What mistakes do students make with Using moles to balance equations (HT only)?

Moving a term without applying the inverse operation to both sides. Put it right by performing the same operation on each side. Rearranging in the wrong order, such as dividing before undoing an addition. Put it right by reversing the order of operations. Dropping or misplacing a factor when a variable appears in a denominator. Put it right by multiplying both sides by that denominator first.

What are the exam tips for Using moles to balance equations (HT only) in Chemistry AQA GCSE?

Write the original equation, then show each rearrangement line so your method is visible. Substitute values only after you have isolated the required variable, reducing arithmetic errors. Check units of the final expression to confirm the rearrangement is dimensionally sensible.

AQA · GCSE · Chemistry

Course version: 8462 · Version from Sept 2018

Students should be able to change the subject of a mathematical equation.

Full specification wording

Using moles to balance equations (HT only): Students should be able to change the subject of a mathematical equation.

Changing the subject means rearranging a formula so a chosen variable stands alone on one side.

Checked against the AQA 8462 specification (opens in a new tab) on 21 September 2026.

Point 3 of 3

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

Identify the variable that must become the subject and the operations currently applied to it.
Apply inverse operations to both sides of the equation in the reverse order of operations.
Keep the equation balanced throughout, showing each rearrangement step clearly.
Substitute known values only after rearranging, or substitute first and then solve consistently.
Check the rearranged equation by substituting simple values to confirm both forms give the same result.

Common mistakes and how to put them right

Moving a term without applying the inverse operation to both sides.
Put it right by performing the same operation on each side.
Rearranging in the wrong order, such as dividing before undoing an addition.
Put it right by reversing the order of operations.
Dropping or misplacing a factor when a variable appears in a denominator.
Put it right by multiplying both sides by that denominator first.

Exam tips

Write the original equation, then show each rearrangement line so your method is visible.
Substitute values only after you have isolated the required variable, reducing arithmetic errors.
Check units of the final expression to confirm the rearrangement is dimensionally sensible.

Try the lesson

The start of the Using moles to balance equations (HT only): Students should be able to change the subject of a mathematical equation. lesson, made from checked teaching notes for this specification point.

Have a go first

The equation for the number of moles is n = m / M, where n is moles, m is mass in grams, and M is molar mass in g/mol. Rearrange this equation to make m the subject.

Tip: press 1 to 4 to choose

Listen

Changing the subject means rearranging a formula so a chosen variable stands alone on one side. Use inverse operations, doing the same to both sides. For example, moles = mass ÷ molar mass can be rearranged to mass = moles × molar mass, or molar mass = mass ÷ moles. In chemistry this allows you to find any missing quantity from the relationship. Work through the equation step by step: identify the variable wanted, undo addition or subtraction first, then multiplication or division, and finally any powers or roots. Keep the equation balanced at every stage and check by substituting simple numbers. This skill underpins mole calculations, concentration, gas volumes and titration work at higher tier. A strong answer earns marks for these points. Identify the variable that must become the subject and the operations currently applied to it. Apply inverse operations to both sides of the equation in the reverse order of operations. Keep the equation balanced throughout, showing each rearrangement step clearly. Substitute known values only after rearranging, or substitute first and then solve consistently. A common slip: Moving a term without applying the inverse operation to both sides. Instead, performing the same operation on each side. Exam tip: Write the original equation, then show each rearrangement line so your method is visible.

See it

A top-to-bottom flowchart with seven nodes. The first node reads 'Original equation' and points to 'Identify the variable wanted'. This points to 'Undo addition or subtraction', which points to 'Undo multiplication or division', which points to 'Undo powers or roots'. That points to 'Variable stands alone', which finally points to 'Check by substituting simple numbers'. — The step-by-step sequence for changing the subject of an equation, from identifying the variable to checking the result.

Put it in order

Put these steps in the order they happen.

1. Check by substituting simple numbers
2. Variable stands alone
3. Undo addition or subtraction
4. Undo powers or roots
5. Original equation
6. Identify the variable wanted
7. Undo multiplication or division

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

The diagram again, with 3 boxes numbered instead of named. A top-to-bottom flowchart with seven nodes. The first node reads 'Original equation' and points to 'Identify the variable wanted'. This points to 'Undo addition or subtraction', which points to 'Undo multiplication or division', which points to 'Undo powers or roots'. That points to 'Variable stands alone', which finally points to 'Check by substituting simple numbers'.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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