Structural units are any particles that make up a substance (atoms, molecules, ions, electrons or any other particles). The unit of measurement of the amount of a substance in the International System of Units (SI) and in the CGS system is mol. Without specifying the object of consideration, the term "amount of substance" is not used.
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✪ Amount of substance
✪ 29. Amount of substance. Tasks (part 3)
✪ Physics. Introduction to mkt, the amount of substance
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Application
This physical quantity is used to measure macroscopic amounts of substances in cases where, for a numerical description of the processes under study, it is necessary to take into account the microscopic structure of a substance, for example, in chemistry, when studying electrolysis processes, or in thermodynamics, when describing the equations of state of an ideal gas.
The amount of a substance is denoted by the Latin n (\displaystyle n)(en) and is not recommended to be denoted by a Greek letter ν (\displaystyle \nu )(nu), since this letter in chemical thermodynamics denotes stoichiometric coefficient substances in the reaction, and it, by definition, is positive for the reaction products and negative for the reactants. However, it is the Greek letter that is widely used in the school course. ν (\displaystyle \nu )(nude).
To calculate the amount of a substance based on its mass, the concept of molar mass is used: n = m / M (\displaystyle n=m/M), where m is the mass of the substance, M is the molar mass of the substance. Molar mass is the mass per mole of a given substance. The molar mass of a substance can be obtained by the product
The amount of substance. A mole is a unit of quantity of a substance. Avogadro's number
In addition to the absolute and relative masses of atoms and molecules considered earlier, in chemistry great importance has a special value - the amount of matter. The amount of a substance is determined by the number of structural units (atoms, molecules, ions or other particles) of this substance. The amount of substance is denoted by the letter ν. You already know that any physical quantity has its own unit of measurement. For example, the length of a body is measured in meters, the mass of a substance is measured in kilograms. How is the amount of a substance measured? To measure the amount of a substance, there is a special unit - the mole.
mole- this is the amount of a substance containing as many particles (atoms, molecules or others) as there are carbon atoms in 0.012 kg (i.e. 12 g of carbon. This means that one mole of zinc, one mole of aluminum, one mole of carbon contain one and the same number of atoms. This also means that one mole of molecular oxygen, one mole of water contain the same number of molecules. Both in the first and in the second cases, the number of particles (atoms, molecules) that is contained in one mole is equal to number of atoms in one mole of carbon.It has been experimentally established that one mole of a substance contains 6.02 1023 particles (atoms, molecules, or others). substance.If a substance consists of atoms (for example, zinc, aluminum, etc.), then one mole of this substance is 6.02 1023 of its atoms.If a substance consists of molecules (for example, oxygen, water, etc.), then one a mole of this substance is 6.02 1023 of its molecules. ina 6.02 1023 is named after the famous Italian scientist Amedeo Avogadro "Avogadro's constant" and is designated NA. Avogadro's number shows the number of particles in one mole of a substance, so it could have the dimension "particles / mole". However, since the particles can be different, the word “particles” is omitted and instead a unit is written in the dimension of the Avogadro number: “1/mol” or “mol-1”. Thus: NA = 6.02 1023.
Avogadro's number very large. Compare: if you collect 6.02 × 1023 balls with a radius of 14 centimeters, then their total volume will be approximately the same volume that our entire planet Earth occupies.
To determine the number of atoms (molecules) in a certain amount of a substance, you must use the following formula: N = ν NA,
where N is the number of particles (atoms or molecules).
For example, let's determine the number of aluminum atoms contained in 2 mol of aluminum substance: N (Al) = ν (Al) · NA.
N (Al) \u003d 2 mol 6.02 1023 \u003d 12.04 1023 (atoms).
In addition, you can determine the amount of a substance by a known number of atoms (molecules): 
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) :
Instruction
One of the formulas for the volume of the solution: V = m/p, where V is the volume of the solution (ml), m is the mass (g), p is the density (g/ml). If you need to additionally find the mass, then this can be done by knowing the formula and the amount of the desired substance. Using the formula of a substance, we will find it molar mass, having added atomic masses all elements included in its . For example, M(AgNO3) = 108+14+16*3 = 170 g/mol. Next, we find the mass according to the formula: m \u003d n * M, where m is the mass (g), n is the amount of the substance (mol), M is the molar mass of the substance (g / mol). It is assumed that the amount of substance is given in the problem.
The next one for finding the volume of the solution is derived from the molar formula: c \u003d n / V, where c is the molar concentration of the solution (mol / l), n is the amount of substance (mol), V is the volume of the solution (l). We deduce: V = n/c. The amount of substance can be additionally found by the formula: n = m/M, where m is the mass, M is the molar mass.
The following are formulas for finding the volume of a gas. V \u003d n * Vm, where V is the volume of gas (l), n is the amount of substance (mol), Vm is the molar volume of gas (l / mol). For normal , i.e. pressure equal to 101 325 Pa 273 K, the molar volume of gas is a constant value and is equal to 22.4 l / mol.
