Calculations in chemistry - Higher - AQAMoles and masses - Higher
The mole is the unit for amount of substance. The number of particles in a substance can be found using the Avogadro constant. The mass of product depends upon the mass of limiting reactant.
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
Example
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
Example 2
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.
Question
Calculate the mass of 0.10 mol of iron. (Ar of Fe = 56)
Mass = 56 × 0.10
= 5.6 g
Calculating amounts in moles
Use Ar instead of Mr for metals or separate atoms.
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)