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How to calculate the molar mass of a substance example. Molar mass, its meaning and calculation

In practical and theoretical chemistry, two concepts exist and are of practical importance: molecular (it is often replaced by the concept of molecular weight, which is not correct) and molar mass. Both of these quantities depend on the composition of a simple or complex substance.

How to determine or molecular? Both of these physical quantities cannot (or almost cannot) be found by direct measurement, for example, by weighing a substance on a scale. They are calculated based on the chemical formula of the compound and the atomic masses of all elements. These quantities are numerically equal, but differ in dimension. expressed in atomic mass units, which are a conventional quantity and are designated a. e.m., as well as another name - “dalton”. The units of molar mass are expressed in g/mol.

The molecular masses of simple substances, the molecules of which consist of one atom, are equal to their atomic masses, which are indicated in the periodic table of Mendeleev. For example, for:

sodium (Na) - 22.99 a. eat.;
iron (Fe) - 55.85 a. eat.;
sulfur (S) - 32.064 a. eat.;
argon (Ar) - 39.948 a. eat.;
potassium (K) - 39.102 a. eat.

Also, the molecular weights of simple substances, the molecules of which consist of several atoms of a chemical element, are calculated as the product of the atomic mass of the element by the number of atoms in the molecule. For example, for:

oxygen (O2) - 16. 2 = 32 a. eat.;
nitrogen (N2) - 14.2 = 28 a. eat.;
chlorine (Cl2) - 35. 2 = 70 a. eat.;
ozone (O3) - 16. 3 = 48 a. eat.

Molecular masses are calculated by summing the product of the atomic mass and the number of atoms for each element included in the molecule. For example, for:

(HCl) - 2 + 35 = 37 a. eat.;
(CO) - 12 + 16 = 28 a. eat.;
carbon dioxide (CO2) - 12 + 16. 2 = 44 a. eat.

But how to find the molar mass of substances?

This is not difficult to do, since it is the mass of a unit amount of a particular substance, expressed in moles. That is, if the calculated molecular mass of each substance is multiplied by a constant value equal to 1 g/mol, then its molar mass will be obtained. For example, how do you find the molar mass (CO2)? It follows (12 + 16.2).1 g/mol = 44 g/mol, that is, MCO2 = 44 g/mol. For simple substances, molecules that contain only one atom of the element, this indicator, expressed in g/mol, numerically coincides with the atomic mass of the element. For example, for sulfur MS = 32.064 g/mol. How to find the molar mass of a simple substance, the molecule of which consists of several atoms, can be considered using the example of oxygen: MO2 = 16. 2 = 32 g/mol.

Examples have been given here for specific simple or complex substances. But is it possible and how to find the molar mass of a product consisting of several components? Like the molecular mass, the molar mass of a multicomponent mixture is an additive quantity. It is the sum of the products of the molar mass of a component and its share in the mixture: M = ∑Mi. Xi, that is, both the average molecular and average molar mass can be calculated.

Using the example of air, which contains approximately 75.5% nitrogen, 23.15% oxygen, 1.29% argon and 0.046% carbon dioxide (the remaining impurities, which are contained in smaller quantities, can be neglected): Mair = 28. 0.755 + 32. 0.2315 + 40 . 0.129 + 44 . 0.00046 = 29.08424 g/mol ≈ 29 g/mol.

How to find the molar mass of a substance if the accuracy of determining the atomic masses indicated in the periodic table is different? For some elements it is indicated with an accuracy of tenths, for others with an accuracy of hundredths, for others to thousandths, and for such elements as radon - to whole ones, for manganese to ten-thousandths.

When calculating molar mass, it does not make sense to carry out calculations with greater accuracy than up to tenths, since they have practical applications when the purity of the chemical substances or reagents themselves will introduce a large error. All these calculations are approximate. But where chemists require greater accuracy, appropriate corrections are made using certain procedures: the titer of the solution is established, calibrations are made using standard samples, etc.

Any substance consists of particles of a certain structure (molecules or atoms). The molar mass of a simple compound is calculated according to the periodic table of elements 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 catalogue, in particular Sigma-Aldrich.

