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The mass of a substance can be calculated from the number of moles, and the number of moles of a substance can be calculated from its mass. The link between the two quantities is the M_r, relative formula mass (or A_r, relative atomic mass for atoms and metals).

Calculating masses

One way to remember this is 'mass equals mister mole'.

Example

Calculate the mass of 0.25 mol of carbon dioxide molecules . (M_r of CO₂ = 44)

mass = 44 × 0.25

= 11 g

The calculation is the same if a substance is a metal or exists as separate atoms, but its A_r is used instead of an M_r.

Calculating moles

Again, use A_r instead of M_r for metals or a separate atom.

Example

Calculate the amount of carbon atoms in 6.0 g of carbon. (A_r of C = 12)

= 0.5 mol