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 With .
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) :
To do this, you need to add the masses of all the atoms in this molecule.
Example 1. In the water molecule H 2 O 2 hydrogen atoms and 1 oxygen atom. The atomic mass of hydrogen \u003d 1, and oxygen \u003d 16. Therefore, the molecular mass of water is 1 + 1 + 16 \u003d 18 atomic mass units, and the molar mass of water \u003d 18 g / mol.
Example 2. In a molecule of sulfuric acid H 2 SO 4 there are 2 hydrogen atoms, 1 sulfur atom and 4 oxygen atoms. Therefore, the molecular weight of this substance will be 1 2 + 32 + 4 16 \u003d 98 amu, and the molar mass will be 98 g / mol.
Example 3. In a molecule of aluminum sulfate Al 2 (SO 4) 3 2 aluminum atoms, 3 sulfur atoms and 12 oxygen atoms. The molecular weight of this substance is 27 2 + 32 3 + 16 12 = 342 amu, and the molar mass is 342 g / mol.
Mole, molar mass
Molar mass is the ratio of the mass of a substance to the amount of a substance, i.e. M(x) = m(x)/n(x), (1)
where M(x) is the molar mass of substance X, m(x) is the mass of substance X, n(x) is the amount of substance X.
The SI unit for molar mass is kg/mol, but the unit g/mol is commonly used. Mass unit - g, kg.
The SI unit for the amount of a substance is the mole.
A mole is such an amount of a substance that contains 6.02 10 23 molecules of this substance.
Any problem in chemistry is solved through the amount of substance. You need to remember the basic formulas:
n(x) =m(x)/ M(x)
or the general formula: n(x) =m(x)/M(x) = V(x)/Vm = N/N A , (2)
where V(x) is the volume of substance X(l), V m is the molar volume of gas at n.o. (22.4 l / mol), N - number of particles, N A - Avogadro's constant (6.02 10 23).
Example 1. Determine the mass of sodium iodide NaI with a quantity of 0.6 mol.
Example 2. Determine the amount of atomic boron substance contained in sodium tetraborate Na 2 B 4 O 7 weighing 40.4 g.
|
m (Na 2 B 4 O 7) \u003d 40.4 g. |
The molar mass of sodium tetraborate is 202 g/mol. Determine the amount of substance Na 2 B 4 O 7: n (Na 2 B 4 O 7) \u003d m (Na 2 B 4 O 7) / M (Na 2 B 4 O 7) \u003d 40.4 / 202 \u003d 0.2 mol. Recall that 1 mol of sodium tetraborate molecule contains 2 mol of sodium atoms, 4 mol of boron atoms and 7 mol of oxygen atoms (see the formula of sodium tetraborate). Then the amount of substance of atomic boron is equal to: n (B) \u003d 4 n (Na 2 B 4 O 7) \u003d 4 0.2 \u003d 0.8 mol. |
And the ability to do calculations, of course. For example, a well-known substance is sulfuric. It finds so wide in a variety of industries that it rightfully bears the name "chemistry". What is her?
Write the exact formula for sulfuric acid: H2SO4. Now take the periodic table and see what are the atomic masses of all the elements that make up it. These three elements are hydrogen, sulfur and oxygen. The atomic mass of hydrogen is 1, sulfur - 32, oxygen - 16. Therefore, the total molecular mass of sulfuric acid, taking into account the indices, is: 1 * 2 + 32 + 16 * 4 = 98 amu (atomic mass units).
And now let's remember one more mole: this is the amount substances, whose mass is numerically equal to its mass expressed in atomic units. Thus, it turns out that 1 mole of sulfuric acid weighs 98 grams. Here is its molar mass. Problem solved.
Suppose you are given the following conditions: there are 800 milliliters of a 0.2 molar solution (0.2 M) of some salt, and it is known that in dry form this salt weighs 25 grams. It is required to calculate its molar mass.
