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Chemistry 0620 How-to guide 6 min read Free

How to do mole calculations in IGCSE Chemistry

One route through the 0620 mole questions, the three conversions you need, and a worked example that shows where each mark comes from.

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What the exam is testing

A mole question looks different each time: a mass of a metal, a volume of gas, a solution of known concentration. Underneath, it is almost always the same task. You are given an amount of one substance and asked about another, and the equation links them.

The mole is the bridge. Everything you are given gets converted to moles, the equation moves you from one substance to the other, and you convert back to whatever the question wants.

The route
Given quantity → moles → mole ratio → moles → quantity asked for. Learn it once and the question types stop feeling separate.
The method

The five-step method

  1. Write the balanced equation

    Check each element has the same number of atoms on both sides before you do anything else.

    Tip. The ratio in step 3 comes from these numbers, so a mistake here carries through.

  2. Convert what you are given into moles

    Use the conversion below that matches the quantity: mass, gas volume or solution.

  3. Use the mole ratio

    Read the ratio from the coefficients in the equation to get the moles of the substance you want.

  4. Convert moles into the quantity asked for

    Reverse the conversion from step 2 to get a mass, a gas volume or a concentration.

  5. Give the answer with its unit

    Round at the end only, and match the unit the question asks for.

    Tip. cm³ and dm³ differ by a factor of 1 000.
What to learn

The three conversions

These cover almost every question. Each one turns a measured quantity into moles, and back again.

Mass
moles = mass ÷ Mr
Mass in g. Mr from the Ar values.
Gas volume
moles = volume ÷ 24
Volume in dm³ at r.t.p. For cm³, divide by 24 000.
Solution
moles = conc. × volume
mol/dm³ and dm³. For cm³, divide by 1 000 first.
Watch the coefficient
Add the Ar values to get Mr, but never multiply Mr by the number in front of the formula. That coefficient only belongs in the mole ratio.

Values to know

Molar gas volume
One mole of any gas at room temperature and pressure.
24 dm³ = 24 000 cm³
Avogadro constant (Extended)
The number of particles in one mole.
6.02 × 10²³ mol⁻¹
Relative formula mass, Mr
The sum of the Ar values in a formula. The Periodic Table in the exam gives you each Ar.

Not 22.4. That figure is for standard temperature and pressure. 0620 uses 24 dm³ at room temperature and pressure.

See it in action

Worked example

Worked example · gas volume[4 marks]

5.0 g of calcium carbonate reacts completely with excess dilute hydrochloric acid. Calculate the volume of carbon dioxide produced at room temperature and pressure, in cm³.

Mass = 5.0 g Ar: Ca = 40, C = 12, O = 16 Find: volume of CO2 in cm³
Mark 1
CaCO3 + 2HCl → CaCl2 + H2O + CO2
Correct balanced equation
Mark 2
Mr(CaCO3) = 40 + 12 + 3 × 16 = 100
moles = 5.0 ÷ 100 = 0.050 mol
Moles of the given substance
Mark 3
CaCO3 : CO2 = 1 : 1, so 0.050 mol CO2
Mole ratio used
Mark 4
Volume = 0.050 × 24 = 1.2 dm³ = 1 200 cm³
Correct volume in the unit asked for
1 200 cm³
The mark split is our own illustration of how a mark scheme typically rewards each stage, not taken from a real paper.

Two things to notice. The acid is in excess, so the calcium carbonate decides how much gas forms. And the last line converts dm³ to cm³ because the question asked for cm³.

Try it yourselfSame route · 3 marks

2.4 g of magnesium burns completely in oxygen: 2Mg + O2 → 2MgO. Calculate the mass of magnesium oxide formed. [Ar: Mg = 24, O = 16]

Show the answer
moles of Mg = 2.4 ÷ 24 = 0.10 mol
Mg : MgO = 2 : 2 = 1 : 1, so 0.10 mol MgO
Mr(MgO) = 40, so mass = 0.10 × 40 = 4.0 g
Exam advice

Mistakes that cost marks

Skipping the mole ratio
Going straight from mass to mass assumes every substance reacts 1 : 1. Always go through moles.
An unbalanced equation
The ratio you read from it is then wrong, and every later line follows it.
Multiplying Mr by the coefficient
M<sub>r</sub> belongs to one formula unit. The coefficient is only for the ratio.
Mixing cm³ and dm³
Gas volumes in cm³ divide by 24 000. Solution volumes in cm³ divide by 1 000 first.
Using 22.4 for gas volume
0620 uses 24 dm³ per mole at room temperature and pressure.
Rounding early
Keep the full value of the moles and round only the final answer.
Keep going

Where to go next

Formula sheet Chemistry formula sheet Moles, concentration, yield and purity on one page. Open the sheet →Free Revision notes Chemistry 0620 notes Twelve topics, including the mole and chemical calculations. Read the notes →Free Past papers Chemistry past papers Practise on the structured theory papers, where mole questions run over several steps. Browse papers →Free
The method gets you the marks. The notes make it make sense.
The mole taught from first principles, with worked examples, exam advice and a recall checklist in every chapter.
Get the Chemistry PDF · $10 Read the notes free