Reacting Mass Calculations
- The number of moles of a substance can be found by using the following equation:
- It is important to be clear about the type of particle you are referring to when dealing with moles
- E.g. 1 mole of CaF2 contains one mole of CaF2 formula units, but one mole of Ca2+ and two moles of F- ions
Reacting masses
- The masses of reactants are useful to determine how much of the reactants exactly react with each other to prevent waste
- To calculate the reacting masses, the chemical equation is required
- This equation shows the ratio of moles of all the reactants and products, also called the stoichiometry, of the reaction
- To find the mass of products formed in a reaction the following pieces of information are needed:
- The mass of the reactants
- The molar mass of the reactants
- The balanced equation
Worked example
Calculate the mass of magnesium oxide that can be made by completely burning 6.0 g of magnesium in oxygen.
magnesium (s) + oxygen (g) → magnesium oxide (s)
Answer:
Step 1: The symbol equation is:
2Mg(s) + O2(g) → 2MgO(s)
Step 2: The relative atomic masses are:
magnesium : 24.31 oxygen : 16.00
Step 3: Calculate the moles of magnesium used in reaction
Step 4: Find the ratio of magnesium to magnesium oxide using the balanced chemical equation
Magnesium | Magnesium oxide | |
Mol | 2 | 2 |
Ratio | 1 | 1 |
Change in mol | -0.25 | +0.25 |
Therefore, 0.25 mol of MgO is formed
Step 5: Find the mass of magnesium oxide
mass = mol x M
mass = 0.25 mol x 40.31 g mol-1
mass = 10.08 g
Therefore, mass of magnesium oxide produced is 10 g (2 sig figs)
Worked example
Calculate the mass of aluminium, in tonnes, that can be produced from 51 tonnes of aluminium oxide. The equation for the reaction is:
2Al2O3 ⟶ 4Al + 3O2
Answer:
Step 1: Calculate the moles if aluminium oxide used
mass of Al2O3 in g = 51 x 106 = 51,000,000 g
moles = = 500,196.16 mol
Step 2: Find the ratio of Al2O3 to Al using the molar ratio from the balanced equation
2Al2O3 : 4Al
Ratio is thus 1 : 2
So 500,196.16 mol moles of Al2O3 produces 100,0392.31 moles of Al
Step 3: Calculate mass of Al
mass = Moles x Mr
mass = 1,000,392.31 mol x 26.98 g mol-1 = 26,990,584.54 g
Step 4: Convert mass from grams to tonnes
= 26.99 tonnes
Examiner Tip
As long as you are consistent it doesn't matter whether you work in grams or tonnes or any other mass unit as the reacting masses will always be in proportion to the balanced equation.