First, remember the definition of a 1-molar (1M) solution. This is a solution in which 1 contains 1 mole of any substances. Accordingly, 1 liter of a 0.2M solution would contain 0.2 mol substances. But you have not 1 liter, but 0.8 liters. Therefore, in fact, you have 0.8 * 0.2 = 0.16 mol substances.
And then everything becomes easier than ever. If 25 grams of salt, according to the conditions of the problem, are 0.16 moles, what is the amount equal to one mole? Having made the calculation in one step, you will find: 25 / 0.16 \u003d 156.25 grams. The molar mass of salt is 156.25 grams/mol. Problem solved.
In your calculations, you used rounded values for the atomic weights of hydrogen, sulfur, and oxygen. If you want to make calculations with high precision, rounding is not allowed.
Sources:
- molar mass of salt
- Calculating the molar mass equivalent
The masses of atoms or molecules are extremely small, therefore, in molecular physics, instead of the masses of molecules and atoms themselves, it is customary to use, at the suggestion of Dalton, their relative values, comparing mass a molecule or atom with 1/12 of the mass of a carbon atom. The amount of a substance that contains as many molecules or atoms as there are in 12 grams of carbon is called a mole. The molar mass of a substance (M) is the mass of one mole. Molar mass is a scalar quantity, it is measured in the international SI system in kilograms divided by a mole.
Instruction
To calculate the molar mass it is enough to know two quantities: mass(m), expressed in kilograms, and the amount of substance (v), measured in moles, substituting them into the formula: M \u003d m / v.
Example. Let it be necessary to determine the molar mass 100 g of water in 3 moles. To do this, you must first mass water in from grams - 100g \u003d 0.01kg. Next, substitute the values \u200b\u200bin the formula, for the molar: M \u003d m / v \u003d 0.01 kg / 3 mol \u003d 0.003 kg / mol.
Any substance consists of particles of a certain structure (molecules or atoms). The molar mass of a simple compound is calculated from the periodic system of elements by D.I. Mendeleev. If it is necessary to find out this parameter for a complex substance, then the calculation turns out to be long, and in this case the figure is looked up in a reference book or chemical catalog, in particular Sigma-Aldrich.
The concept of molar mass
Molar mass (M) - the weight of one mole of a substance. This parameter for each atom can be found in the periodic system of elements, it is located right under the name. When calculating the mass of compounds, the figure is usually rounded to the nearest whole or tenth. For a final understanding of where this value comes from, it is necessary to understand the concept of "mole". This is the amount of a substance containing the number of particles of the latter, equal to 12 g of a stable carbon isotope (12 C). Atoms and molecules of substances vary in size over a wide range, while their number in the mole is constant, but the mass increases and, accordingly, the volume.
The concept of "molar mass" is closely related to the Avogadro number (6.02 x 10 23 mol -1). This figure indicates a constant number of units (atoms, molecules) of a substance in 1 mole.
The value of molar mass for chemistry
Chemical substances enter into various reactions with each other. Usually, the equation of any chemical interaction indicates how many molecules or atoms are used. Such designations are called stoichiometric coefficients. Usually they are specified before the formula. Therefore, the quantitative characteristic of reactions is based on the amount of substance and molar mass. They clearly reflect the interaction of atoms and molecules with each other.
Molar mass calculation
The atomic composition of any substance or mixture of components of a known structure can be viewed from the Periodic Table of the Elements. Inorganic compounds, as a rule, are written by the empirical formula, that is, without designating the structure, but only the number of atoms in the molecule. Organic substances for calculating molar mass are designated in the same way. For example, benzene (C 6 H 6).
How is molar mass calculated? The formula includes the type and number of atoms in the molecule. According to the table D.I. Mendeleev, the molar masses of the elements are checked, and each figure is multiplied by the number of atoms in the formula.
Based on the molecular weight and type of atoms, you can calculate their number in a molecule and draw up a formula for the compound.
