Target: To acquaint students with the concepts of "amount of substance", "molar mass" to give an idea of ​​the Avogadro constant. Show the relationship between the amount of a substance, the number of particles and the Avogadro constant, as well as the relationship between the molar mass, mass and amount of a substance. Learn to do calculations.

Lesson type: lesson of studying and primary consolidation of new knowledge.

During the classes

I. Organizational moment

II. Checking d / z on the topic: "Types of chemical reactions"

III. Learning new material

1. Amount of substance - mole

Substances react in strictly defined proportions. For example, to obtain the substance water, you need to take so much hydrogen and oxygen that for every two molecules of hydrogen there is one molecule of oxygen:

2H 2 + O 2 \u003d 2H 2 O

To obtain the substance iron sulfide, you need to take so much iron and sulfur that for each atom of iron there is one atom of sulfur.

To obtain the substance phosphorus oxide, you need to take so many molecules of phosphorus and oxygen that for four molecules of phosphorus there are five molecules of oxygen.

It is impossible to determine the number of atoms, molecules and other particles in practice - they are too small and not visible to the naked eye. To determine the number of structural units (atoms, molecules) in chemistry, a special value is used - amount of matter ( v - nude). The unit of quantity of a substance is mole.

• A mole is the amount of a substance that contains as many structural particles (atoms, molecules) as there are atoms in 12 g of carbon.

It has been experimentally established that 12 g of carbon contains 6·10 23 atoms. This means that one mole of any substance, regardless of its state of aggregation, contains the same number of particles - 6 10 23.

• 1 mole of oxygen (O 2) contains 6 10 23 molecules.
• 1 mol of hydrogen (H 2) contains 6 10 23 molecules.
• 1 mol of water (H 2 O) contains 6 10 23 molecules.
• 1 mole of iron (Fe) contains 6 10 23 molecules.

Exercise: Using the information you received, answer the following questions:

a) how many oxygen atoms are there in 1 mole of oxygen?

– 6 10 23 2 = 12 10 23 atoms.

b) how many hydrogen and oxygen atoms are there in 1 mole of water (H 2 O)?

– 6 10 23 2 = 12 10 23 hydrogen atoms and 6 10 23 oxygen atoms.

Number 6 10 23 is called Avogadro's constant in honor of the Italian scientist of the 19th century and is designated NA. Units of measurement are atoms/mol or molecules/mol.

2. Solving problems for finding the amount of substance

Often you need to know how many particles of a substance are contained in a certain amount of a substance. Or to find the amount of substance by a known number of molecules. These calculations can be done using the formula:

where N is the number of molecules, NA is the Avogadro constant, v- amount of substance. From this formula, you can express the amount of substance.

v= N / NA

Task 1. How many atoms are there in 2 moles of sulfur?

N = 2 6 10 23 = 12 10 23 atoms.

Task 2. How many atoms are there in 0.5 moles of iron?

N = 0.5 6 10 23 = 3 10 23 atoms.

Task 3. How many molecules are there in 5 moles of carbon dioxide?

N = 5 6 10 23 = 30 10 23 molecules.

Task 4. How much of a substance is 12 10 23 molecules of this substance?

v= 12 10 23 / 6 10 23 \u003d 2 mol.

Task 5. What amount of a substance is 0.6 10 23 molecules of this substance?

v= 0.6 10 23 / 6 10 23 \u003d 0.1 mol.

Task 6. How much of a substance is 3 10 23 molecules of this substance?

v= 3 10 23 / 6 10 23 \u003d 0.5 mol.

3. Molar mass

For chemical reactions, you need to take into account the amount of substance in moles.

Q: But how in practice to measure 2, or 2.5 moles of a substance? What is the best unit to measure the mass of substances?

For convenience in chemistry, molar mass is used.

Molar mass is the mass of one mole of a substance.

It is designated - M. It is measured in g / mol.

