IGCSE Chemistry 0620 · Common mistakes
Common mistakes in IGCSE Chemistry mole calculations
Updated · by the Fahmazing team
Mole questions are worth a lot on the Extended papers, and the same handful of slips appear again and again. Each one below shows a wrong answer next to a right one, so you can spot the trap before it catches you.
Dividing the wrong way
Moles = mass ÷ molar mass. Mass always goes on top. A quick check: if the mass is bigger than the molar mass, you must get more than 1 mol.
How many moles are in 36 g of water (Mr = 18)?
✗ Wrong: 18 ÷ 36 = 0.5 mol
✓ Right: 36 ÷ 18 = 2.0 mol
Free lesson: The mole and the Avogadro constant
Multiplying grams by the Avogadro constant
The Avogadro constant (6.02 × 10²³) counts particles per mole, not per gram. Change grams into moles first.
How many molecules are in 9.0 g of water?
✗ Wrong: 9.0 × 6.02 × 10²³ = 5.42 × 10²⁴
✓ Right: 9.0 ÷ 18 = 0.50 mol; 0.50 × 6.02 × 10²³ = 3.01 × 10²³ molecules
Free lesson: The mole and the Avogadro constant
Forgetting to change cm³ to dm³
Concentration in mol/dm³ needs the volume in dm³. Divide cm³ by 1000. The same goes for gas volumes: use 24 dm³ with dm³, or 24 000 cm³ with cm³.
0.10 mol of solute is dissolved to make 250 cm³ of solution. Find the concentration.
✗ Wrong: 0.10 ÷ 250 = 0.0004 mol/dm³
✓ Right: 250 cm³ = 0.250 dm³; 0.10 ÷ 0.250 = 0.40 mol/dm³
Tip: A tiny answer like 0.0004 mol/dm³ is a warning sign that the volume was not converted.
Free lesson: Moles of gases and solutions
Using the mass of a gas instead of moles
At room temperature and pressure one mole of any gas fills 24 dm³. Change grams to moles first, then multiply by 24.
What volume does 8.8 g of carbon dioxide (Mr = 44) occupy at r.t.p.?
✗ Wrong: 8.8 × 24 = 211 dm³
✓ Right: 8.8 ÷ 44 = 0.20 mol; 0.20 × 24 = 4.8 dm³
Free lesson: Moles of gases and solutions
Skipping the equation ratio
Moles of one substance are not always equal to moles of another. Check the numbers in front of both substances in the balanced equation.
0.40 mol of magnesium burns: 2Mg + O₂ → 2MgO. How many moles of oxygen react?
✗ Wrong: 0.40 mol of O₂
✓ Right: Mg : O₂ is 2 : 1, so 0.40 ÷ 2 = 0.20 mol of O₂
Free lesson: Reacting masses, yield and empirical formulae
Upside-down percentage yield
Percentage yield = actual mass ÷ predicted mass × 100. A yield over 100% is impossible, so if you get one, you have divided the wrong way.
Predicted mass 8.0 g, actual mass 6.0 g
✗ Wrong: 8.0 ÷ 6.0 × 100 = 133%
✓ Right: 6.0 ÷ 8.0 × 100 = 75%
Free lesson: Reacting masses, yield and empirical formulae
Using the mass ratio as the empirical formula
Formulae count atoms, not grams. Divide each mass by its Ar before finding the ratio.
An oxide contains 11.2 g of iron and 4.8 g of oxygen. Find its empirical formula.
✗ Wrong: 11.2 : 4.8 simplifies to about 2 : 1, so Fe₂O
✓ Right: Fe: 11.2 ÷ 56 = 0.20; O: 4.8 ÷ 16 = 0.30; ratio 2 : 3, so Fe₂O₃
Free lesson: Reacting masses, yield and empirical formulae
Missing atoms after a bracket
A number after a bracket multiplies everything inside it. Mr has no unit.
Find the Mr of Mg(OH)₂
✗ Wrong: 24 + 16 + 1 × 2 = 42
✓ Right: 24 + 2 × (16 + 1) = 58
Free lesson: Relative masses of atoms and molecules
Questions students ask
What is the most common mole mistake?
Dividing the wrong way. Moles = mass ÷ molar mass, so the mass always goes on top.
What volume does one mole of gas take up?
At room temperature and pressure, one mole of any gas occupies 24 dm³, which is 24 000 cm³.
Why is my concentration answer 1000 times too small?
You probably divided by the volume in cm³. Change cm³ to dm³ by dividing by 1000 first.