The concept of molar mass

Molar mass (M) is the weight of one mole of a substance. This parameter for each atom can be found in the periodic table of elements; it is located directly under the name. When calculating the mass of compounds, the figure is usually rounded to the nearest whole or tenth. To fully understand where this meaning 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 the stable isotope of carbon (12 C). Atoms and molecules of substances vary in size over a wide range, while their number in a mole is constant, but the mass increases and, accordingly, the volume.

The concept of “molar mass” is closely related to Avogadro’s number (6.02 x 10 23 mol -1). This figure denotes a constant number of units (atoms, molecules) of a substance in 1 mole.

Importance of Molar Mass for Chemistry

Chemical substances enter into various reactions with each other. Typically, the equation for any chemical interaction specifies how many molecules or atoms are involved. Such designations are called stoichiometric coefficients. They are usually indicated before the formula. Therefore, the quantitative characteristics of reactions are based on the amount of substance and molar mass. They clearly reflect the interaction of atoms and molecules with each other.

Calculation of molar mass

The atomic composition of any substance or mixture of components of a known structure can be viewed using the periodic table of elements. Inorganic compounds, as a rule, are written with a gross formula, that is, without designating the structure, but only the number of atoms in the molecule. Organic substances are designated in the same way for calculating molar mass. 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 the molecule and create a formula for the compound.

Molar mass of elements

Often, to carry out reactions, calculations in analytical chemistry, and arranging 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 two or more elements are present, the molar mass is multiplied by their number.

The value of molar mass when calculating concentrations

This parameter is used to recalculate almost all methods of expressing the concentrations of substances. For example, situations often arise in determining the mass fraction based on the amount of a substance in a solution. The last parameter is expressed in the unit of measurement mol/liter. To determine the required weight, the amount of substance is multiplied by the molar mass. The resulting value is reduced by 10 times.

Molar mass is used to calculate the normality of a substance. This parameter is used in analytical chemistry to carry out titration and gravimetric analysis methods when it is necessary to accurately carry out a reaction.

Molar mass measurement

The first historical experiment was to measure the density of gases relative 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 particles of matter 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 is to place a solution of a substance in a chamber saturated with solvent vapor. 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 by the change in the heating and cooling rate 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 you initially need to select the conditions for the separation of compounds in the sample. The mass spectrometry method 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.

Determination of molar mass 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 amount to another at the same temperature. A well-known way to measure the volume of steam is to determine the amount of displaced air. This process is carried out using a side branch leading to a measuring device.

Practical uses of molar mass

Thus, the concept of molar mass is used everywhere in chemistry. To describe the process, create polymer complexes and other reactions, it is necessary to calculate this parameter. An important point is to determine the concentration of the active substance in the pharmaceutical substance. For example, the physiological properties of a new compound are studied using cell culture. In addition, molar mass is important when conducting biochemical studies. For example, when studying the participation of an element in metabolic processes. 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 are synonymous in certain cases. Their differences lie in the fact that the last parameter is the average for all isotopes of the atom.

Any microbiological experiments in accurately determining the effect of a substance on an enzyme system are carried out using molar concentrations. For example, in biocatalysis and other areas where the study of enzymatic activity is necessary, concepts such as inducers and inhibitors are used. To regulate enzyme activity at the biochemical level, research using molar masses is necessary. This parameter has become firmly established in the fields of natural and engineering sciences such as physics, chemistry, biochemistry, and biotechnology. Processes characterized in this way become more understandable from the point of view of mechanisms and determination of their parameters. The transition from fundamental to applied science is not complete without an indicator of molar mass, starting from physiological solutions, buffer systems and ending with determining the dosages of pharmaceutical substances for the body.

Instructions

To find a mole of a substance, you need to remember a very simple rule: the mass of one mole of any substance is numerically equal to its molecular mass, only expressed in other quantities. How is it determined? Using the periodic table, you will find out the atomic mass of each element included in the molecules of a substance. Next, you need to add the atomic masses, taking into account the index of each element, and you will get the answer.

Calculate its molecular weight taking into account the index of each element: 12*2 + 1*4 + 16*3 = 76 amu. (atomic mass units). Therefore, its molar mass (that is, the mass of one mole) is also 76, only its dimension is grams/mol. Answer: one mole of ammonium nitrate weighs 76 grams.