The molar mass is equal to the ratio of the mass of a substance to the corresponding amount of the substance.

Molar mass is a constant value. The numerical value of the molar mass corresponds to the value of the relative atomic or relative molecular weight.

Q: How can I find relative atomic or relative molecular weights?

Mr(S) = 32; M (S) \u003d 32 g / mol - which corresponds to 1 mole of sulfur

Mr (H 2 O) = 18; M (H 2 O) \u003d 18 g / mol - which corresponds to 1 mole of water.

4. Solving problems on finding the mass of matter

Task 7. Determine the mass of 0.5 mol of iron.

Task 8. Determine the mass of 0.25 mol of copper

Task 9. Determine the mass of 2 moles of carbon dioxide (CO 2)

Task 10. How many moles of copper oxide - CuO make up 160 g of copper oxide?

v= 160 / 80 = 8 mol

Task 11. How many moles of water correspond to 30 g of water

v= 30/18 = 1.66 mol

Task 12. How many moles of magnesium corresponds to its 40 grams?

v= 40/24 = 1.66 mol

IV. Anchoring

Front poll:

1. What is the amount of substance?
2. What is 1 mole of any substance equal to?
3. What is molar mass?
4. Is there a difference between the terms "mole of molecules" and "mole of atoms"?
5. Explain using the example of the ammonia molecule NH3.
6. Why is it important to know formulas when solving problems?

Tasks:

1. How many molecules are there in 180 grams of water?
2. How many molecules make up 80 g of carbon dioxide?

V. Homework

Study the text of the paragraph, make two tasks: to find the amount of substance; to find the mass of a substance.

Literature:

1. Gara N.N. Chemistry. Lessons in Grade 8: A Teacher's Guide. _ M.: Enlightenment, 2009.
2. Rudzites G.E., Feldman F.G. Chemistry. Grade 8: Textbook for general educational institutions - M .: Education, 2009.

The most typical processes carried out in chemistry are chemical reactions, i.e. interactions between some initial substances, leading to the formation of new substances. Substances react in certain quantitative relationships, which must be taken into account in order to obtain the desired products using the minimum amount of starting substances and not creating useless production waste. To calculate the masses of reacting substances, it turns out that one more physical quantity is needed, which characterizes a portion of a substance in terms of the number of structural units contained in it. In itself, the ego number is unusually large. This is obvious, in particular, from Example 2.2. Therefore, in practical calculations, the number of structural units is replaced by a special value called quantity substances.

The amount of substance is a measure of the number of structural units, determined by the expression

where N(X)- the number of structural units of the substance X in a real or mentally taken portion of a substance, N A = 6.02 10 23 - Avogadro's constant (number), widely used in science, one of the fundamental physical constants. If necessary, a more accurate value of the Avogadro constant 6.02214 10 23 can be used. A portion of a substance containing N a structural units, represents a single amount of a substance - 1 mol. Thus, the amount of a substance is measured in moles, and the Avogadro constant has a unit of 1/mol, or in another notation, mol -1.

With all sorts of reasoning and calculations related to the properties of matter and chemical reactions, the concept amount of substance completely replaces the concept number of structural units. This eliminates the need to use large numbers. For example, instead of saying "taken 6.02 10 23 structural units (molecules) of water", we say: "taken 1 mole of water."

Each portion of a substance is characterized by both mass and quantity of the substance.

The ratio of the mass of a substanceXto the amount of the substance is called the molar massM(X):

The molar mass is numerically equal to the mass of 1 mol of a substance. This is an important quantitative characteristic of each substance, depending only on the mass of structural units. The Avogadro number is set in such a way that the molar mass of a substance, expressed in g / mol, numerically coincides with the relative molecular mass M g For a water molecule M g = 18. This means that the molar mass of water is M (H 2 0) \u003d 18 g / mol. Using the data of the periodic table, it is possible to calculate more accurate values M g and M(X), but in teaching tasks in chemistry this is usually not required. From all that has been said, it is clear how easy it is to calculate the molar mass of a substance - it is enough to add the atomic masses in accordance with the formula of the substance and put the unit g / mol. Therefore, formula (2.4) is practically used to calculate the amount of a substance:

Example 2.9. Calculate the molar mass of baking soda NaHC0 3 .

