Calculations in chemistry - AQA SynergyMoles and masses - Higher
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 massThe amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). of a substance can be calculated from the number of moleThe amount of substance that contains the same number of particles as there are atoms in 12 g of carbon-12 (contains the Avogadro's constant 6.0 ×10²³ number of particles)., and the number of moles of a substance can be calculated from its mass. The link between the two quantities is the relative formula massThe sum of the relative atomic masses of the atoms in a chemical formula..
Calculating masses
The mass of a given amount of substance is calculated using:
mass = relative atomic or formula mass Ă— amount
Worked example
Calculate the mass of 0.25 mol of carbon dioxide molecules. (Mr of CO2 = 44)
Mass = relative formula mass Ă— amount
= 44 Ă— 0.25
= 11 g
Worked example
Calculate the mass of 0.10 mol of iron. (Ar of Fe = 56)
Mass = 56 Ă— 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.
Calculating amounts in moles
Use Ar instead of Mr for metals or separate atoms.
Worked example
Calculate the amount of carbon atoms in 6.0 g of carbon. (Ar of C = 12)
Amount = \(\frac{\textup{mass}}{A_{\textup{r}}}\)
Amount = \(\frac{\textup{6.0}}{\textup{12}} \)
= 0.5 mol
Question
Calculate the amount of water molecules in 36 g of water. (Mr of water = 18)