Molar table. Relative molecular weight. In general, why

Mass of atoms and molecules are very small, so as a unit of measurement it is convenient to choose a mass of one of the atoms and express the masses of the remaining atoms relative to it. This is how the founder of the atomic theory of Dalton, which was the table of atomic masses, received, adopting a mass of hydrogen atom per unit.

Until 1961, in physics for the atomic unit of mass (A.E.M. Abbreviated), 1/16 masses of the oxygen atom 16 o, and in chemistry - 1/16 of the average atomic mass of natural oxygen, which is a mixture of three isotopes. The chemical unit of mass was 0.03% more than physical.

Atomic mass and relative atomic weight of the element

Currently, a single measurement system is adopted for physics and chemistry. As a standard unit of atomic mass, 1/12 part of the mass of carbon atom 12 S.

1 A.E.M. \u003d 1/12 m (12 s) \u003d 1,66057 × 10 -27 kg \u003d 1,66057 × 10 -24

Definition

Relative atomic weight of the element (A R) - this is a dimensionless value equal to the relation medium mass Atom of an element to 1/12 mass of atom 12 C.

When calculating the relative atomic mass, the prevalence of isotopes of elements in earth Kore. For example, chlorine has two isotope 35 CL (75.5%) and 37 CL (24.5%). The relative atomic mass of chlorine is equal to:

A R (Cl) \u003d (0.755 × m (35 ° C) + 0.245 × m (37 CL)) / (1/12 × m (12 s) \u003d 35.5.

From the determination of the relative atomic mass it follows that the average absolute mass of the atom is equal to the relative atomic mass, multiplied by A.Е.M.:

m (CL) \u003d 35.5 × 1,66057 × 10 -24 \u003d 5.89 × 10 -23

Relative molecular weight of the element

Definition

Relative molecular weight of substance (M R) - this is a dimensionless value equal to the ratio of the mass of the substance molecule to 1/12 mass of atom 12 C.

The relative molecular weight of the molecule is equal to the sum of the relative atomic masses of atoms included in the molecule, for example:

M R (n 2 O) \u003d 2 × a r (n) + a r (o) \u003d 2 × 14.0067 + 15,994 \u003d 44,0128.

The absolute mass of the molecule is equal to the relative molecular weight multiplied by A.Y.m.

The number of atoms and molecules in conventional samples of substances is very large, therefore, as a characteristic of the amount of substance, a special unit of measurement is used - mole.

Mol is the amount of substance that contains as many particles (molecules, atoms, ions, electrons), how many carbon atoms are contained in 12 g of isotope 12 C.

The mass of one atom 12 s is 12 AE.m., therefore, the number of atoms in 12 g of isotope 12 s is:

N a \u003d 12 g / 12 × 1,66057 × 10 -24 g \u003d 1 / 1,66057 × 10 -24 \u003d 6,0221 × 10 -23.

Thus, the mole of substances contains 6.0221 × 10 -23 particles of this substance.

The physical value of n a is called constant avogadro, it has dimension \u003d mol -1. The number 6,0221 × 10 -23 is called the Nogadro number.

Definition

Molar mass (M) - This is a mass of 1 mol of substance.

It is easy to show that the numerical values \u200b\u200bof the molar mass M and the relative molecular weight M r are equal, however the first value has dimensions [M] \u003d g / mol, and the second is dimensionless:

M \u003d n a × m (1 molecules) \u003d n a × m × 1 AE.m. \u003d (N A × 1 AE.M.) × M r \u003d × m r.

This means that if the mass of some molecule is equal, for example, 44 AE.M., then the mass of one mole of molecules is 44 g.

Permanent Avogadro is a ratio of proportionality that ensures the transition from molecular relationship to molar.

One of the main units in the international system of units (C) is the unit of the amount of substance is mole.

Mole – this is an amount of substance that contains so many structural units of this substance (molecules, atoms, ions, etc.), how many carbon atoms are contained in 0.012 kg (12 g) carbon isotope 12 FROM .

Considering that the value of the absolute atomic mass for carbon is equal m.(C) \u003d 1.99 · 10  26 kg, you can calculate the number of carbon atoms N. BUT contained in 0.012 kg of carbon.

Mol of any substance contains the same number of particles of this substance (structural units). The number of structural units contained in substance by the amount of one mol is equal to 6.02 · 10 23 and called avogadro's number (N. BUT ).

For example, one mole of copper contains 6.02 · 10 23 copper atoms (Cu), and one mooss of hydrogen (H 2) is 6.02 · 10 23 hydrogen molecules.