Solution. According to the formula of the substance M g = 23 + 1 + 12 + 3 16 = 84. Hence, by definition, M(NaIIC0 3) = 84 g/mol.

Example 2.10. What is the amount of substance in 16.8 g of baking soda? Solution. M(NaHC0 3) = 84 g/mol (see above). By formula (2.5)

Example 2.11. How many fractions (structural units) of drinking soda are in 16.8 g of a substance?

Solution. Transforming formula (2.3), we find:

AT(NaHC0 3) = N a n(NaHC0 3);

tt(NaHC0 3) = 0.20 mol (see example 2.10);

N (NaHC0 3) \u003d 6.02 10 23 mol "1 0.20 mol \u003d 1.204 10 23.

Example 2.12. How many atoms are there in 16.8 g of baking soda?

Solution. Baking soda, NaHC0 3 , is made up of sodium, hydrogen, carbon and oxygen atoms. In total, there are 1 + 1 + 1 + 3 = 6 atoms in the structural unit of matter. As was found in example 2.11, this mass of drinking soda consists of 1.204 10 23 structural units. Therefore, the total number of atoms in a substance is

One of the basic units in the International System of Units (SI) is the unit of quantity of a substance is the mole.

mole – this is such an amount of a substance that contains as many structural units of a given substance (molecules, atoms, ions, etc.) as there are carbon atoms in 0.012 kg (12 g) of a carbon isotope 12 FROM .

Given that the value of the absolute atomic mass for carbon is 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.

A mole of any substance contains the same number of particles of this substance (structural units). The number of structural units contained in a substance with an amount of one mole is 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 mole of hydrogen (H 2) contains 6.02 10 23 hydrogen molecules.

molar mass(M) is the mass of a substance taken in an amount of 1 mol.

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

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

For example, the relative molecular weight of water is:

Mr (H 2 O) \u003d 2Ar (H) + Ar (O) \u003d 2 1 + 16 \u003d 18 a.m.u.

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

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

Thus, a 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 mass of 18 grams. 1 mole of water contains 2 moles of hydrogen atoms and 1 mole of oxygen atoms.

1.3.4. The relationship between the mass of a substance and its quantity

Knowing the mass of a substance and its chemical formula, and hence the value of its molar mass, one can determine the amount of a substance and, conversely, knowing the amount of a substance, one can determine its mass. For such calculations, you should use the formulas:

where ν is the amount of substance, [mol]; m is the mass of the 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) in the amount of 5 mol, we find:

1) the value of the relative molecular weight of Na 2 SO 4, which is the sum of the rounded values ​​of the relative atomic masses:

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

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

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

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

m = ν M = 5 mol 142 g/mol = 710 g

Answer: 710.

1.3.5. The relationship between the volume of a substance and its quantity

Under normal conditions (n.o.), i.e. at pressure R , equal to 101325 Pa (760 mm Hg), and temperature T, equal to 273.15 K (0 С), one mole of various gases and vapors occupies the same volume, equal to 22.4 l.

The volume occupied by 1 mole of gas or vapor at n.o. is called molar volumegas and has the dimension of a liter per mole.

V mol \u003d 22.4 l / mol.