Suppose you are given such a task. It is known that the mass of 179.2 liters of some gas is 352 grams. It is necessary to determine how much one mole of this gas weighs. It is known that under normal conditions one mole of any gas or mixture of gases occupies a volume approximately equal to 22.4 liters. And you have 179.2 liters. Do the calculation: 179.2/22.4 = 8. Therefore, this volume contains 8 moles of gas.

Dividing the mass known according to the conditions of the problem by the number of moles, you get: 352/8 = 44. Therefore, one mole of this gas weighs 44 grams - this is carbon dioxide, CO2.

If there is a certain amount of gas of mass M, enclosed in a volume V at a given temperature T and pressure P. It is required to determine its molar mass (that is, find what its mole is equal to). The universal Mendeleev-Clapeyron equation will help you solve the problem: PV = MRT/m, where m is the very molar mass that we need to determine, and R is the universal gas constant equal to 8.31. Transforming the equation, you get: m = MRT/PV. By substituting known quantities into the formula, you will find what a mole of gas is equal to.

Helpful advice

Calculations usually use rounded values for the atomic weights of elements. If higher precision is required, rounding is not acceptable.

A. Avogadro in 1811, at the very beginning of the development of atomic theory, made the assumption that an equal number of ideal gases at the same pressure and temperature contain the same number of molecules. Later this assumption was confirmed and became a necessary consequence for the kinetic theory. Now this theory is called Avogadro.

Instructions

Avogadro's constant shows the number of atoms or molecules that are contained in one mole of a substance.

The number of molecules, provided that the system is one-component and the molecules or atoms of the same type contained in it, can be found using a special formula

Video on the topic

First, determine the chemical composition and state of aggregation of the substance. If you are testing a gas, measure its temperature, volume, and pressure, or place it under normal conditions and measure only the volume. After this, calculate the number of molecules and atoms. To determine the number of atoms in a solid or liquid, find its mass and molar mass, and then the number of molecules and atoms.

You will need

pressure gauge, thermometer, scales and periodic table, find out Avogadro's constant.

Instructions

Determining the mass of one mole from a known amount of a substance If the amount of a substance in moles is known, the molar mass of which needs to be found, use a scale to find its actual mass, expressing it in grams. To determine the mass of one mole, divide the mass of the substance by its amount M=m/υ.

Determining the mass of one mole of a substance by the mass of a molecule If the mass of one molecule of a substance, expressed in grams, is known, find the mass of one mole by multiplying the mass of this molecule by the number of molecules in one mole (Avogadro's number), which is equal to 6.022 10^23, M = m0 NA .

Determining the mass of one mole of gas Take a sealed vessel of known volume, expressed in cubic meters. Pump out the gas from it and weigh it on a scale. Pump gas into it and weigh it again, the difference between the empty and filled cylinders will be equal to the mass of the gas. Convert it to kilograms.
Measure the temperature of the gas in the cylinder; if you wait a little after pumping, it will become equal to the ambient air temperature, and convert it to kelvins by adding the number 273 to degrees Celsius. Measure the gas pressure with a pressure gauge, in pascals. Find the molar mass of a gas (mass of one mole) by multiplying the mass of the gas by its temperature and 8.31 (the universal gas constant), and dividing the result by the pressure and volume M=m R T/(P V).

Sometimes researchers are faced with the following problem: how to determine the number of atoms of a particular substance? Initially, it may seem extremely complex, because the number of atoms even in a tiny sample of any substance is simply enormous. How to count them?

Instructions

Suppose you need to count the number of atoms in a piece of pure copper, for example, or even gold. Yes, imagine yourself in the place of the great scientist Archimedes, to whom King Hiero gave a completely different assignment, saying: “You know, Archimedes, in vain I suspected my jeweler of fraud, the crown turned out to be made of pure gold! Our royal majesty now wants to know the atoms in it.”

The task, naturally, would have plunged the real Archimedes into a stupor, even though he was. Well, you could deal with it in no time. First you need to accurately weigh the crown. Suppose it weighed exactly 2 kg, that is, 2000 grams. Then, using the periodic table, set the molar mass of gold (approximately 197 grams/mol.) To simplify the calculations, round up a little - let it be 200 grams/mol. Therefore, there are exactly 10 moles of gold in the ill-fated crown. Well, then take Avogadro’s universal number (6.022x1023), multiply by 10 and triumphantly take the result to King Hieron.

