Determining the amount of a substance
Let's talk about what such a quantity of substance as this term is used in the subjects of the natural science cycle. Since serious attention is paid to quantitative relations in chemistry and physics, it is important to know the physical meaning of all quantities, their units of measurement, and areas of application.
Designation, definition, units of measurement
In chemistry, quantitative relationships are of particular importance. Special quantities are used to perform calculations according to the equations. In order to understand what the amount of a substance in chemistry is, let's define the term. This is a physical quantity that characterizes the number of similar structural units (atoms, ions, molecules, electrons) present in a substance. To understand what the amount of a substance is, we note that this quantity has its own designation. When making calculations involving the use of this value, use the letter n. Units of measurement - mol, kmol, mmol.
The value of the quantity
Eighth graders who do not yet know how to write chemical equations do not know what the amount of a substance is, how to use this quantity in calculations. After getting acquainted with the law of constancy of the mass of substances, the meaning of this quantity becomes clear. For example, in the combustion reaction of hydrogen in oxygen, the ratio of reactants is two to one. If the mass of hydrogen that entered the process is known, it is possible to determine the amount of oxygen that took part in the chemical reaction.
The use of formulas for the amount of a substance makes it possible to reduce the ratio between the initial reagents and simplify calculations. What is the amount of a substance in chemistry? From the point of view of mathematical calculations, these are the stereochemical coefficients put in the equation. They are used to carry out certain calculations. Since it is inconvenient to count the number of molecules, it is Mole that is used. Using the Avogadro number, one can calculate that 1 mol of any reagent includes 6 1023 mol−1.
Computing
Do you want to understand what the amount of a substance is? In physics, this quantity is also used. It is needed in molecular physics, where pressure and volume of gaseous substances are calculated according to the Mendeleev-Clapeyron equation. To perform any quantitative calculations, the concept of molar mass is used.
By it is meant the mass that corresponds to one mole of a particular chemical substance. You can determine the molar mass through the relative atomic masses (their sum, taking into account the number of atoms in the molecule) or determine through the known mass of the substance, its amount (mol).
Not a single task of a school chemistry course related to calculations according to an equation is complete without the use of such a term as “amount of substance”. Knowing the algorithm, you can cope not only with ordinary software calculations, but also with complex Olympiad tasks. In addition to calculations through the mass of a substance, it is also possible, using this concept, to carry out calculations through the molar volume. This is relevant in cases where gaseous substances are involved in the interaction.
Test on the topic "Basic chemical concepts"
(Multiple correct answers possible)
1. The volume fractions of nitrogen and ethylene (C 2 H 4) in the mixture are the same. Mass fractions of gases in the same mixture:
a) are the same; b) more in nitrogen;
c) more for ethylene; d) depend on pressure.
2. Mass of 10 m3 of air at n.o.s. is equal to (in kg):
a) 20.15; b) 16.25; c) 14.50; d) 12.95.
3. 465 mg of calcium phosphate contains the following number of cations and anions, respectively:
a) 2.7 1021 and 1.8 1021; b) 4.5 1020 and 3.0 1020;
c) 2.7 1025 and 1.8 1025; d) 1.2 1025 and 1.1 1025.
4. The number of moles of water molecules contained in 18.06 1022 water molecules is:
a) 0.667; b) 0.5; c) 0.3; d) 12.
5. Of the following substances, simple ones include:
a) sulfuric acid; b) sulfur;
c) hydrogen; d) bromine.
6. An atom having a mass of 2.66 10–26 kg corresponds to the element:
a) sulfur; b) magnesium;
c) oxygen; d) zinc.
7. A particle that is chemically divisible is:
a) a proton; b) a molecule;
c) positron; d) an atom.
8. Carbon as a simple substance is stated in the statement:
a) carbon is distributed in nature in the form of an isotope with a mass number of 12;
b) during combustion, depending on the conditions, carbon can form two oxides;
c) carbon is a part of carbonates;
d) carbon has several allotropic modifications.
9. The valency of an atom is:
a) the number of chemical bonds formed by a given atom in the compound;
b) the oxidation state of the atom;
c) the number of given or received electrons;
d) the number of electrons missing before obtaining the electron configuration of the nearest inert gas.