Knowing the amount of gaseous substance (ν ) and molar volume value (V mol) you can calculate its volume (V) under normal conditions:

V = ν V mol,

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

Conversely, knowing the volume ( V) of a gaseous substance under normal conditions, you can calculate its amount (ν) :

The seventh basic unit of the SI system - the unit of quantity of a substance, the mole - occupies a very special place among the basic units. There are several reasons for this. The first reason is that this value practically duplicates the existing basic unit, the unit of mass. Mass, defined as a measure of the inertia of a body or a measure of gravitational forces, is a measure of the amount of matter. The second reason, caused by the first and closely related to it, is that there is still no implementation of the standard unit of this physical quantity. Numerous attempts to independently reproduce the mole led to the fact that the accumulation of an accurately measured amount of a substance was eventually reduced to other standards of basic physical quantities. For example, attempts to electrolytically isolate a substance led to the need to measure the mass and strength of the electric current. Accurate measurement of the number of atoms in crystals led to the measurement of the linear dimensions of the crystal and its mass. In all other similar attempts to independently reproduce the mole, metrologists have encountered the same difficulties.

The question naturally arises: why did the metrological services of the most developed countries agree that among the basic units there were two different ones characterizing the same physical concept? The answer to this question is obvious if we start from the basic principle of constructing systems of units of physical quantities - the convenience of practical use. Indeed, to describe the parameters of mechanical processes, it is most convenient to use an arbitrary artificial measure of mass - a kilogram. To describe chemical processes, it is very important to know the number of elementary particles, atoms or molecules that take part in chemical reactions. For this reason, the mole is called the chemical basic unit of the SI system, emphasizing the fact that it is introduced not to describe some new phenomena, but to serve specific measurements related to the chemical interaction of substances and materials.

This specificity gave rise to another very important quality of the unit of quantity of a substance - the mole. It consists in the fact that with the introduction of the chemical definition of a unit, not just the amount of any substance is regulated, but the amount of a substance in the form of atoms or molecules of a given type. Therefore, a mole can be called a unit of quantity of an individual substance. With this definition, the mole becomes a more universal unit of quantity of a substance than the kilogram. In fact, individual substances have the properties of inertia and gravity, so that the standard mole, provided that it is implemented at the required level of accuracy, can be used as a mass standard. The opposite is impossible, since a mass measure made, for example, from an alloy of platinum and iridium, can never be a carrier of properties inherent, for example, to silicon or carbon.

In addition to the convenience of using the unit of quantity of a substance in carrying out chemical reactions, the introduction of the second basic unit of the quantity of a substance is justified by one more circumstance. It consists in the fact that measurements of the amount of a substance must be carried out in a very wide range of changes in this value. In macroscopic phenomena, measurement objects in the form of solids contain about 10 23 atoms. This is an order of magnitude of the number of atoms in the gram equivalent of a substance. In microscopic phenomena, there is even the problem of detecting individual atoms. Therefore, the amount of a substance must be measured in a range of more than 20 orders of magnitude! Naturally, not a single device, not a single device at the reference level will provide such an opportunity.

For this reason, the desire of metrologists to have as basic units two units of the amount of a substance becomes obvious, one of which allows accurate measurements in the field of large quantities, and the second allows measuring particles of a certain substance one by one.

The unwillingness of metrologists to abandon any basic unit of the quantity of a substance, for example, the kilogram, is due to the fact that the reproduction of this unit by making a copy of the prototype is possible with very high accuracy. Reproducing mass by independent methods, such as taking one liter of water or electrolytically depositing a certain mass of metal from solution, is much less accurate than making a copy of a kilogram by weighing.

In connection with the listed difficulties, the implementation of the basic unit of the quantity of a substance in the form of a standard does not exist. The definition of a mole reads:

A mole is the amount of a substance that has as many structural units as there are in 12 grams of the carbon monoisotope C 12 .

It clearly follows from the definition that this value has not been precisely established. In physical terms, it is equal to the Avogadro constant - the number of atoms in a gram equivalent of carbon. This makes it possible to define the mole as the reciprocal of Avogadro's constant. For 12 grams of carbon with a mass number of 12, the number of atoms will be N A .

In accordance with this, the problem of creating a standard for the amount of a substance is reduced to refining the Avogadro constant. Technically, the following procedure is currently used:

A certain amount (hundreds of grams) of ultra-pure silicon is produced.