Molar mass (M) the mass of the substance taken in the amount of 1 mol is called.

The molar mass is denoted by the letter M and has dimension [g / mol]. In physics, they use dimension [kg / kmol].

In the general case, the numerical value of the molar mass of the substance is numerically coincided with the value of its relative molecular (relative atomic) mass.

For example, the relative molecular weight of the water is equal to:

Mr (H 2 O) \u003d 2AR (H) + AR (O) \u003d 2 ∙ 1 + 16 \u003d 18 A.E.M.

The molar mass of water has the same magnitude, but expressed in g / mol:

M (H 2 O) = 18 g / mol.

Thus, mole of water containing 6.02 · 10 23 water molecules (respectively 2 · 6.02 · 10 23 hydrogen atoms and 6.02 · 10 23 oxygen atoms), has a lot of 18 grams. In water, the amount of substance is 1 mol, contains 2 mol of hydrogen atoms and one mole of oxygen atoms.

1.3.4. Communication between the mass and its number

Knowing the mass of the substance and its chemical formula, which means the value of its molar mass, one can determine the amount of substance and, on the contrary, knowing the amount of substance, can be determined by its mass. For such calculations, the formulas should be used:

where ν is the amount of substance [mol]; m. - mass of substance, [g] or [kg]; M is the molar mass of the substance, [g / mol] or [kg / kmol].

For example, to find the mass of sodium sulfate (Na 2 SO 4), 5 mole will find:

1) The value of the relative molecular weight of Na 2 SO 4, representing the sum of the rounded values \u200b\u200bof the relative atomic masses:

Mr (Na 2 SO 4) \u003d 2AR (Na) + Ar (S) + 4Ar (O) \u003d 142,

2) numerically equal to the value of the molar mass of the substance:

M (Na 2 SO 4) = 142 g / mol

3) and, finally, mass 5 mol sodium sulfate:

m \u003d ν · m = 5 mol · 142 g / mol \u003d 710

Answer: 710.

1.3.5. Communication between the volume of substance and its number

Under normal conditions (N.U.), i.e. With pressure r equal to 101325 PA (760 mm. RT. Art.), and temperature T, equal to 273.15 K (0 c), one mole of various gases and vapors takes the same volume equal to 22.4 liters.

Volume occupied by 1 mol of gas or a pair of n., Called molar volume Gas and has a dimension liter on mole.

V mol \u003d 22.4 l / mol.

Knowing the amount of gaseous substance (ν ) and the value of the molar volume (V mol) it is possible to calculate its volume (V) under normal conditions:

V \u003d ν · v

where ν is the amount of substance [mol]; V is the volume of the gaseous substance [l]; V mol \u003d 22.4 l / mol.

And, on the contrary, knowing the volume ( V.) gaseous substance under normal conditions, it is possible to calculate its number (ν) :

It's no secret that chemical signs allow you to portray the composition of a complex substance in the form of formulas.

Chemical formula - This is a conditional record of the composition of the substance through chemical signs and indices.

Formulas are distinguished by molecular, structural, electronic and others.

Molecular formulas (H3P04, FE203, A1 (OH) 3, Na2S04, 02, etc.) show high-quality (i.e., from which elements the substance consists) and quantitative (that is, how many atoms of each element are in the substance) composition.

Structural formulasshow the order of the compound of atoms in the molecule, connecting atoms to inks (one shorter - one chemical communications Between two atoms in the molecule).

Relative Atomic and Molecule Mass

Relative atomic massthe substitute or element is a dimensionless value. Why is dimensionless, because the mass should have dimension?

The reason is that the atomic mass of the substance in kg is very small and is expressed in order 10 in minus 27 degrees. That in the calculations not to take into account this indicator, the mass of each element led to the ratio of 1/12 of the mass of carbon isotope. For this reason, the relative atomic mass of carbon is 12 units.

The current values \u200b\u200bof the relative atomic masses are given in periodic system Elements D.I. Imeleeev. For most elements are indicated

The medium-gradatic values \u200b\u200bof the atomic masses of the natural mixture of isotopes of these elements.

For example, the relative mass of hydrogen is equal to 1, and oxygen 16.

Relative molecular weight of simple and complex substances Numerically equal to the sum of the relative atomic masses of atoms included in the molecule.

For example, the relative molecular weight of water consisting of two hydrogen atoms and one oxygen atom is equal to

By chemical formula can be calculated as chemical compositionand molecular weight.