10. Which of the following is a chemical phenomenon?
a) melting ice b) water electrolysis;
c) sublimation of iodine; d) photosynthesis.
Key to the test
Tasks for determining the amount of a substance using basic formulas
(By known mass, volume, number of structural units)
Level A
1. How many chromium atoms are there in 2 g of potassium dichromate?
Answer. 8,19 1021.
2. What atoms - iron or magnesium - are more in the earth's crust and how many times? The mass fraction of iron in the earth's crust is 5.1%, magnesium - 2.1%.
Answer. There are 1.04 times more iron atoms than magnesium atoms.
3. What volume (in liters) do:
a) 1.5 1022 fluorine molecules;
b) 38 g of fluorine;
c) 1 1023 oxygen molecules?
Answer. a) 0.558; b) 22.4; c) 3.72.
4. Find the mass (in g) of one molecule: a) water;
b) hydrofluoric acid; c) nitric acid.
Answer. a) 2.99 10–23; b) 3.32 10–23; c) 1.046 10–22.
5. How many moles of a substance are contained in:
a) 3 g of boron trifluoride;
b) 20 liters of hydrogen chloride;
c) 47 mg of phosphorus pentoxide;
d) 5 ml of water?
Answer. a) 0.044; b) 0.893; c) 0.33; d) 0.28.
6. A metal weighing 0.4 g contains 6.021021 atoms. Define metal.
Given:
N= 6.02 1021 atoms, m(M) = 0.4 g.
To find:
metal.
Decision
The desired metal is Ca.
Answer. Calcium.
7. On one pan of the scales there is a certain amount of copper shavings, on the other pan of the scales there is a portion of magnesium containing 75.25 1023 magnesium atoms, while the scales are in a state of equilibrium. What is the mass of a portion of copper chips?
Answer. 300 g.
8. Calculate the amount of calcium substance contained in 62 kg of calcium phosphate.
Answer. 600 mol.
9. In a copper-silver alloy sample, the number of copper atoms is equal to the number of silver atoms. Calculate the mass fraction of silver in the alloy.
Answer. 62.8%.
10. Find the mass of one structural unit of table salt NaCl.
Answer. 9.72 10–23 G.
11. Find the molar mass of a substance if the mass of one of its molecules is 5.31 10–23 G.
Answer. 32 g/mol.
12. Find the molar mass of a gaseous substance if 112 ml of it at n.o. have a mass of 0.14 g.
Answer. 28 g/mol.
13. Find the molar mass of a gaseous substance, if at n.o. 5 g of this substance occupy a volume of 56 liters.
Answer. 2 g/mol.
14. Where are more hydrogen atoms found: in 6 g of water or in 6 g of ethyl alcohol?
Answer. In 6 g of ethyl alcohol.
15. How many grams of calcium are in 1 kg of gypsum?
Answer. 232.5
16. Calculate in Mohr's salt which has the formula Fe(NH 4 ) 2 (SO 4 ) 2 6H 2 O, mass fractions (in%):
a) nitrogen; b) water; c) sulfate ions.
Answer. a) 7.14; b) 27.55; c) 48.98.
Level B
1. To 100 g of a 20% hydrochloric acid solution was added 100 g of a 20% sodium hydroxide solution. How many structural units of NaCl salt and water molecules does the resulting solution contain?
Answer. 5.65 1024 water molecules and 3.01 1023 structural units of NaCl salt.
2. Determine the mass of 8.2 liters of a gas mixture of helium, argon and neon (n.o.), if there are two neon atoms and three argon atoms per helium atom in this mixture.
Answer. 10 y.
3. In what ratio by mass should 2% solutions of potassium chloride and sodium sulfate be mixed so that the final solution contains four times more sodium ions than potassium ions?
Answer. 6.46:1.
4. The density of liquid oxygen at a temperature of –183 °C is 1.14 g/cm3 . How many times will the volume of oxygen increase during its transition from a liquid state to a gaseous state at n.o.?
Answer. 798 times.
5. What is the mass fraction of sulfuric acid in a solution in which the numbers of hydrogen and oxygen atoms are equal to each other?
