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Approximately 20% of the marks across both Edexcel GCSE Chemistry papers require mathematical skills. That is roughly 40 marks out of 200 — the equivalent of several grade boundaries. Many students find calculations the most daunting part of the exam, but with a systematic approach and thorough practice, they become some of the most reliable marks available.
This lesson teaches you how to approach chemistry calculations for maximum marks, covering the key formulae, common errors, and the step-by-step method that earns full credit every time.
Edexcel provides a formulae sheet, but you should know these by heart so you do not waste time looking them up:
| Formula | When to Use It |
|---|---|
| moles = mass ÷ Mr | Finding moles from mass, or mass from moles |
| concentration (mol/dm³) = moles ÷ volume (dm³) | Solution concentration in mol/dm³ |
| concentration (g/dm³) = mass (g) ÷ volume (dm³) | Solution concentration in g/dm³ |
| percentage yield = (actual yield ÷ theoretical yield) × 100 | How much product was obtained vs. expected |
| atom economy = (Mr of desired product ÷ sum of Mr of all products) × 100 | Efficiency of a reaction |
| Rf = distance moved by substance ÷ distance moved by solvent | Chromatography |
| mean rate = amount of product formed ÷ time | Rate of reaction |
Exam tip: Even though a formula sheet is provided, practising without it means you will be faster and more confident in the exam. Time saved on calculations can be spent on 6-mark questions.
Use this method for every calculation question. It maximises your marks because you earn credit for each step, even if you make an arithmetic error at the end.
graph TD
A["Step 1: Write the formula"] --> B["Step 2: Substitute the values"]
B --> C["Step 3: Calculate the answer"]
C --> D["Step 4: Add the correct units"]
This means on a 3-mark calculation, you could get 2 marks even if your final number is wrong, as long as your method and substitution are correct.
Question: Calculate the number of moles of sodium hydroxide (NaOH) in 8.0 g of sodium hydroxide. (Relative atomic masses: Na = 23, O = 16, H = 1)
Step 1: Write the formula moles = mass ÷ Mr
Step 2: Calculate Mr Mr of NaOH = 23 + 16 + 1 = 40
Step 3: Substitute and calculate moles = 8.0 ÷ 40 = 0.20 mol
Step 4: Units The answer is 0.20 mol
Exam tip: Always calculate Mr first and show this working separately. This often earns its own mark.
Question: A student dissolves 5.85 g of sodium chloride (NaCl) in water to make 500 cm³ of solution. Calculate the concentration in mol/dm³. (Relative atomic masses: Na = 23, Cl = 35.5)
Step 1: Convert volume to dm³ 500 cm³ = 500 ÷ 1000 = 0.500 dm³
Step 2: Calculate Mr of NaCl Mr = 23 + 35.5 = 58.5
Step 3: Calculate moles moles = mass ÷ Mr = 5.85 ÷ 58.5 = 0.10 mol
Step 4: Calculate concentration concentration = moles ÷ volume = 0.10 ÷ 0.500 = 0.20 mol/dm³
Exam tip: The most common error in concentration questions is forgetting to convert cm³ to dm³. Always check: divide cm³ by 1000 to get dm³.
Question: A student reacts magnesium with hydrochloric acid. The theoretical yield of magnesium chloride is 4.75 g. The student actually obtains 3.80 g. Calculate the percentage yield.
Step 1: Write the formula percentage yield = (actual yield ÷ theoretical yield) × 100
Step 2: Substitute percentage yield = (3.80 ÷ 4.75) × 100
Step 3: Calculate percentage yield = 0.800 × 100 = 80.0%
Question: In the thermite reaction, aluminium reacts with iron(III) oxide: 2Al + Fe₂O₃ → Al₂O₃ + 2Fe Calculate the atom economy for producing iron. (Relative atomic masses: Al = 27, Fe = 56, O = 16)
Step 1: Calculate Mr of desired product Mr of 2Fe = 2 × 56 = 112
Step 2: Calculate sum of Mr of all products Mr of Al₂O₃ = (2 × 27) + (3 × 16) = 54 + 48 = 102 Mr of 2Fe = 112 Total = 102 + 112 = 214
Step 3: Calculate atom economy atom economy = (112 ÷ 214) × 100 = 52.3%
Understanding how marks are allocated helps you maximise your score:
| Mark Type | What It Rewards | How to Earn It |
|---|---|---|
| Method mark (M) | Using the correct formula or approach | Write the formula clearly |
| Substitution mark (S) | Putting the right numbers into the formula | Show your substitution step |
| Accuracy mark (A) | Getting the correct final answer | Complete the arithmetic correctly |
| Unit mark (U) | Giving the correct units | Always write units with your answer |
Exam tip: If a question is worth 3 marks and you cannot remember the exact formula, write down what you think it is and substitute the numbers. You will likely still earn 1–2 marks for a sensible attempt.
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