How do you work out the moles of something?

1 mole of a pure substance has a mass equal to its molecular mass expressed in grams.

This is known as the molar mass, M, and has the units g mol-1

So 2 moles of a substance would have a mass = 2 x molar mass

3 moles of a substance would have a mass = 3 x molar mass etc

This leads to the formula: mass = moles x molar mass

If we let:

m = mass of substance in grams,

n = moles of pure substance,

M = molar mass of the pure substance in g mol-1

we can write the equation: m = n x M

This equation can be rearranged to give the following:

n = m : M (moles = mass : molar mass)

M = m : n (molar mass = mass : moles)