Reacting Masses
Recap Reacting Masses for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Use a balanced equation and simple ratios to work out the mass of product formed or the mass of reactant needed.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Reacting masses calculations use a balanced equation to work out how much product a given mass of reactant will make, or how much reactant is needed for a given product.
The method has four steps: write the balanced equation, calculate the relative formula masses, convert the known mass into moles, then use the ratio in the equation to find the moles of the other substance and convert back to a mass.
The ratio comes from the large numbers in the balanced equation. If the equation shows 2Mg + O₂ → 2MgO, then two moles of magnesium produce two moles of magnesium oxide — a one to one ratio — but only one mole of oxygen is required.
Revision notes
The four steps
1. Write the balanced equation. 2. Calculate the relative formula masses you need.
3. Convert the known mass into moles using moles = mass ÷ Mr. 4. Use the ratio from the equation to find the moles of the other substance, then convert back to mass using mass = moles × Mr.
Reading the ratio
The large numbers in front of the formulae give the ratio in moles.
In 2Mg + O₂ → 2MgO, the ratio of magnesium to magnesium oxide is 2:2, which simplifies to 1:1. The ratio of magnesium to oxygen is 2:1.
Limiting reactants
When two reactants are given, one will be used up first — this is the limiting reactant.
The amount of product formed is determined by the limiting reactant, because the reaction stops when it runs out. Calculate the moles of each and compare against the equation ratio.
Key points
- Use a balanced equation to find reacting masses.
- Convert the known mass to moles first.
- Use the ratio from the balanced equation.
- Convert the moles back to a mass at the end.
- The large numbers give the mole ratio.
- The limiting reactant determines how much product forms.
Worked examples
Example 1
Calculate the mass of magnesium oxide formed when 48 g of magnesium burns. 2Mg + O₂ → 2MgO. Ar: Mg = 24, O = 16. [4 marks]
Model answer
Moles of Mg = 48 ÷ 24 = 2 moles (1 mark). The ratio of Mg to MgO is 2:2, which is 1:1, so 2 moles of MgO are formed (1 mark). Mr of MgO = 24 + 16 = 40 (1 mark). Mass = 2 × 40 = 80 g (1 mark).
Example 2
Explain what is meant by the limiting reactant. [2 marks]
Model answer
The limiting reactant is the reactant that is completely used up first (1 mark), and it determines the maximum amount of product that can be formed (1 mark).
Example 3
Calculate the moles of oxygen needed to react completely with 4 moles of magnesium. 2Mg + O₂ → 2MgO. [2 marks]
Model answer
The ratio of Mg to O₂ in the equation is 2:1 (1 mark), so 4 moles of magnesium require 2 moles of oxygen (1 mark).
Common mistakes
Using masses directly in the ratio.
The ratio applies to moles, not masses. Convert to moles first.
Forgetting to balance the equation.
The ratio is meaningless unless the equation is balanced.
Using the wrong Mr at the final step.
Convert back using the Mr of the substance you are calculating, not the starting one.
Assuming a 1:1 ratio.
Always read the large numbers in the balanced equation.
Exam tips
- Follow the four steps in order every time.
- Convert to moles before applying any ratio.
- Read the ratio from the balanced equation, never assume it.
- Show every stage of working so method marks are available.
Key terms
- Reacting mass
- The mass of a substance taking part in a reaction.
- Mole ratio
- The ratio of amounts shown by a balanced equation.
- Limiting reactant
- The reactant used up first, limiting the product formed.
- Balanced equation
- An equation with equal atoms of each element on both sides.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.