Most Common Calculation Mistakes

The short answerThe five most common mistakes are using an unbalanced equation, using the wrong Mr for diatomic gases, mixing up cm3 and dm3, rounding too early, and using the Mr of the wrong substance. Each one is easy to fix once you know what to look for.

Mole calculations are where many students lose marks in IGCSE Chemistry, even when they understand the chemistry. The good news is that the same few errors appear again and again. Here are the five to watch for, each with a wrong and a right way to do it. The relative atomic masses used are H = 1, O = 16, Mg = 24 and Al = 27.

Mistake 1: using an unbalanced equation

The numbers in front of the formulae give the mole ratio. If the equation is not balanced, the ratio is wrong. Always balance the equation first.

Question: 4Al + 3O2 → 2Al2O3. How many moles of Al2O3 form from 0.4 mol of aluminium?

✗ Wrong

Assumes 1 : 1, so 0.4 mol of Al gives 0.4 mol of Al2O3.

✓ Right

The ratio Al : Al2O3 is 4 : 2, so 0.4 × 2 ÷ 4 = 0.2 mol.

Mistake 2: the wrong Mr for diatomic gases

Hydrogen, oxygen, nitrogen, chlorine and the other halogens exist as pairs of atoms, so the formula is H2, O2, N2 or Cl2. Use the Mr of the whole molecule.

Question: How many moles are in 4.0 g of hydrogen gas?

✗ Wrong

Uses Mr = 1, so 4.0 ÷ 1 = 4.0 mol.

✓ Right

H2 has Mr = 2, so 4.0 ÷ 2 = 2.0 mol.

Mistake 3: mixing up cm3 and dm3

Concentration is measured in mol per dm3. If a volume is given in cm3, divide it by 1000 first.

Question: How many moles of NaOH are in 25 cm3 of a 0.20 mol/dm3 solution?

✗ Wrong

0.20 × 25 = 5.0 mol.

✓ Right

25 ÷ 1000 = 0.025 dm3, so 0.20 × 0.025 = 0.0050 mol.

Mistake 4: rounding too early

Rounding in the middle of a calculation can change your final answer. Keep the full number in your calculator and round only at the end, usually to two or three significant figures.

Question: 2Mg + O2 → 2MgO. What mass of MgO forms from 1.00 g of magnesium? (Mr of MgO = 40)

✗ Wrong

1.00 ÷ 24 rounded to 0.04 mol, so 0.04 × 40 = 1.6 g.

✓ Right

1.00 ÷ 24 = 0.041666… mol, so 0.041666… × 40 = 1.67 g.

Mistake 5: using the Mr of the wrong substance

When you find the mass of a product, use the Mr of the product, not the reactant you started with.

Question: What mass of MgO forms from 2.4 g of magnesium, using 2Mg + O2 → 2MgO?

✗ Wrong

0.10 mol × 24 (the Mr of Mg) = 2.4 g, which is the mass you started with.

✓ Right

0.10 mol of Mg gives 0.10 mol of MgO. Mr of MgO = 40, so 0.10 × 40 = 4.0 g.

A quick checklist before you move on

  1. Is the equation balanced?
  2. Did you use the right Mr for the right substance?
  3. Are all volumes in dm3 where they need to be?
  4. Did you keep full digits until the final answer?
  5. Is the answer sensible, and does it have the right units?
Practise it: Try the interactive mole calculator and ten worked examples on our IGCSE Chemistry mole and stoichiometry notes, then work through the practice questions.

Frequently asked questions

Why do I lose marks in mole calculations?

Usually from small slips, such as the wrong Mr, unit conversions or early rounding, and not from a lack of understanding. Showing every step also helps you earn method marks.

How many significant figures should I give?

Usually two or three, depending on the data in the question. Check your exam instructions.

What is the best way to practise?

Do a few calculations every day, check each against the checklist above, and redo any you got wrong a few days later.

Want help with mole calculations?

Book a 1-to-1 online lesson with Ms. Asim and get step-by-step guidance, scheduled around your time zone.

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