For a gas system, there is a formula: q(x) = V(x)/V, where q(x)(phi) is the volume fraction of the component, V(x) is the volume of the component (l), V is the volume of the system (l) . From this formula, 2 others can be derived: V(x) = q*V, and also V = V(x)/q.
If there is a reaction equation in the condition of the problem, the problem should be solved using it. From the equation you can find the amount of any substance, it is equal to the coefficient. For example, CuO + 2HCl = CuCl2 + H2O. From here we see that during the interaction of 1 mol of copper oxide and 2 mol of hydrochloric acid 1 mol of copper chloride and 1 mol of water were obtained. Knowing by the condition of the problem the amount of a substance of only one component of the reaction, one can easily find the amounts of all substances. Let the amount of copper oxide substance be 0.3 mol, then n(HCl) = 0.6 mol, n(CuCl2) = 0.3 mol, n(H2O) = 0.3 mol.
note
Don't forget the units of measurement!
Sources:
- "Collection of Problems in Chemistry", G.P. Khomchenko, I.G. Khomchenko, 2002.
- volume formula from mass
The mass of any substance, molecule is equal to the sum of the masses of its constituent atoms. If the calculation uses relative atomic masses, then the relative molecular mass of the substance is obtained. Relative molecular weight shows how many times the absolute mass of a molecule of a given substance is greater than 1/12 of the absolute mass of a carbon atom. Typically, approximate values of relative atomic and molecular weights are used. These quantities are dimensionless.
Instruction
Calculate the value of each element in the molecule. To find out relative mass one atom look into periodic system elements. The serial number is the atomic mass. You can also calculate it using the formula Ar(element)=m(element)/1a.e.m. For ease of calculation, approximate values are used.
Ar(H)=1?2=2;Ar(O)=16?1=16Ar(Fe)=56?2=112;Ar(S)=32?3=96;Ar(O)=16?12 =192
Add up the results. This will be the molecular weight of the substance.
Mr(H2O)=2Ar(H)+Ar(O)=2+16=18
Mr(Fe2(SO4)3)=2Ar(Fe)+3Ar(S)+12Ar(O)=112+96+192=400
In addition to the relative molecular weight, the molar mass is often used in calculations. Its unit of measurement is g/mol. It is numerically equal to the relative molecular weight of the substance.
M(H2O)=18 g/mol
M(Fe2(SO4)3=400 g/mol
Related videos
In the course of a chemical reaction, a variety of substances can be formed: gaseous, soluble, slightly soluble. In the latter case, they precipitate. Often there is a need to know what is the exact mass of the precipitate formed. How can this be calculated?
You will need
- - glass funnel;
- - paper filter;
- - laboratory scales.
Instruction
You can act by experience. That is, carry out a chemical, carefully separate the precipitate formed from the filtrate using an ordinary glass funnel and a paper filter, for example. A more complete separation is achieved by vacuum filtration (on a Buchner funnel).
After that, dry the precipitate - naturally or under vacuum, and weigh it as accurately as possible. Best of all, on sensitive laboratory scales. This is how the task will be solved. This method is resorted to when the exact quantities of the initial substances that have entered into the reaction are unknown.
If you know these quantities, then the problem can be solved much easier and faster. Suppose you need to calculate how much chloride formed 20 grams of chloride - table salt - and 17 grams of silver nitrate. First of all, write the equation: NaCl + AgNO3 = NaNO3 + AgCl.
During this reaction, a very slightly soluble compound is formed - silver chloride, which precipitates as a white precipitate.
Calculate the molar masses of the starting materials. For sodium chloride, it is approximately 58.5 g / mol, for silver nitrate - 170 g / mol. That is, initially, according to the conditions of the problem, you had 20/58.5 = 0.342 moles of sodium chloride and 17/170 = 0.1 moles of silver nitrate.
Thus, it turns out that sodium chloride was initially taken in excess, that is, the reaction on the second starting material will go to the end (all 0.1 moles of silver nitrate will react, “binding” the same 0.1 moles of common salt). How much silver chloride is formed? To answer this question, find the molecular weight of the precipitate formed: 108 + 35.5 = 143.5. Multiplying the initial amount of silver nitrate (17 grams) by the ratio of the molecular weights of the product and the starting substance, you will get the answer: 17 * 143.5/170 = 14.3 grams. This is the exact mass of the precipitate formed during the reaction.
Of course, the answer you get is not very accurate, since you used rounded values for the atomic masses of the elements in your calculations. If greater accuracy is required, it must be taken into account that the atomic mass of silver, for example, is not 108, but 107.868. Accordingly, the atomic mass of chlorine is not 35.5, but 35, 453, etc.
Sources:
- Calculate the mass of the precipitate formed during the interaction
In school chemistry problems, as a rule, it is required to calculate the volume for the gaseous reaction product. You can do this if you know the number of moles of any participant chemical interaction. Or find this amount from other task data.
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, concept amount of substance completely replaces the concept number of structural units. This eliminates the need to use big 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 weight 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. That's why total number atoms in matter is
