Calculating Masses from Balanced Equations
- Chemical equations can be used to calculate the moles or masses of reactants and products
- To do this, information given in the question is used to find the amount in moles of the substances being considered
- Then, the ratio between the substances is identified using the balanced chemical equation
- Once the moles have been determined they can then be converted into grams using the relative atomic or relative formula masses
Worked example
Example 1
Calculate the mass of magnesium oxide that can be made by completely burning 6.0 g of magnesium in oxygen in the following reaction:
2Mg (s) + O2 (g) ⟶ 2 MgO (s)
Answer
Worked example
Example 2
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
( 1 tonne = 1,000,000 g)
Answer
Examiner Tip
Don't be worried about working in mass units other than grams, as the mass ratios don't change. You can see in the second example that 51g of aluminium oxide gives 27 g of aluminium metal.
