# Mole and the Avagadro's Number

## The mole is the SI base unit for an amount of a 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 (^{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 H_{2} molecules. 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 }H_{2}-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*10 ^{25} [molecules]*