# The mole and molar mass

A mole is a quantity used to designate an amount of particles.

One mole of a substance is equal to $$6.02 \times 10^{23}$$ particles (such as molecules, atoms or ions) of the substance. This number is also known as Avogadro's constant. In practical situations, you can often round to $$6 \times 10^{23}$$.

One mole of water contains $$6.02 \times 10^{23}$$ molecules of water. One mole of helium contains $$6.02 \times 10^{23}$$ atoms of helium.

The number of moles of substance can be calculated using the following formula:

$$$\text{Number of moles}=\displaystyle{\frac{\text{Mass (in g)}}{\text{Molar mass (g/mol)}}}$$$The molar mass is the number of grams in one mole of a substance. The relative atomic or molecular mass of a substance is equal to the molar mass.

Water has a relative molecular mass of approximately 18. There are thus **18 grams** of water in **1 mole of water**. 18 grams of water therefore contain $$6.02 \times 10^{23}$$ molecules of water.

The unit of the molar mass is grams per mole (g/mol).

This relationship holds because a mole was specifically defined as **the number of atoms in 12 grams of carbon-12** (carbon atoms with 12 nucleons).

Because protons and neutrons have the same mass in all atoms, the relative atomic or molecular mass of all particles is equal to the molar mass of the particles.