Molar mass of elements
Often, to carry out reactions, calculations in analytical chemistry, and the arrangement of coefficients in equations, knowledge of the molecular mass of elements is required. If the molecule contains one atom, then this value will be equal to that of the substance. If there are two or more elements, the molar mass is multiplied by their number.
Molar mass value when calculating concentrations
This parameter is used to convert almost all ways of expressing concentrations of substances. For example, situations often arise to determine the mass fraction based on the amount of a substance in a solution. The last parameter is expressed in the unit mol/liter. To determine the desired weight, the amount of a substance is multiplied by the molar mass. The received value is reduced by 10 times.
Molar mass is used to calculate the normality of a substance. This parameter is used in analytical chemistry for carrying out titri- and gravimetric analysis methods, if it is necessary to accurately carry out the reaction.
Molar mass measurement
The first historical experience was to measure the density of gases in relation to hydrogen. Further studies of colligative properties were carried out. These include, for example, osmotic pressure, determining the difference in boiling or freezing between a solution and a pure solvent. These parameters directly correlate with the number of substance particles in the system.
Sometimes the measurement of molar mass is carried out on a substance of unknown composition. Previously, a method such as isothermal distillation was used. Its essence lies in placing a solution of a substance in a chamber saturated with solvent vapors. Under these conditions, vapor condensation occurs and the temperature of the mixture rises, reaches equilibrium and begins to decrease. The released heat of evaporation is calculated from the change in the heating and cooling index of the solution.
The main modern method for measuring molar mass is mass spectrometry. This is the main way to identify mixtures of substances. With the help of modern instruments, this process occurs automatically, only initially it is necessary to select the conditions for the separation of compounds in the sample. The method of mass spectrometry is based on the ionization of a substance. As a result, various charged fragments of the compound are formed. The mass spectrum indicates the ratio of mass to charge of ions.
Molar mass determination for gases
The molar mass of any gas or vapor is simply measured. It is enough to use control. The same volume of a gaseous substance is equal in quantity to another at the same temperature. A known way to measure the volume of steam is to determine the amount of displaced air. This process is carried out using a side outlet leading to the measuring device.
Practical uses of molar mass
Thus, the concept of molar mass in chemistry is used everywhere. To describe the process, create polymer complexes and other reactions, it is necessary to calculate this parameter. An important point is the determination of the concentration of the active substance in the pharmaceutical substance. For example, using a cell culture, the physiological properties of a new compound are investigated. In addition, molar mass is important in biochemical research. For example, when studying the participation in the metabolic processes of the element. Now the structure of many enzymes is known, so it is possible to calculate their molecular weight, which is mainly measured in kilodaltons (kDa). Today, the molecular weights of almost all components of human blood, in particular, hemoglobin, are known. Molecular and molar mass of a substance in certain cases are synonymous. Their differences lie in the fact that the last parameter is the average for all isotopes of the atom.
Any microbiological experiments with an accurate determination of the effect of a substance on an enzyme system are carried out using molar concentrations. For example, in biocatalysis and other areas where it is necessary to study enzymatic activity, concepts such as inductors and inhibitors are used. To regulate the activity of the enzyme at the biochemical level, it is necessary to study using precisely molar masses. This parameter has firmly entered the field of such natural and engineering sciences as physics, chemistry, biochemistry, biotechnology. Processes characterized in this way become more understandable from the point of view of mechanisms, determination of their parameters. The transition from fundamental science to applied science is not complete without a molar mass indicator, ranging from physiological solutions, buffer systems, and ending with the determination of dosages of pharmaceutical substances for the body.
In chemistry, the values of the absolute masses of molecules are not used, but the value of the relative molecular mass is used. It shows how many times the mass of a molecule is greater than 1/12 of the mass of a carbon atom. This value is denoted by M r .
The relative molecular weight is equal to the sum of the relative atomic masses of its constituent atoms. Calculate the relative molecular weight of water.
