoms, molecules, ions). This number is called "Avogadro's number", and is named after the famous scientist Amedeo Avogadro. The molar mass of a substance numerically coincides with its molecular mass, but has a different dimension: not atomic mass units (amu), but gram / mol. Knowing this, determine the molar mass hydrogen as easy as pie.

What does the molecule have hydrogen? It is diatomic, with the formula H2. Immediately: Consider a molecule consisting of two atoms of the lightest and most abundant hydrogen isotope, protium, and not of the heavier

In chemistry, the values ​​of the absolute masses of molecules are not used, but the value of the relative molecular weight is used. It shows how many times the mass of a molecule is more than 1/12 of the mass of a carbon atom. This value is designated M r.

The relative molecular weight is equal to the sum of the relative atomic masses of its constituent atoms. Let's calculate the relative molecular weight of water.

You know that a water molecule contains two hydrogen atoms and one oxygen atom. Then its relative molecular mass will be equal to the sum of the products of the relative atomic mass of each chemical element by the number of its atoms in a water molecule:

Knowing the relative molecular weights of gaseous substances, one can compare their densities, i.e., calculate the relative density of one gas by another - D (A / B). The relative density of gas A over gas B is equal to the ratio of their relative molecular masses:

Let's calculate the relative density of carbon dioxide by hydrogen:

Now we calculate the relative density of carbon dioxide by hydrogen:

D (coal year / hydrogen) = M r (coal year): M r (hydrogen) = 44: 2 = 22.

Thus, carbon dioxide is 22 times heavier than hydrogen.

As you know, Avogadro's law applies only to gaseous substances. But chemists need to have an idea of ​​the number of molecules and in portions of liquid or solid substances. Therefore, to compare the number of molecules in substances, chemists introduced the value - molar mass .

Molar mass is denoted M, it is numerically equal to the relative molecular weight.

The ratio of the mass of a substance to its molar mass is called the amount of substance .

The amount of substance is indicated n... This is a quantitative characteristic of a portion of a substance, along with mass and volume. The amount of substance is measured in moles.

The word "mole" comes from the word "molecule". The number of molecules in equal amounts of a substance is the same.

It has been experimentally established that 1 mole of a substance contains particles (for example, molecules). This number is called Avogadro's number. And if you add a unit of measurement to it - 1 / mol, then it will be a physical quantity - Avogadro's constant, which is denoted by N A.

Molar mass is measured in g / mol. The physical meaning of molar mass is that this mass is 1 mole of a substance.

According to Avogadro's law, 1 mole of any gas will occupy the same volume. The volume of one mole of gas is called the molar volume and is denoted by V n.

Under normal conditions (which is 0 ° C and normal pressure is 1 atm. Or 760 mm Hg. Or 101.3 kPa), the molar volume is 22.4 l / mol.

Then the amount of the gas substance at n.u. can be calculated as the ratio of gas volume to molar volume.

PROBLEM 1... What amount of substance corresponds to 180 g of water?

OBJECTIVE 2. Let us calculate the volume at standard conditions, which will be occupied by carbon dioxide in the amount of 6 mol.

Bibliography

1. Collection of tasks and exercises in chemistry: 8th grade: to the textbook by P.A. Orzhekovsky et al. "Chemistry, grade 8" / P.А. Orzhekovsky, N.A. Titov, F.F. Hegel. - M .: AST: Astrel, 2006. (p. 29-34)
2. Ushakova O.V. Chemistry workbook: grade 8: to the textbook by P.A. Orzhekovsky and others. "Chemistry. Grade 8 "/ О.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M .: AST: Astrel: Profizdat, 2006. (p. 27-32)
3. Chemistry: 8th grade: textbook. for general institutions / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M .: AST: Astrel, 2005. (§§ 12, 13)
4. Chemistry: nonorg. chemistry: textbook. for 8 cl. general institution / G.E. Rudzitis, F.G. Feldman. - M .: Education, JSC "Moscow textbooks", 2009. (§§ 10, 17)
5. Encyclopedia for children. Volume 17. Chemistry / Chap. ed. by V.A. Volodin, led. scientific. ed. I. Leenson. - M .: Avanta +, 2003.

1. Single collection of digital educational resources ().
2. Electronic version of the journal "Chemistry and Life" ().
3. Chemistry tests (online) ().

Homework

1.p.69 No. 3; p.73 No. 1, 2, 4 from the textbook "Chemistry: 8th grade" (PA Orzhekovsky, LM Meshcheryakova, LS Pontak. M .: AST: Astrel, 2005).

2. №№ 65, 66, 71, 72 from the Collection of tasks and exercises in chemistry: 8th grade: to the textbook by P.A. Orzhekovsky et al. "Chemistry, grade 8" / P.А. Orzhekovsky, N.A. Titov, F.F. Hegel. - M .: AST: Astrel, 2006.