And then use the well-known Mendeleev–Clapeyron equation: PV = MRT/m. Note that M/m is nothing more than the number of moles of a given gas, since M is its actual mass and m is its molar mass.

Substitute the values you know into the fraction PV/RT, multiply the result found by Avogadro’s universal number (6.022*1023) and get the number of gas atoms at a given volume, pressure and temperature.

What if you need to count the number of atoms in a sample of a complex substance? And there is nothing particularly difficult here. Weigh the sample, then write its exact chemical formula, use the Periodic Table to clarify the molar mass of each component and calculate the exact molar mass of this complex substance (taking into account the elemental indices if necessary).

Well, then find out the number of moles in the sample under study (by dividing the mass of the sample by the molar mass) and multiply the result by the value of Avogadro’s number.

In chemistry, the mole is used as a unit of quantity of a substance. A substance has three characteristics: mass, molar mass, and amount of substance. Molar mass is the mass of one mole of a substance.

Instructions

One mole of a substance represents its quantity, which contains as many structural units as there are atoms contained in 0.012 kg of an ordinary (non-radioactive) isotope. The structural units of matter are molecules, atoms, ions. When the conditions of the problem are given with the relative atomic mass of Ar, from the formula of the substance, depending on the formulation of the problem, either the mass of one mole of the same substance or its molar mass is found by performing calculations. The relative atomic mass of Ar is a value equal to the ratio of the average mass of an isotope of an element to 1/12 of the mass of carbon.

Both organic and inorganic substances have molar mass. For example, calculate this parameter in relation to water H2O and methane CH3. First find the molar mass of water:
M(H2O)=2Ar(H)+Ar(O)=2*1+16=18 g/mol
Methane is a gas of organic origin. This means that its molecule contains hydrogen and carbon atoms. Just one molecule of this gas contains three hydrogen atoms and one carbon atom. Calculate the molar mass of this substance as follows:
M(CH3)=Ar(C)+2Ar(H)=12+3*1=15 g/mol
Calculate the molar masses of any other substances in the same way.

Also, the mass of one mole of a substance or molar mass is found by knowing the mass and quantity of the substance. In this case, molar mass is calculated as the ratio of the mass of a substance to its quantity. The formula looks like this:
M=m/ν, where M is molar mass, m is mass, ν is the amount of substance.
The molar mass of a substance is expressed in grams or kilograms per mole. If the mass of a molecule of a substance is known, then, knowing Avogadro’s number, you can find the mass of one mole of the substance as follows:
Mr=Na*ma, where Mr is the molar mass, Na is Avogadro's number, ma is the mass of the molecule.
So, for example, knowing the mass of a carbon atom, you can find the molar mass of this substance:
Mr=Na*ma=6.02*10^23*1.993*10^-26=12 g/mol

Video on the topic

The mass of 1 mole of a substance is called its molar mass and is designated by the letter M. The units of measurement of molar mass are g/mol. The method for calculating this value depends on the specified conditions.

You will need

- periodic table of chemical elements D.I. Periodic table (periodic table);
- calculator.

Instructions

If a substance is known, its molar mass can be calculated using the periodic table. The molar mass of a substance (M) is equal to its relative molecular mass (Mr). In order to calculate it, find in the periodic table the atomic masses of all elements that make up the substance (Ar). Typically this is a number written in the lower right corner of the cell of the corresponding element under its serial number. For example, the atomic mass is 1 - Ar (H) = 1, the atomic mass of oxygen is 16 - Ar (O) = 16, the atomic mass of sulfur is 32 - Ar (S) = 32.

In order to find out the molecular and molar mass of a substance, you need to add up the relative atomic masses of the elements included in it, taking into account their number. Mr = Ar1n1+Ar2n2+…+Arxnx. Thus, the molar mass of water (H2O) is equal to the sum of the atomic mass of hydrogen (H) multiplied by 2 and the atomic mass of oxygen (O). M(H2O) = Ar(H)?2 + Ar(O) = 1?2 +16=18(g/mol). The molar mass of (H2SO4) is equal to the sum of the atomic mass of hydrogen (H) multiplied by 2, the atomic mass of sulfur (S) and the atomic mass of oxygen (O) multiplied by 4. M (H2SO4) = Ar (H) ?2 + Ar( S) + Ar (O) ?4=1?2 + 32 + 16?4 = 98(g/mol). The molar mass of simple substances consisting of one element is calculated in the same way. For example, the molar mass of oxygen gas (O2) is equal to the atomic mass of the element oxygen (O) multiplied by 2. M (O2) = 16?2 = 32 (g/mol).

