Calculating Masses from Balanced Equations (OCR GCSE Chemistry A (Gateway))
Revision Note
Calculating Masses from Balanced Equations
Higher Tier Only
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 Tips and Tricks
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.
You've read 0 of your 5 free revision notes this week
Sign up now. It’s free!
Did this page help you?