AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Using moles to balance equations (HT only): Students should be able to balance an equation given the masses of reactants and products.
Balancing an equation from masses is a Higher Tier skill that involves converting each substance's mass into moles, then finding the simplest whole-number ratio.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Balancing an equation from masses is a Higher Tier skill that involves converting each substance's mass into moles, then finding the simplest whole-number ratio. For example, if 4.8 g of magnesium reacts with 3.2 g of oxygen to form 8.0 g of magnesium oxide, divide each mass by its relative formula mass: Mg: 4.8 ÷ 24 = 0.20 mol; O₂: 3.2 ÷ 32 = 0.10 mol; MgO: 8.0 ÷ 40 = 0.20 mol. The mole ratio Mg:O₂:MgO is 0.20:0.10:0.20, which simplifies to 2:1:2. The balanced equation is 2Mg + O₂ → 2MgO. This method works because the law of conservation of mass means the number of atoms of each element is the same before and after reaction, so the mole ratio must be a simple whole-number ratio.
What earns the marks
- Calculate the amount in moles of each substance by dividing its mass by its relative formula mass or relative atomic mass.
- Use the mole ratios obtained to deduce the simplest whole-number ratio of reactants and products.
- Write the balanced symbol equation with the correct state symbols if they are given or can be deduced.
- Check that the final equation has the same number of atoms of each element on both sides and that any charges balance.
- Show working clearly, including units and the relative formula masses used, so that the method can be followed.
Common mistakes and how to put them right
Dividing mass by relative atomic mass for a molecular substance such as O₂ instead of by its relative formula mass of 32.
Put it right: Use the relative formula mass for the substance as written in the equation.
Forgetting to simplify the mole ratio to the simplest whole numbers, for example leaving 0.20.
Put it right: Divide all values by the smallest value and multiply to remove fractions.
Assuming the masses given are always in grams and using them directly as moles.
Put it right: Convert mass to moles using n = m ÷ Mᵣ before finding the ratio.
Exam tips
- Write down the relative formula mass for every substance before doing any division, as this reduces arithmetic errors and makes your method clear.
- If the mole ratio is not immediately whole numbers, divide by the smallest value and then multiply all values by the same factor to obtain the simplest whole-number ratio.
- After balancing, do a quick atom count for each element on both sides; this catches most errors and takes only a few seconds.
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
A metal M reacts completely with oxygen (O₂) to form a single oxide. When 5.60 g of M reacts with 2.40 g of oxygen, all of the M and all of the oxygen are used up. Using these masses, which equation is correctly balanced? (Relative atomic masses: M = 56, O = 16)
Listen
Balancing an equation from masses is a Higher Tier skill that involves converting each substance's mass into moles, then finding the simplest whole-number ratio. For example, if 4.8 g of magnesium reacts with 3.2 g of oxygen to form 8.0 g of magnesium oxide, divide each mass by its relative formula mass: Mg: 4.8 ÷ 24 = 0.20 mol; O₂: 3.2 ÷ 32 = 0.10 mol; MgO: 8.0 ÷ 40 = 0.20 mol. The mole ratio Mg:O₂:MgO is 0.20:0.10:0.20, which simplifies to 2:1:2. The balanced equation is 2Mg + O₂ → 2MgO. This method works because the law of conservation of mass means the number of atoms of each element is the same before and after reaction, so the mole ratio must be a simple whole-number ratio. A strong answer earns marks for these points. Calculate the amount in moles of each substance by dividing its mass by its relative formula mass or relative atomic mass. Use the mole ratios obtained to deduce the simplest whole-number ratio of reactants and products. Write the balanced symbol equation with the correct state symbols if they are given or can be deduced. Check that the final equation has the same number of atoms of each element on both sides and that any charges balance. A common slip: Dividing mass by relative atomic mass for a molecular substance such as O₂ instead of by its relative formula mass of 32. Instead, use the relative formula mass for the substance as written in the equation. Exam tip: Write down the relative formula mass for every substance before doing any division, as this reduces arithmetic errors and makes your method clear.
See it
Put it in order
Put these steps in the order they happen.
- 1. Divide each mass by its relative formula mass or relative atomic mass
- 2. Amount in moles of each substance
- 3. Mole ratio of reactants and products
- 4. Simplify to simplest whole-number ratio
- 5. Write balanced symbol equation
- 6. Check same number of atoms of each element on both sides
- 7. Masses of reactants and products
Key words
Say what each word means before you turn the card.
mole
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mole ratio
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conservation of mass
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balanced symbol equation
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See every definition
- mole
- Amount of a substance found by dividing its mass by its relative formula mass or relative atomic mass.
- mole ratio
- Ratio of the amounts in moles of reactants and products, which simplifies to the simplest whole-number ratio.
- conservation of mass
- Law meaning the number of atoms of each element is the same before and after reaction, so the mole ratio must be a simple whole-number ratio.
- balanced symbol equation
- Equation written with the correct state symbols if they are given or can be deduced, with the same number of atoms of each element on both sides.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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