You know that a water molecule contains two hydrogen atoms and one oxygen atom. Then its relative molecular mass will be equal to the sum of the products of the relative atomic mass of each chemical element and the number of its atoms in a water molecule:
Knowing the relative molecular weights of gaseous substances, one can compare their densities, i.e., calculate the relative density of one gas from another - D (A / B). The relative density of gas A for gas B is equal to the ratio of their relative molecular masses:
Calculate the relative density of carbon dioxide for hydrogen:
Now we calculate the relative density of carbon dioxide for hydrogen:
D(co.g./hydrogen.) = M r (co. g.) : M r (hydrogen.) = 44:2 = 22.
Thus, carbon dioxide is 22 times heavier than hydrogen.
As you know, Avogadro's law applies only to gaseous substances. But chemists need to have an idea about the number of molecules and in portions of liquid or solid substances. Therefore, to compare the number of molecules in substances, chemists introduced the value - molar mass .
Molar mass is denoted M, it is numerically equal to the relative molecular weight.
The ratio of the mass of a substance to its molar mass is called amount of substance .
The amount of a substance is denoted n. This is a quantitative characteristic of a portion of a substance, along with mass and volume. The amount of a substance is measured in moles.
The word "mole" comes from the word "molecule". The number of molecules in equal amounts of a substance is the same.
It has been experimentally established that 1 mol of a substance contains particles (for example, molecules). This number is called Avogadro's number. And if you add a unit of measurement to it - 1 / mol, then it will be a physical quantity - the Avogadro constant, which is denoted N A.
Molar mass is measured in g/mol. The physical meaning of the molar mass is that this mass is 1 mole of a substance.
According to Avogadro's law, 1 mole of any gas will occupy the same volume. The volume of one mole of gas is called the molar volume and is denoted by V n .
Under normal conditions (and this is 0 ° C and normal pressure - 1 atm. Or 760 mm Hg or 101.3 kPa), the molar volume is 22.4 l / mol.
Then the amount of gas substance at n.o. can be calculated as the ratio of gas volume to molar volume.
TASK 1. What amount of substance corresponds to 180 g of water?
TASK 2. Let us calculate the volume at n.o., which will be occupied by carbon dioxide in the amount of 6 mol.
Bibliography
- Collection of tasks and exercises in chemistry: 8th grade: to the textbook by P.A. Orzhekovsky and others. "Chemistry, Grade 8" / P.A. Orzhekovsky, N.A. Titov, F.F. Hegel. - M.: AST: Astrel, 2006. (p. 29-34)
- Ushakova O.V. Chemistry workbook: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry. Grade 8” / O.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M.: AST: Astrel: Profizdat, 2006. (p. 27-32)
- Chemistry: 8th grade: textbook. for general institutions / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M.: AST: Astrel, 2005. (§§ 12, 13)
- Chemistry: inorg. chemistry: textbook. for 8 cells. general institution / G.E. Rudzitis, F.G. Feldman. - M .: Education, JSC "Moscow textbooks", 2009. (§§ 10, 17)
- Encyclopedia for children. Volume 17. Chemistry / Chapter. edited by V.A. Volodin, leading. scientific ed. I. Leenson. - M.: Avanta +, 2003.
- A single collection of digital educational resources ().
- Electronic version of the journal "Chemistry and Life" ().
- Chemistry tests (online) ().
Homework
1.p.69 No. 3; p.73 Nos. 1, 2, 4 from the textbook "Chemistry: 8th grade" (P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M .: AST: Astrel, 2005).
2. №№ 65, 66, 71, 72 from the Collection of tasks and exercises in chemistry: 8th grade: to the textbook by P.A. Orzhekovsky and others. "Chemistry, Grade 8" / P.A. Orzhekovsky, N.A. Titov, F.F. Hegel. - M.: AST: Astrel, 2006.