Chemical formulas quantitative composition It is of great importance for numerous calculations that are manufactured by chemical composition.

The calculation of the relative molecular weight of the substance according to the chemical formula is made by adding the products of the relative atomic masses of the elements to the corresponding indices in the chemical formula.

How the molecular weight of the substance is calculated, we looked at a little higher.

And it is this task that automates our chemical calculator.

Knowing the molecular weight of the substance, we should not calculate anything and the molar mass.

Mole - there is the amount of substance of the system containing the same structural elementshow many contain atoms in carbon-12 weighing 12 grams

Thus, the molar mass of the substance with accuracy corresponds to the relative molecular weight and has a dimension of gram / mole

Thus, the molar mass of water is 18 grams / mol.

The molar mass of the substance can be defined as an attitude mass this portion of substance to the number of substances In this portion

Distinctive features

With regard to other calculators, calculating the molar mass of the substance, this calculator has the following features:

The formula may contain brackets for example

The formula may contain a coefficient

If there is a need to calculate the mass fractions of each chemical element in the formula, it is worth using the calculator mass fraction of the substance online

Molar masses of which chemical elements are not rounded?

It is logical, assume if you have read, from where the concept of "relative mass" appears, that the "non-rounded" mass will have one element - carbon . Will other chemical elements with "not rounded" masses? I doubt.

syntax

molar formula [!]

where the formula is an arbitrary formula of a chemical substance.

Attention! Chemical elements in the formula must be indicated as in the Mendeleev table.

a simple example will show what the error price does not follow the register (uppercase or lowercase characters) letters

If you write Co, it is carbon and oxygen, and if you write a CO is cobalt.

In any part of the formula, you can insert a service symbol (exclamation mark).

What does he give us?

It all parameters reloads to the level of accuracy that is used in school Program. It is very convenient for those who solve school challenges.

For example, the molar mass of water in school textbooks is 18, and if we take more accurate algorithms, we obtain that the molar mass is equal to 18.01528. The difference is small, but if you make a calculation of the mass fraction of a chemical substance, it turns out a small, but very unpleasant discrepancy in the output parameters, which can be misleading inexperienced users of the calculator.

Examples

we write request mOLAR NAMGU3O24C18H27.

we get the answer

If you write an exclamation mark in the input parameters, we will receive such an answer

The purpose of the lesson.

Pacquaint students with the concept of "amount of substance", "mole"; form an idea of \u200b\u200bthe molar mass of the substance; Teach to calculate the amount of substance at a known mass of the substance and a mass of a substance according to a known amount of substance.

Type of lesson: Study lesson and primary consolidation of knowledge.

Technologies: Elements of cooperation technology and problem learning.

Methods: Heuristic conversation, search engines,

Basic concepts.The amount of substance, mole, the number of Avogadro, the constant Avogadro, the molar mass.

Planned learning outcomes.Know the number of Avogadro, determining the amount of substance and praying. To be able to determine the number of structural units on this number of substance and vice versa. Know about equality numeric values molar and relative molecular masses. Be able to calculate the mass of this amount of substance.

Equipment: Multimedia - equipment, periodic table DI. Mendeleeva.

During the classes

1. Organizational moment.

Hello expensive guys. My name is Alla Stanislavovna, and today I will spend your chemistry lesson.

My friends! I am very glad
Log in Friendly Your Class
And for me already award
Attention your smart eye
I know: each in the genius class,
But without difficulty, talent is not
Cross the swords of your opinions -
We are writing a lesson together!

2. Setting the problem of lesson and goal.

And we will begin our lesson with a funny, non-standard situation that once happened in the store.

The eighth grader Kostya went to the store and asked the saleswoman to sell him 10 mours crash salt . What answered the cruise saleswoman?

You will respond to this question after studying a new topic.

And what term is new for you?

I'll tell you today, or about the malicious role of moths.

Mol eats wool and fur - just a panic at all ...

Well, in chemistry - please! There is another word "mole".

And today at the lesson, we will meet with this concept.

Our lesson is called "the amount of substance. Molar mass" ( write to notebook).

The purpose of our lesson:

first: to get acquainted with the concept of "amount of substance", "mole";

secondly: to form an idea of \u200b\u200bthe molar weight of the substance;

thirdly: learn how to calculate the amount of substance at a well-known mass of the substance and mass of the substance according to the number of substance.

