Amounts of Substances in Equations (AQA GCSE Chemistry)
Revision Note
Author
StewartExpertise
Chemistry Lead
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
Question 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
Question 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
Answer:
Balancing Equations using Reacting Masses
- If the masses of reactants and products of a reaction are known then we can use them to write a balanced equation for that reaction
- This is done by converting the masses to moles and simplifying to find the molar ratios
Worked example
Question3 3
A student reacts 1.2 g of carbon with 16.2 g of zinc oxide. The resulting products are 4.4 g of carbon dioxide and 13 g of zinc. Determine the balanced equation for the reaction.
Answer:
Exam Tip
These questions look hard but they are actually quite easy to do, as long as you follow the steps and organise your work neatly.
Remember the molar ratio of a balanced equation gives you the ratio of the amounts of each substance in the reaction.
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