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. (Mr of CO2 = 44)
Mass = relative formula mass × amount
= 44 × 0.25
= 11 g
Calculate the mass of 0.10 mol of iron. (Ar 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 Ar is used instead of an Mr.
Use Ar instead of Mr for metals or separate atoms.
Calculate the amount of carbon atoms in 6.0 g of carbon. (Ar of C = 12)
= 0.5 mol
Calculate the amount of water molecules in 36 g of water. (Mr of water = 18)
= 2.0 mol