3. Studying a new material.

Everything is subject to measurement. And you are already familiar with the units of mass measurement or volume. For example,

When buying sugar, we define it ___ (weight) using weights using the measurement units -________ (kilograms, grams).

When buying a spill milk, we define it _____ (volume) with the help of measuring dishes, using units of measurement ______ (liter, milliliter)

We can also determine how many pieces (particles) in 1 kilogram?

Chemistry Science about substances. Substances consist of atoms or molecules. Which units can be measured, substances? After all, atoms and molecules cannot be calculated and weighed.

And then, a special unit was selected for measuring the substance, in which two values \u200b\u200bwere connected - the number of molecules and mass of the substance.

Such a unit is called the amount of substance or mole.

To measure 1 mol of substances, you need to take as many grams of it, what is the relative mass of the substance:

1 mol H 2 weighs 2 g (Mr (H 2) \u003d 2)

1mol o 2 weighs 32 g (Mr (O 2) \u003d 32)

1mol H 2 O weighs 18 g (Mr (H 2 O) \u003d 18)

And how many real particles - molecules are contained in 1 mole of any substance?

It has been established that 1 mol of any substance always contains the same number of molecules. This number is 6 10 23. For example,

1 mole of water \u003d 6 . 10 23 molecules H 2 o,

1 mole iron \u003d 6 . 10 23 FE atoms

1 mole chlorine \u003d 6 . 10 23 CL 2 molecules,

1 mole of chlorine ions Cl - \u003d 6 . 10 23 CL ions -.

In honor of the Italian scientist Amedeo Avogadro, this number was called Permanent Avogadro

Denotes N a \u003d 6? 10 23

Constant Avogadro is so great that it is difficult to imagination.

The sugar desert contains less than three moles of the smallest grain.

If you take 1 mole of dollar paper, then they will cover all the continents of the Earth with a 2-kilometer dense layer.

Now we can write down the definition of the concept of "mole".

Mol is the amount of substance that contains 6 10 23structural units of this substance - molecules oratoms.

The amount of substance is denoted by the letter - n, measured in a mole

To find out the number of molecules (N), you can use the formula:

knowing the number of molecules you can find the amount of substance:

And what needs to be done to measure 1 mol of substances?

You need to take so many grams of this substance, what is its relative molecular weight.

Mass 1 mol of matter is called molar mass. Denotes the letter - M. located by the formula:

Suppose in which units will the molar mass be measured?

measured in (g / mol)

The molar mass by value coincides with the relative atomic or molecular weight, but differ by units of measurement (M - g / mol; Mr, Ar - dimensionless values).

M (g / mol) \u003d mr

The table for illustration shows the molar masses M. For several substances of different buildings.

Table. Molar masses of various substances.

Substance	Molecular or atomic mass MR, (AR)	Molar mass M.	Number of Avogadro
Water n 2 o			6,02? 10 23 molecules
Calcium Oxide SAO			6,02? 10 23 molecules
Carbon 12 S.			6,02? 10 23 atoms
			6,02? 10 23 atoms
Atom chlorine CL.		35.5 g / mol	6,02? 10 23 atoms
Chlorine CL 2 molecule			6,02? 10 23 molecules

Molar masses of substances differ from each other, but the amount of substance remains the same - 1 mol.

Number of moles of matter n. find mass m. this substance (d) to its molar mass M. (g / mol).

Consequently, the mass can be found by the formula:

Set the relationship of the main values: m \u003d n? M, n \u003d m / m, m \u003d m / n, n \u003d n / n a, n \u003d n? N a, where n a 6,02? 10 23 mol -1

4. Fastening material

We determined how the number and mass of the substance are related. Now we will solve the tasks using the concepts discussed above.

Task number 1. . Determine the mass of oxygen by the amount of substance 3.6 mol.

Task number 2. . What amount of substance will contain 64 g of oxygen?

Task number 3. . Calculate the amount of substance and the number of molecules contained in carbon dioxide weighing 11 g.

Task 4. . Find a lot of 24. 10 23 ozone molecules about 3.

Let's try to answer the question given at the beginning of the lesson:

If the saleswoman studied well in the eighth grade, then she quickly considers weight (? ACL) \u003d 58.5 (g / mol)? 10 (mol) \u003d 585 grams.

After that, it pours salt into the package, weighing and politely say "Pay on the cashier."

5. Homework.

So friends came to say goodbye.
And I want to wish you:
Always learning with the hunt
Always work with the hunt.
And never lose heart.

