AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Students should be able to balance an equation given the masses of…
Full specification wording
Using moles to balance equations (HT only): Students should be able to balance an equation given the masses of reactants and products.
Balancing from masses means converting each substance's mass into moles, then finding the simplest whole-number ratio.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 2 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Balancing from masses means converting each substance's mass into moles, then finding the simplest whole-number ratio. Divide each mass by its molar mass (Ar or Mr), giving moles of atoms or molecules. For example, if 4.8 g of magnesium reacts with 3.2 g of oxygen to form 8.0 g of magnesium oxide, moles Mg = 4.8 ÷ 24 = 0.20 mol and moles O₂ = 3.2 ÷ 32 = 0.10 mol. The ratio Mg : O₂ is 2 : 1, so the equation is 2Mg + O₂ → 2MgO. Check that atoms balance on both sides. This skill is examined in higher-tier calculations where masses are supplied and the balanced equation must be deduced.
What earns the marks
- Convert each given mass to moles using moles = mass ÷ molar mass, selecting the correct Ar or Mr for each species.
- Compare the mole values by dividing each by the smallest value to obtain the simplest whole-number ratio.
- Write the balanced equation using the ratio as coefficients, including correct formulae and state symbols where required.
- Verify that the number of each type of atom is equal on both sides of the equation.
- Use the total mass of reactants equal to the total mass of products as a consistency check where all masses are given.
Common mistakes and how to put them right
Using the mass ratio directly as the equation coefficients instead of converting to moles first.
Put it right by dividing mass by molar mass before comparing.
Dividing by the wrong molar mass, such as using Ar of a diatomic element rather than its Mr.
Put it right by identifying whether the substance is an atom or molecule, for example O₂ has Mr = 32.
Forgetting to simplify the mole ratio to the smallest whole numbers.
Put it right by dividing all values by the smallest and multiplying if a fraction remains.
Exam tips
- Show each step: mass, molar mass, moles, ratio, equation, so method marks can be awarded even if the final equation is wrong.
- Label units throughout and round only at the end to avoid losing accuracy.
- Check the final equation by counting atoms of each element on both sides before writing your answer.
Try the lesson
The start of the Using moles to balance equations (HT only): Students should be able to balance an equation given the masses of reactants and products. lesson, made from checked teaching notes for this specification point.
Have a go first
When 4.6 g of ethanol (C₂H₅OH) burns completely in oxygen, 8.8 g of carbon dioxide and 5.4 g of water are produced. Using these masses, what is the simplest whole-number mole ratio of ethanol to oxygen in the balanced equation? (Ar: C = 12, H = 1, O = 16)
Listen
Balancing from masses means converting each substance's mass into moles, then finding the simplest whole-number ratio. Divide each mass by its molar mass (Ar or Mr), giving moles of atoms or molecules. For example, if 4.8 g of magnesium reacts with 3.2 g of oxygen to form 8.0 g of magnesium oxide, moles Mg = 4.8 ÷ 24 = 0.20 mol and moles O₂ = 3.2 ÷ 32 = 0.10 mol. The ratio Mg : O₂ is 2 : 1, so the equation is 2Mg + O₂ → 2MgO. Check that atoms balance on both sides. This skill is examined in higher-tier calculations where masses are supplied and the balanced equation must be deduced. A strong answer earns marks for these points. Convert each given mass to moles using moles = mass ÷ molar mass, selecting the correct Ar or Mr for each species. Compare the mole values by dividing each by the smallest value to obtain the simplest whole-number ratio. Write the balanced equation using the ratio as coefficients, including correct formulae and state symbols where required. Verify that the number of each type of atom is equal on both sides of the equation. A common slip: Using the mass ratio directly as the equation coefficients instead of converting to moles first. Instead, dividing mass by molar mass before comparing. Exam tip: Show each step: mass, molar mass, moles, ratio, equation, so method marks can be awarded even if the final equation is wrong.
See it
Put it in order
Put these steps in the order they happen.
- 1. Check total mass of reactants equals total mass of products
- 2. Verify atoms balance on both sides
- 3. Compare mole values
- 4. Write balanced equation using ratio as coefficients
- 5. Given masses of reactants and products
- 6. Convert each mass to moles: moles = mass ÷ molar mass
- 7. Divide each by the smallest value to get simplest whole-number ratio
Key words
Say what each word means before you turn the card.
Mole
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Molar mass
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Simplest whole-number ratio
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Conservation of mass
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See every definition
- Mole
- The amount of a substance obtained by dividing its mass by its molar mass, giving moles of atoms or molecules.
- Molar mass
- The Ar or Mr of a substance, by which its mass is divided to convert that mass into moles.
- Simplest whole-number ratio
- The result of comparing mole values by dividing each by the smallest value, used as the coefficients in the balanced equation.
- Conservation of mass
- The total mass of reactants equals the total mass of products, used as a consistency check where all masses are given.
Label the diagram
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1Box 1
2Box 2
3Box 3
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