Using Moles to Balance Equations
Revise Using Moles to Balance Equations for GCSE Chemistry with this free worksheet and full mark scheme — Higher-tier exam-style questions with worked answers for AQA, Edexcel and OCR. Divide each reacting mass by its relative formula mass to find the mole ratio, then use that ratio to balance the equation.
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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.
This is a Higher tier only topic, so there's no Foundation paper — only the Higher worksheet and mark scheme below.
Topic overview
Balanced equations can be deduced from the masses of reactants and products, by converting each mass into moles and finding the simplest whole number ratio. This is a Higher-only topic.
The method has three steps. Convert each mass into moles using moles = mass ÷ Mr. Then divide every value by the smallest to give the simplest ratio. Finally round to whole numbers and use these as the balancing numbers in the equation.
Small rounding differences are expected because experimental masses are never exact. A ratio of 1.98 to 1 should be read as 2 to 1, not written as a decimal in the equation.
Revision notes
The method
Convert each mass into moles: moles = mass ÷ Mr.
Divide every value by the smallest number of moles. Round to the nearest whole numbers. These become the large numbers in the balanced equation.
Interpreting the ratio
Experimental masses are never perfectly accurate, so the ratio will rarely be exact.
A value of 1.98 should be read as 2, and 2.97 as 3. Never leave decimals as balancing numbers in a chemical equation.
Checking the answer
Once the equation is written, count the atoms of each element on both sides.
If they match, the equation is balanced. This check catches rounding errors before they cost marks.
Key points
- Convert each mass to moles first.
- Moles = mass ÷ Mr.
- Divide all values by the smallest.
- Round to the nearest whole numbers.
- Use these as the balancing numbers.
- Check by counting atoms on both sides.
Worked examples
Example 1
0.24 g of magnesium reacts with 0.16 g of oxygen. Ar: Mg = 24, O = 16. Find the mole ratio. [3 marks]
Model answer
Moles of Mg = 0.24 ÷ 24 = 0.01 (1 mark). Moles of O atoms = 0.16 ÷ 16 = 0.01 (1 mark). Dividing both by 0.01 gives a ratio of 1 to 1 (1 mark).
Example 2
A calculation gives a ratio of 1.98 to 1. State how this should be written in the balanced equation. [2 marks]
Model answer
It should be rounded to 2 to 1 (1 mark), because experimental masses are never exact and balancing numbers must be whole numbers (1 mark).
Example 3
Explain why the number of moles is used rather than the masses directly. [2 marks]
Model answer
A balanced equation shows the ratio in which particles react (1 mark), and moles count particles whereas masses do not, because different substances have different relative formula masses (1 mark).
Common mistakes
Using masses directly as the ratio.
Masses must be converted to moles first, because different substances have different Mr values.
Leaving decimals in the equation.
Balancing numbers must be whole numbers, so round sensibly.
Dividing by the largest value.
Divide by the smallest so the ratio starts from 1.
Not checking the final equation.
Count atoms on both sides to confirm it balances.
Exam tips
- Convert to moles before doing anything else.
- Divide by the smallest value to simplify.
- Round to whole numbers and explain why if asked.
- Remember this is a Higher-only topic.
Key terms
- Mole ratio
- The ratio in which substances react, measured in moles.
- Simplest ratio
- A ratio reduced to the smallest whole numbers.
- Balancing number
- The large number in front of a formula in an equation.
- Relative formula mass
- The mass of one mole of a substance in grams.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.