Literature:

1. Alikbelova L.Yu. Entertaining chemistry, M, "AST-PRESS", 1999
2. Berdonosov S.S., Chemistry 8kl, Miros, 1994;
3. Newspaper "Chemistry at School" №44 1996 P. 9.
4. Gabrielyan O.S. Chemistry grade 8. M.: Drop, 2007.
5. Ivanova R.G. Chemistry 8-9 class. M.: Enlightenment, 2005.
6. Novoshinsky I.I. Novoshinskaya N.S. Types of chemical problems and methods for their solutions 8-11 classes. M.: Onyx 21 century.
7. Educational collection. Chemistry. Basic course. 8-9 class. Laboratory of Multimedia MargTu systems. Yoshkar-Ola, 2003.

Try to evaluate your own knowledge and skills after today's lesson.

• i understand everything, I can teach another.
• i can explain the new topic with some help.
• it is difficult for me to figure it out on my own new topic, need help.

Formula			Molar mass (m, g / mol)	Mass (m, gram)	Number of particles (N)

Formula	Relative Molecular Mass (MR)	Number of substance (N, mol))	Molar mass (m, g / mol)	Mass (m, gram)	Number of particles (N)

Formula	Relative Molecular Mass (MR)	Number of substance (N, mol))	Molar mass (m, g / mol)	Mass (m, gram)	Number of particles (N)

In chemistry, the concept of "molecular weight" is extremely important. Molecular weight is often confused with a molar mass. What is the difference between these values, and what properties do you have?

Molecular mass

Atoms and molecules are the smallest particles of any chemical substances. If you try to express their mass in grams, then the number in which about 20 zeros will be before the comma. Therefore, measure the mass in such units as grams is inconvenient. To get out of this situation, it follows some very small mass to take per unit, and all other masses to express in relation to it. As this unit, 1/12 masses of carbon atom are used.

Relative molecular weight is a mass of a substance molecule, which is measured in atomic units of mass. The molecular weight is equal to the ratio of the mass of a molecule of one or another substance to 1/12 part of the mass of the carbon atom. It shows how many times the mass of the molecule of a certain substance is more than 1/12 mass of the carbon atom.

Fig. 1. Table of molecular masses of organic substances.

Atomic unit of mass (a. E. M.) Is 1.66 * 10 B -24 degree and is 1/12 part of the mass of carbon atom, that is, the carbon element isotope isotope atom is a mass number of 12. The chemical element in nature may have Several stable isotopes, therefore, when they talk about the relative atomic weight of the element, or, as often indicated, the atomic mass of the element A, then necessarily take into account the atomic mass of all stable nuclides.

The molecular weight is often confused with a molar mass, the unit of measurement of which is g / mol. And really numerically, these two values \u200b\u200bare absolutely identical, but they have absolutely different dimensions.

Relative molecular weight can be found by resolving atomic masses

In order to calculate the molecular weight of simple and complex substances, it is necessary to find the sum of the relative atomic masses of atoms included in the molecule. For example, the relative molecular weight of the Mr (H 2 O) water, which consists, as is known, from two hydrogen atoms and one oxygen atom, equal to 1 * 2 + 16 \u003d 18.

This means that the mass of water molecules is 18 times more than 1/12 mass of carbon atom. And the molecular weight of the air is 29.

Fig. 2. Formula Relative molecular weight.

Atomic mass

Atomic mass chemical element - Also one of the most important designations in chemistry. Atomic mass is the average value of atomic masses of stable natural isotopes of this element, taking into account their relative content in nature (their natural distribution). So, in nature there are two stable isotopes of chlorine element Cl with mass numbers 35 and 37:

Ar (CL) \u003d (34.97 * 0.7553) + (36.95 * 0.2447) \u003d 35.45 - This is precisely such a value for chlorine element as its relative atomic mass.

For the first time, the calculation of atomic scales was made by D. Dalton. Atomic weights of the elements he attributed to the atomic weight of hydrogen, accepting it per unit. However, the weight of the oxygen atom and some other elements calculated in accordance with its principle of the "greatest simplicity" were incorrect.

Fig. 3. D. Dalton.

True atomic masses are meager. A hydrogen atom weighs 1.674 * 10 B -24 degrees of grams, oxygen 26.67 * 10 B -24 degrees of grams, and carbon 19,993 * 10 B -24 degrees of grams.

What did we know?

In the school curriculum in chemistry (grade 8), much attention is paid to such a concept as a relative atomic and molecular weight of the substance. Students learn their differences and features, and also learn to determine the masses of gases and substances.

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