If the chemical formula of a substance is unknown, but its quantity and mass are known, the molar mass can be found using the formula: M = m/n, where M is the molar mass, m is the mass of the substance, n is the amount of the substance. For example, it is known that 2 moles of a substance have a mass of 36 g, then its molar mass is M = m/n = 36 g? 2 mol = 18 g/mol (most likely this is water H2O). If 1.5 moles of a substance has a mass of 147 g, then its molar mass is M = m/n = 147 g? 1.5 mol = 98 g/mol (most likely this is sulfuric acid H2SO4).

Video on the topic

Sources:

Talitsa Mendeleev

August 23, 2012

How to determine molar mass or molecular mass? Both of these physical quantities cannot (or almost cannot) be found by direct measurement, for example, by weighing a substance on a scale. They are calculated based on the chemical formula of the compound and the atomic masses of all elements. These quantities are numerically equal, but differ in dimension. Molecular mass is expressed in atomic mass units, which are a conventional quantity and are designated a. e.m., as well as another name - “dalton”. The units of molar mass are expressed in g/mol.

The molecular masses of simple substances, the molecules of which consist of one atom, are equal to their atomic masses, which are indicated in the periodic table of Mendeleev. For example, for:

sodium (Na) - 22.99 a. eat.;
iron (Fe) - 55.85 a. eat.;
sulfur (S) - 32.064 a. eat.;
argon (Ar) - 39.948 a. eat.;
potassium (K) - 39.102 a. eat.

oxygen (O2) - 16. 2 = 32 a. eat.;
nitrogen (N2) - 14.2 = 28 a. eat.;
chlorine (Cl2) - 35. 2 = 70 a. eat.;
ozone (O3) - 16. 3 = 48 a. eat.

The molecular masses of complex substances are calculated by summing the products of the atomic mass and the number of atoms for each element included in the molecule. For example, for:

hydrochloric acid (HCl) - 2 + 35 = 37 a. eat.;
carbon monoxide (CO) - 12 + 16 = 28 a. eat.;
carbon dioxide (CO2) - 12 + 16. 2 = 44 a. eat.

But how to find the molar mass of substances?

This is not difficult to do, since it is the mass of a unit amount of a particular substance, expressed in moles. That is, if the calculated molecular mass of each substance is multiplied by a constant value equal to 1 g/mol, then its molar mass will be obtained. For example, how do you find the molar mass of carbon dioxide (CO2)? It follows (12 + 16.2).1 g/mol = 44 g/mol, that is, MCO2 = 44 g/mol. For simple substances, molecules that contain only one atom of the element, this indicator, expressed in g/mol, numerically coincides with the atomic mass of the element. For example, for sulfur MS = 32.064 g/mol. How to find the molar mass of a simple substance, the molecule of which consists of several atoms, can be considered using the example of oxygen: MO2 = 16. 2 = 32 g/mol.

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Current

The molar mass of a substance, denoted M, is the mass that 1 mole of a certain chemical substance has. Molar mass is measured in kg/mol or g/mol.

Instructions

To determine the molar mass of a substance, it is necessary to know its qualitative and quantitative composition. The molar mass expressed in g/mol is numerically equal to the relative molecular mass of the substance - Mr.
Molecular mass is the mass of a molecule of a substance, expressed in atomic mass units. Molecular weight is also called molecular weight. To find the molecular mass of a molecule, you need to add up the relative masses of all the atoms that make up its composition.
Relative atomic mass is the mass of an atom expressed in atomic mass units. The atomic mass unit is a common unit of measurement for atomic and molecular masses, equal to 1/12 the mass of a neutral 12C atom, the most common isotope of carbon.
The atomic masses of all chemical elements present in the earth's crust are presented in the periodic table. By adding up the relative atomic masses of all the elements that make up a chemical substance or molecules, you will find the molecular mass of the chemical substance, which will be equal to the molar mass, expressed in g/mol.
Also, the molar mass of a substance is equal to the ratio of the mass of the substance m (measured in kilograms or grams) to the amount of substance ν (measured in moles).

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