Decision
Solution H 2 SO 4 consists of H 2 SO 4 and H 2 O. Let (H 2 SO 4 ) = x mol, then (H in H 2 SO 4 ) = 2xmol;
(H 2 O) = y mol, then (H in H 2 O) = 2y mol.
Sum (H in solution) = (2x + 2y) mol.
Let us determine the amount of atomic oxygen substance:
(O to H 2 SO 4 ) = 4x mol, (O in H 2 O) = y mol.
Sum (O in solution) = (4x + y) mol.
Since the numbers of O and H atoms are equal, then 2x + 2y = 4x + y.
Solving the equation, we get: 2x = y. If a
Determining the equivalent amount of a substance from a secondary cloud
Determination of the equivalent amount of a substance from the primary cloud
Determination of quantitative characteristics of the release
Forecasting the depths of SDYAV infection zones
Initial data for predicting the scale of infection with SDYAV
1. The total number of SDYAV at the facility and data on the placement of their stocks in tanks and process pipelines.
2. The amount of SDYAV released into the atmosphere, and the nature of their spill on the underlying surface (“loose”, “into a pallet” or “bund”).
3. The height of the pallet or bunding of storage tanks.
4. Meteorological conditions: air temperature, wind speed (at the height of the weather vane), degree of vertical air stability.
When predicting the scale of infection in advance in case of industrial accidents, it is recommended to take as initial data: for the amount of SDYAV release ( Q about ) - its content in the maximum capacity (technological, storage, transport, etc.), meteorological conditions - the degree of vertical air stability, wind speed and temperature. To predict the extent of contamination immediately after the accident, specific data should be taken on the amount of released (spilled) SDYAV, the time elapsed after the accident, and the nature of the spill on the underlying surface. The external boundaries of the SDYAV infection zone are calculated according to the threshold toxodose during inhalation exposure to the human body.
The calculation of the depth of the SDYAV contamination zone is carried out using the data given in tables 11-13, the value of the depth of the zone of contamination in case of an accidental release (spill) of SDYAV is determined according to Table 8, depending on the quantitative characteristics of the release and wind speed.
Quantitative characteristics of the release of SDYAV to calculate the scale of infection are determined by their equivalent values.
For compressed gases, the equivalent amount of a substance is determined only by the primary cloud.
For liquefied SDYAV, the boiling point of which is higher than the ambient temperature, the equivalent amount of the substance is determined only by the secondary cloud. For SDYAV, the boiling point of which is below the ambient temperature, the equivalent amount of a substance is determined by the primary and secondary clouds.
The equivalent amount of matter in the primary cloud (in tons) is determined by the formula
where K 1 - coefficient depending on storage conditions SDYAV, table 12;
K 3- coefficient equal to the ratio of the threshold toksodose of chlorine to the threshold toksodose of another SDYAV, table 12;
K 5- coefficient taking into account the degree of vertical air stability (taken equal to 1 for inversion; 0.23 for isotherm; 0.08 for convection), table 11;
K 7- coefficient taking into account the influence of air temperature, table 12;
Qo- the amount of substance ejected (spilled) during the accident, i.e.
The equivalent amount of matter in the secondary cloud is calculated by the formula
where K 2 - coefficient depending on the physicochemical properties of SDYAV, table 12;
K 4- coefficient taking into account wind speed, table 13;
K 6– coefficient depending on the time elapsed since the beginning of the accident; N , K 6 determined after calculating the duration t And the time of evaporation of the substance, at N = t And;
h is the thickness of the SDYAV layer, m;
d- SDYAV density, t/m3, table 12.
The height of the spilled liquid during free spillage is taken to be 0.05 m. If there is a pallet or the container is bunded, then
where H is the height of the pallet or bunding.
The evaporation time of SDYAV is calculated by the formula
, (h). (4)
Table 11
Determination of the degree of vertical air stability according to the weather forecast
NOTE:
1. Designation: in - inversion; from– isotherm; to- convection, letters in brackets - with snow cover.
2. Under the term "morning" means a period of time within two hours after sunrise; under the term "evening"- within two hours after sunset.
The period from sunrise to sunset minus two hours in the morning - day, and the period from sunset to sunrise minus two evening hours - night.
