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Mole and the Avagadro's Number

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The SI unit for amount of substance is the mole , defined as

  • the amount of substance that has an equal number of elementary entities as there are atoms in 12 g of carbon-12  ( 12 C -  the isotope of carbon with standard atomic weight 12 by definition).

That number of elementary entities in carbon-12 is the Avogadro constant (number), N A.

N A = 6.022141 10 23 [molecules/mol]

The mole is one of the base units of the SI, and has the unit symbol [ mol] .

Standard volume of 1 mole of an ideal gas at 1 atmosphere and 0°C is 22.414 liters.
See also Molecular Weight - Common Substances


Example: Number of molecules in 1 mole of hydrogen gas.

Hydrogen gas contains H2molecules.  1 mole of a substance is 6.022141*10 23 molecules of the substance.

So; 1 mole of hydrogen gas contains 6.022141*10 23 H2-molecules


Example: Number of molecules in ethyl alcohol

The molecular weight of ethyl alcohol is 46.07 kg/kmol and the number of molecules in 1 kg can be calculated as

n = (1 [kg] / 46.07 [kg/kmol])* 6.022 10 23 [molecules/mol] * 1000 [mol/kmol]

= 1.31*1 0 25 [molecules]

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