Precise mass spectrometers measure the isotopic composition of this silicon.

A single crystal of ultrapure silicon is grown.

The volume of a single crystal is measured by measuring its mass and density V.

The X-ray interferometer measures the size of the unit cell of a cube in a single crystal of silicon - a.

Since the crystal lattice in silicon has the shape of a cube, the number of structural units in a single crystal is equal to

By measuring the mass and equivalent atomic weight, the number of moles of silicon in the crystal is determined

where m is the mass of the crystal, c. - atomic weight of the sample, taking into account the different percentages of isotopes.

The Avogadro constant is defined as the number of structural units in one gram equivalent of silicon

Work to refine the Avogadro constant is constantly being carried out by international metrological centers. The national physical laboratory of Germany PTB in Braunschweig is especially active. There is a constant struggle for the purity of the starting material (silicon), both due to purification from impurities and due to the homogeneity of the isotopic composition. The present level of impurity content for most elements is no more than one particle per million silicon particles, and for some impurities that interfere with crystal formation, one particle per billion silicon particles.

With the repetition of work on the refinement of the Avogadro constant, the means for measuring the mass of a crystal, its density, isotopic composition, and crystal lattice dimensions are improved. At present, it is possible to guarantee the reliability of the determination of the Avogadro constant at the level of 10 -6 -10 -7 by relative error. Nevertheless, this value is much larger than the error in making copies of the kilogram standard by weighing.

In addition to accuracy inferior to the accuracy of reproduction of the kilogram, the described procedure for determining the mole suffers from a number of significant drawbacks. The most important of them is the impossibility of creating a measure equal to any part of a mole or several moles, i.e., creating measures of multiples and submultiples. Any attempts to do this lead to the need for weighing, i.e., determining the mass and reaching the kilogram standard. Naturally, the meaning of reproducing the mole is lost in this case. Another fundamental flaw in the procedure for using the mole is that the measurements of the number of particles on silicon are very difficult, and sometimes impossible, to compare with any other particles, and primarily with carbon, by which the mole is actually determined. In the general case, any ultra-precise procedure for determining the number of particles of a substance may be completely unsuitable for another substance. We can compare the mass of any substances with each other, but the number of particles of one substance may not be comparable with the number of particles of another substance. In the ideal case, to ensure the uniformity of measurements of the composition of substances and materials, one should have a universal method for reproducing the mole of any substance, but most often such a task turns out to be impossible. A very large number of substances do not enter into chemical interactions with each other.

Despite all these problems in the implementation of the standard of the mole, "chemical metrology" exists, and it is very convenient for chemists to use a unit of the amount of a substance, defined as the number of particles of a given type. That is why the mole is widely used in measurements of the composition of substances and materials, and especially in environmental measurements. Currently, the problems of ecology, both interethnic and interstate, are one of the main points of application of the achievements of metrology as a science of ensuring the uniformity of measurements.

Lesson Objectives:

• Introduce the concept of the amount of a substance and its units of measurement: mol, mmol, kmol.
• Give an idea of ​​the Avogadro constant.
• Show the relationship between mass, amount of matter and number of particles.

Lesson objectives:

• 1. Contribute to the formation of students' worldview ideas about the relationship between different properties of the phenomena of the surrounding world.
• 2. To develop the ability of students to establish causal relationships, as well as to observe, generalize and draw conclusions.

Basic terms:

• non-metals - chemical elements that form in free form simple substances that do not have the physical properties of metals.
• mole is the amount of any substance that contains as many structural elements as atoms contained in 12g. carbon-12 nuclide

  DURING THE CLASSES

Amount of substance

In chemistry (as well as in physics and other natural sciences) one has to deal with large quantities of the smallest particles - with the so-called structural elements of matter (molecules, atoms, ions, electrons, etc.).
In order to express the number of such particles, the unit of quantity, the mole, was introduced. 1 mole is the amount of any substance that contains as many structural elements as there are atoms in 12g. carbon-12 nuclide. It was experimentally found that the number of structural elements corresponding to 1 mol is 6.02∙1023 (the constant 6.02∙1023 mol-1 is called the Avogadro constant. Cylinders with substances in 1 mol).

