How to Answer IGCSE Chemistry Calculation Questions

Calculation questions on IGCSE Chemistry 0620 follow predictable patterns. The content is finite — a small set of formulae applied to different chemical contexts. The challenge is not the mathematics (rarely beyond basic arithmetic and simple algebra) but the method: knowing which formula to use, how to extract the right values from the question, and how to present working in a way that earns all available marks.

This guide provides a step-by-step answering framework that works for every calculation type on the exam.

The universal five-step framework

Use this for every calculation, regardless of the topic or difficulty.

Step 1: Write the balanced equation

If the question provides an equation, copy it into your working area. If it does not, write it yourself. The equation gives you the mole ratio, which is the link between what you know and what you need to find.

Even if the question does not seem to need an equation (e.g., “Calculate the Mr of CaCO3”), identify the relevant formula or relationship before starting any arithmetic.

Step 2: Identify what you know and what you need

Read the question and list:

• What data is given? (mass, volume, concentration, Mr values, percentage)
• What are you asked to find? (moles, mass, volume, concentration, percentage)

This step takes ten seconds and prevents the common error of calculating the wrong thing.

Step 3: Select and write the formula

Choose the correct formula based on what you know and what you need:

I have…	I need…	Formula
Mass and Mr	Moles	moles = mass / Mr
Concentration and volume	Moles	moles = concentration x volume (dm3)
Gas volume (at r.t.p.)	Moles	moles = volume / 24
Moles and Mr	Mass	mass = moles x Mr
Moles and volume	Concentration	concentration = moles / volume (dm3)
Moles	Gas volume	volume = moles x 24

Write the formula on the page before substituting. This earns the first method mark.

Step 4: Substitute and calculate

Replace the variables with the values from the question. Check units before substituting:

• Is volume in dm3 or cm3? Convert if needed (divide cm3 by 1000).
• Is mass in grams?
• Is Mr calculated correctly?

Write the substitution on the page: “moles = 4.0 / 40 = 0.10 mol”

This earns the second method mark. If your arithmetic is wrong but the setup is correct, you still score method marks.

Step 5: Write the answer with units

State the final answer clearly, with the correct units and appropriate significant figures:

• “mass = 7.2 g”
• “concentration = 0.50 mol/dm3”
• “volume = 2.4 dm3”

This earns the answer mark. Missing units may lose the mark even if the number is correct.

Worked example: multi-step calculation

Question: 25.0 cm3 of sodium hydroxide solution of concentration 0.10 mol/dm3 is exactly neutralised by 20.0 cm3 of hydrochloric acid. Calculate the concentration of the hydrochloric acid.

Step 1: NaOH + HCl → NaCl + H2O (ratio is 1:1)

Step 2: Known: NaOH volume = 25.0 cm3, NaOH concentration = 0.10 mol/dm3, HCl volume = 20.0 cm3. Find: HCl concentration.

Step 3: moles of NaOH = concentration x volume (dm3)

Step 4: Volume of NaOH in dm3 = 25.0 / 1000 = 0.0250 dm3 Moles of NaOH = 0.10 x 0.0250 = 0.00250 mol From the equation, moles of HCl = moles of NaOH = 0.00250 mol (1:1 ratio) Volume of HCl in dm3 = 20.0 / 1000 = 0.0200 dm3 Concentration of HCl = moles / volume = 0.00250 / 0.0200

Step 5: Concentration of HCl = 0.125 mol/dm3

Each step is visible. If you made an arithmetic error in the final division, the examiner can still award marks for the method.

Common calculation types on 0620

Relative formula mass (Mr)

Add up the Ar values for each atom in the formula, multiplying by subscripts. Mr of H2SO4 = (2 x 1) + 32 + (4 x 16) = 98

Moles from mass

moles = mass / Mr Example: moles of CaCO3 in 10 g = 10 / 100 = 0.10 mol

Reacting masses

Use the mole ratio from the balanced equation to convert moles of one substance to moles of another, then convert back to mass.

Concentration and volume (titration calculations)

Calculate moles of one solution using moles = concentration x volume, use the mole ratio to find moles of the other, then calculate its concentration.

Percentage yield

percentage yield = (actual yield / theoretical yield) x 100 The theoretical yield comes from stoichiometric calculation assuming 100% conversion.

Percentage purity

percentage purity = (mass of pure substance / total mass of sample) x 100

Empirical formula

Divide each element’s mass by its Ar to get moles, then divide each by the smallest mole value to get the simplest whole number ratio.

Gas volume calculations

At room temperature and pressure (r.t.p.), one mole of any gas occupies 24 dm3. volume = moles x 24 dm3

Presenting working for maximum marks

Do

• Write one step per line
• Label each calculation (e.g., “moles of NaOH = …”)
• Show unit conversions explicitly (e.g., “25 cm3 = 0.025 dm3”)
• Circle or underline your final answer
• Include units with every answer

Do not

• Write multiple calculations on one line
• Skip the formula and jump to the answer
• Write the answer without showing how you got it
• Round intermediate values (keep full precision until the final answer)
• Forget to convert units before substituting

When you are stuck

If you cannot see how to complete the calculation:

• Write the balanced equation (this alone may earn a mark)
• Calculate whatever you can (Mr, moles of a known substance)
• State the mole ratio from the equation
• Attempt the next step even if uncertain

A partial calculation that shows correct method can score 2 or 3 marks out of 4. A blank answer always scores zero. For a full treatment of calculation methods, see the stoichiometry study guide and the guide on avoiding common calculation errors.