Relative formula masses can be calculated and used in conservation of mass calculations. Calculations can be carried out to find out concentrations of solutions.

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 relative formula mass.

The mass of a given amount of substance is calculated using:

mass = relative atomic or formula mass × amount

Calculate the mass of 0.25 mol of carbon dioxide molecules. (*M*_{r} of CO_{2} = 44)

Mass = relative formula mass × amount

= 44 × 0.25

= 11 g

Calculate the mass of 0.10 mol of iron. (*A*_{r} of Fe = 56)

Mass = 56 × 0.10

= 5.6 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}.

The amount of a given mass substance is calculated using:

Use *A*_{r} instead of *M*_{r} for metals or separate atoms.

Calculate the amount of carbon atoms in 6.0 g of carbon. (*A*_{r} of C = 12)

Amount =

Amount =

= 0.5 mol

- Question
Calculate the amount of water molecules in 36 g of water. (

*M*_{r}of water = 18)Amount =

Amount =

= 2.0 mol