Rice. 1. Avogadro's constant
Illustration of the corollary of Avogadro's law

Rice. 2. - unit of the amount of substance

Mole is a unit of quantity of a substance


Rice. 3. Amount of substance
This portion of the substance has a mass, which is called the molar mass. It is denoted by M, which is found by the formula M \u003d m / n. What units will the molar mass be measured in?
The molar mass coincides in value with the relative atomic or molecular mass, but differ in units of measurement (M - g / mol; Mr, Ar - dimensionless quantities).


Rice. 4. Amount of substance in moles


Rice. 5. Molar mass

Control block

№1.
The mass of 3 mol H2O is ____ g
The mass of 20 mol H2O is ____ g
№2.
36 g of H2O are ______ mol
180 g of H2O are _______ mol

Homework

How many molecules are there in 180 g of water?
Find the mass of 24x1023 ozone molecules?

Oxygen is the most abundant chemical element in the earth's crust. Oxygen is part of almost all the substances around us. For example, water, sand, many rocks and minerals that make up the earth's crust contain oxygen. Oxygen is also an important part of many organic compounds, such as proteins, fats and carbohydrates, which are of exceptional importance in the life of plants, animals and humans.
In 1772, the Swedish chemist K.V. Scheele found that air is made up of oxygen and nitrogen. In 1774, D. Priestley obtained oxygen by decomposition of mercury oxide (2). Oxygen is a colorless gas, tasteless and odorless, relatively slightly soluble in water, slightly heavier than air: 1 liter of oxygen under normal conditions weighs 1.43 g, and 1 liter of air weighs 1.29 g. (Normal conditions - abbreviated: n. u . – temperature 0 °C and pressure 760 mm Hg, or 1 atm). At a pressure of 760 mm Hg. Art. and a temperature of -183 °C, oxygen liquefies, and when the temperature drops to -218.8 °C, it solidifies.
The chemical element oxygen O, in addition to ordinary oxygen O2, exists in the form of another simple substance - ozone O3. Oxygen O2 is converted to ozone in a device called an ozonator.
It is a gas with a sharp characteristic odor (the name “ozone” in Greek means “smelling”). You have probably smelled ozone more than once during a thunderstorm. Ozone is made up of three atoms of the element oxygen. Pure ozone is a blue gas, one and a half times heavier than oxygen, it dissolves better in water.
There is an ozone layer in the air atmosphere above the Earth at an altitude of 25 km. There, ozone is formed from oxygen under the influence of ultraviolet radiation from the sun. In turn, the ozone layer delays this radiation, which is dangerous for all living beings, which ensures normal life on Earth.
Ozone is used to disinfect drinking water, as ozone oxidizes harmful impurities in natural water. In medicine, ozone is used as a disinfectant.

Bibliography

1. Lesson on the topic “Amount of substance”, teacher of biology and chemistry Yakovleva Larisa Aleksandrovna, Kurgan region, Petukhovsky district, municipal educational institution “Novogeorgievskaya secondary school”
2. F. A. Derkach "Chemistry", - scientific and methodological manual. - Kyiv, 2008.
3. L. B. Tsvetkova "Inorganic Chemistry" - 2nd edition, corrected and supplemented. – Lvov, 2006.
4. V. V. Malinovsky, P. G. Nagorny "Inorganic Chemistry" - Kyiv, 2009.
4. Glinka N.L. General chemistry. - 27 ed. / Under. ed. V.A. Rabinovich. - L .: Chemistry, 2008. - 704 pages.

Edited and sent by Borisenko I.N.