Moth. Mass, volume, concentration

The mole is one of the basic units of the Si system. What is the molar mass? How to calculate the molar volume? How to record the molar concentration?

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1. Mol. Definition

The name mole comes from the word molecule. A mole is the unit of quantity of matter and it is the amount of a substance containing as many molecules as there are atoms in 12 g of carbon 12C. In other words, one mole is as many pieces as there are atoms in 12 g of carbon 12C.

The choice of the carbon isotope was not a matter of chance. 12C is also the basis for the definition of atomic mass. The mole is one of the basic units of the Si system.

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2. What is the Avorgad constant?

One mole contains exactly 6.02214076 × 1023 molecules. This number is called Avogadro's number or Avogadro's constant and is denoted NA.

In other words, in 0.012 kg of 12C there are 6.022 × 1023 atoms of the carbon 12C isotope. 1 mole of the substance contains 6.02 × 1023 molecules. It can be compared to a dozen, which always contain 12 elements. Amadeo Avogadro was an Italian physicist and founder of Avogadro's law.

The first term "Avogadro's number" was used in 1909 by Jean Baptiste Perrina, a French physicist and Nobel laureate. Perrin defined the Avogadro number as the number of molecules in one gram of oxygen (then exactly 32 g).

The Avogadro number was used as a unit for the quantity of matter until October 1971, when the XIV General Conference of Measures introduced the concept of a mole.

3. Molar mass

The mass of atoms, molecules or ions, expressed in grams, is called the molar mass. Often confused with molecular weight, but numerically equal to this value.

The SI unit for molar mass is kg / mol, but is more commonly expressed as the derived unit g / mol. This is because the unit g / mol is much more convenient to use because the molar mass is numerically equal to the relative molar mass and similar to the mass number value.

The molar mass is marked with the symbol M. The molar mass of a substance is calculated by summing up the molar masses of all atoms included in the molecule.

Examples of molar masses (approximate values):

  • for hydrogen (H2): 0.002 kg / mol = 2 g / mol;
  • for water (H2O): 0.018 kg / mol = 18 g / mol;
  • for potassium (K): 0.039 kg / mol = 39 g / mol.

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4. Molar volume

The molar volume is the volume that one mole of a substance occupies. The SI unit for molar volume is m³ / mol.

The molar volume is written as follows:

Vm = V / n = M / p

Where: Vm - molar volume V - the volume of n moles of the substance M - molar mass (unit: kg / mol, in practice g / mol is used - see below), ρ - density (unit: kg / m³).

The molar volume is given under certain conditions, usually under normal conditions.

Condensed phase. For liquids and solids, molar volumes are often reported using the more convenient subunits of cm³ / mol or dm³ / mol.

For example, for water 18 cm³ / mol = 0.018 dm³ / mol = 0.000018 m³ / mol. Gas phase. For gases and vapors (there are some reservations here), the molar volume changes approximately linearly with temperature. This phenomenon is defined by the Clapeyron equation:

Vm = V / n = RT / p

where: ρ - pressure, R– (universal) gas constant, T - temperature.

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5. Molar concentration

Molar concentration, also called molarity or molarity, is one way to express the concentration of substances in solutions.

CB = n_B / V = ​​m_B / (M_B V)


CB - molar concentration of component B nB - the number of moles of component B V - the volume of the solution mB - mass of component B MB - molar mass of component B.

Most often, the molar concentration is used for liquid solutions. The molar concentration of the solution is the number of moles of the dissolved substance in 1 dm³ of the solution. For example, a 5-molar solution contains 5 moles of a substance dissolved in 1 liter of solution.

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