3. Wind speed and the degree of vertical stability of the air are taken into account at the time of accidents.
Table 9
Table 13
The value of the coefficient K 4 depending on the wind speed
| Wind speed, m/s | |||||||||||
| K 4 | 1,0 | 1,33 | 1,67 | 2,0 | 2,34 | 2,67 | 3,0 | 3,34 | 3,67 | 4,0 | 5,68 |
Formula for finding the amount of a substance?
Irina Ruderfer
The amount of a substance is a physical quantity that characterizes the number of structural units of the same type contained in a substance. Structural units are any particles that make up a substance (atoms, molecules, ions, electrons or any other particles). The SI unit for measuring the amount of a substance is mol.
[edit] Application
This physical quantity is used to measure macroscopic quantities of substances in those cases when, for the numerical description of the processes under study, it is necessary to take into account the microscopic structure of the substance, for example, in chemistry, when studying electrolysis processes, or in thermodynamics, when describing the equations of state of an ideal gas.
When describing chemical reactions, the amount of a substance is a more convenient quantity than the mass, since the molecules interact regardless of their mass in quantities that are multiples of integers.
For example, the hydrogen combustion reaction (2H2 + O2 → 2H2O) requires twice as much hydrogen substance as oxygen. In this case, the mass of hydrogen involved in the reaction is approximately 8 times less than the mass of oxygen (since the atomic mass of hydrogen is approximately 16 times less than the atomic mass of oxygen). Thus, the use of the amount of a substance facilitates the interpretation of the reaction equations: the ratio between the amounts of reacting substances is directly reflected by the coefficients in the equations.
Since it is inconvenient to use the number of molecules directly in calculations, because this number is too large in real experiments, instead of measuring the number of molecules "in pieces", they are measured in moles. The actual number of units of a substance in 1 mole is called the Avogadro number (NA \u003d 6.022 141 79 (30) × 1023 mol-1) (more correctly, the Avogadro constant, since, unlike the number, this value has units of measurement).
The amount of a substance is denoted by the Greek letter ν (nu) or, simplified, the Latin n (en). To calculate the amount of a substance based on its mass, the concept of molar mass is used: ν \u003d m / M where m is the mass of the substance, M is the molar mass of the substance. Molar mass is the total mass of one mole of the molecules of a given substance. The molar mass of a substance can be obtained by multiplying the molecular weight of that substance by the number of molecules in 1 mole - by Avogadro's number.
According to Avogadro's law, the amount of a gaseous substance can also be determined based on its volume: ν \u003d V / Vm - where V is the volume of gas (under normal conditions), Vm is the molar volume of gas at N. W., equal to 22.4 l / mol.
Thus, a formula is valid that combines the basic calculations with the amount of substance: 
Diana tangatova
designation: mol, international: mol - a unit of measurement of the amount of a substance. Corresponds to the amount of a substance that contains NA particles (molecules, atoms, ions). Therefore, a universal value was introduced - the number of moles. A frequently encountered phrase in tasks is “it was obtained ... a mole of a substance”
NA = 6.02 1023
NA - Avogadro's number. Also "number by agreement". How many atoms are there in the tip of a pencil? About a thousand. It is not convenient to operate with such values. Therefore, chemists and physicists around the world agreed - let's designate 6.02 1023 particles (atoms, molecules, ions) as 1 mole of a substance.
1 mol = 6.02 1023 particles
It was the first of the basic formulas for solving problems.
Molar mass of a substance
The molar mass of a substance is the mass of one mole of the substance.
Referred to as Mr. It is located according to the periodic table - this is simply the sum of the atomic masses of a substance.
For example, we are given sulfuric acid - H2SO4. Let's calculate the molar mass of a substance: atomic mass H = 1, S-32, O-16.
Mr(H2SO4)=1 2+32+16 4=98 g/mol.
The second necessary formula for solving problems is
The formula for the mass of a substance:
That is, in order to find the mass of a substance, it is necessary to know the number of moles (n), and we find the molar mass from the Periodic system.
The law of conservation of mass - the mass of substances that have entered into a chemical reaction is always equal to the mass of the formed substances.
