The mole is the SI base unit for amount of substance
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 (12C - the isotope of carbon with standard atomic weight 12 by definition).
That number of elementary entities in carbon-12 is the Avogadro constant (number), NA.
NA = 6.022141 1023 [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 H2 molecules. 1 mole of a substance is 6.022141*1023 molecules of the substance.
So; 1 mole of hydrogen gas contains 6.022141*1023 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 1023 [molecules/mol] * 1000 [mol/kmol]
= 1.31*1025 [molecules]