The mole is a base unit of the International System of Units (SI) for the amount of a substance. Mole provides a standard of measurement for atoms and other elementary particles (e.g. protons, electrons, ions, and molecules). It is a very useful concept in chemical reactions.

A mole means gram molecular weight or molecular weight of a substance in grams. Therefore the molecular weight of a substance is also its molar mass. For example, the molecular weight of NaOH is 40 and the molar mass is 40 g/mole.

A mole of any substance contains 6.02214076×10^{23} particles. The number 6.02214076×10^{23} is Avogadro number. This means 1 mole of NaOH is equal to 6.02214076×10^{23} NaOH molecules and the total weight of these many molecules of NaOH is 40 g.

Formula to calculate number of moles in a given amount of a substance:

Number of moles : $\frac{\mathrm{Amount}\mathrm{of}\mathrm{a}\mathrm{substance}(\mathrm{in}\mathrm{gram}}{\mathrm{Molecular}\mathrm{weight}}$

For example, calculate how many moles are in 80 g of NaOH:

Amount of NaOH: 80 g

Molecular (formula) weight of NaOH: 40

Number of Moles = $\frac{80}{40}$ = 2

So 80 g of NaOH will contain 2 moles of NaOH.

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