If we know the mass (masses) of substances that have entered into a reaction, we can find the mass (masses) of the products of this reaction. And vice versa.
The third formula for solving problems in chemistry is
Volume of substance:
Basic formulas for solving problems in chemistry
Where did the number 22.4 come from? From Avogadro's law:
Equal volumes of different gases, taken at the same temperature and pressure, contain the same number of molecules.
According to Avogadro's law, 1 mole of an ideal gas under normal conditions (n.a.) has the same volume Vm = 22.413 996 (39) l
That is, if in the problem we are given normal conditions, then, knowing the number of moles (n), we can find the volume of the substance.
So, the basic formulas for solving problems in chemistry
NotationFormulasAvogadro NumberNA
6.02 1023 particles
Amount of substance n (mol)
n=m\Mr
n=V\22.4 (l\mol)
Mass of substance m (g)
m=n Mr
Substance volumeM (l)
V=n 22.4 (l\mol)
Or here's another handy one:
Basic formulas for solving problems in chemistry
These are formulas. Often, to solve problems, you must first write the reaction equation and (necessarily!) Arrange the coefficients - their ratio determines the ratio of moles in the process.
Formula to find the number of moles in terms of mass and molar mass. Please give the formula tomorrow exam!!!
Ekaterina Kurganskaya
Mole, molar mass
The smallest particles - molecules, atoms, ions, electrons - participate in chemical processes. The number of such particles, even in a small portion of matter, is very large. Therefore, in order to avoid mathematical operations with large numbers, a special unit, the mole, is used to characterize the amount of a substance participating in a chemical reaction.
A mole is such an amount of a substance that contains a certain number of particles (molecules, atoms, ions) equal to the Avogadro constant
The Avogadro constant NA is defined as the number of atoms contained in 12 g of the 12C isotope:
Thus, 1 mol of a substance contains 6.02 1023 particles of this substance.
Based on this, any amount of substance can be expressed by a certain number of moles ν (nu). For example, a sample of a substance contains 12.04 1023 molecules. Therefore, the amount of substance in this sample is:
In general:
Where N is the number of particles of a given substance;
NA is the number of particles that 1 mole of a substance contains (Avogadro's constant).
The molar mass of a substance (M) is the mass that 1 mole of a given substance has.
This value, equal to the ratio of the mass m of a substance to the amount of substance ν, has the dimension kg/mol or g/mol. The molar mass, expressed in g/mol, is numerically equal to the relative relative molecular mass Mr (for substances of atomic structure, the relative atomic mass Ar).
For example, the molar mass of methane CH4 is defined as follows:
Mr(CH4) \u003d Ar (C) + 4 Ar (H) \u003d 12 + 4 \u003d 16
M(CH4)=16 g/mol, i.e. 16 g of CH4 contain 6.02 1023 molecules.
The molar mass of a substance can be calculated if its mass m and quantity (number of moles) ν are known, using the formula:
Accordingly, knowing the mass and molar mass of a substance, we can calculate the number of its moles:
Or find the mass of a substance by the number of moles and the molar mass:
m = ν M
It should be noted that the value of the molar mass of a substance is determined by its qualitative and quantitative composition, i.e., depends on Mr and Ar. Therefore, different substances with the same number of moles have different masses m.
Example
Calculate the masses of methane CH4 and ethane C2H6, taken in the amount of ν = 2 mol each.
Decision
The molar mass of methane M(CH4) is 16 g/mol;
molar mass of ethane M(С2Н6) = 2 12+6=30 g/mol.
From here:
m(CH4) = 2 mol 16 g/mol = 32 g;
m (C2H6) \u003d 2 mol 30 g / mol \u003d 60 g.
Thus, a mole is a portion of a substance containing the same number of particles, but having a different mass for different substances, since the particles of a substance (atoms and molecules) are not the same in mass.
n(CH4) = n(С2Н6), but m(CH4)< m(С2Н6)
The calculation of ν is used in almost every computational problem.
Ivan Knyazev
mass is measured in grams, the amount of a substance in moles, molar mass in grams divided by a mole. It is clear that to get the molar mass, you need to divide the mass by the amount, respectively, the amount is the mass divided by the molar mass
