How to Avoid Common Chemistry Calculation Errors

Calculation questions on IGCSE Chemistry 0620 are the most reliable source of marks for students who practise them and the most reliable source of lost marks for students who do not. The content is finite (a handful of formulae), the method is systematic, and the errors are predictable. By learning to recognise and prevent common errors, you can convert calculation questions from a source of anxiety into free marks.

Error category 1: Unit conversion

This is the single most common source of lost marks in chemistry calculations.

Volume: cm3 vs dm3

The concentration formula is: moles = concentration (mol/dm3) x volume (dm3)

Volume in exam questions is frequently given in cm3. If you substitute cm3 directly, your answer is 1000 times too large.

Rule: If volume is in cm3, divide by 1000 to get dm3 before substituting.

• 25 cm3 = 0.025 dm3
• 250 cm3 = 0.250 dm3
• 50 cm3 = 0.050 dm3

Write the conversion explicitly: “25 cm3 = 25/1000 = 0.025 dm3.” This takes three seconds and prevents the most common calculation error in the exam.

Gas volume: cm3 vs dm3

The molar volume formula uses dm3: moles = volume / 24 dm3

If the gas volume is given in cm3, convert first. Or use: moles = volume in cm3 / 24000 cm3.

Mass: grams vs kilograms

Most 0620 calculations use grams. If a question gives mass in kilograms (rare but possible), convert to grams first: 1 kg = 1000 g.

Error category 2: Relative formula mass (Mr) mistakes

Forgetting to multiply by subscripts

The Mr of H2SO4 is NOT 1 + 32 + 16 + 4. It IS (2 x 1) + 32 + (4 x 16) = 2 + 32 + 64 = 98.

Write out the full calculation, showing every multiplication:

• Mr of Ca(OH)2 = 40 + 2 x (16 + 1) = 40 + 34 = 74
• Mr of Mg(NO3)2 = 24 + 2 x (14 + 3 x 16) = 24 + 2 x 62 = 24 + 124 = 148

Misreading brackets

Ca(OH)2 has two OH groups, not one O and two H atoms. The brackets mean everything inside is multiplied by the subscript outside.

• Ca(OH)2: Ca = 40, O x 2 = 32, H x 2 = 2. Mr = 74.
• Al2(SO4)3: Al x 2 = 54, S x 3 = 96, O x 12 = 192. Mr = 342.

Using the wrong Ar value

The periodic table is provided, but students sometimes read the wrong number. Ar is the larger number shown for each element (the relative atomic mass). The atomic number (smaller) is the number of protons, not the mass. Always double-check which number you are using.

Error category 3: Mole ratio errors

Ignoring the balanced equation

The balanced equation gives the mole ratio. If you skip this step, you are guessing at the ratio, which often gives the wrong answer.

Example: How many moles of H2 are produced when 0.1 mol of Mg reacts with excess HCl?

Equation: Mg + 2HCl → MgCl2 + H2 Ratio: 1 mol Mg : 1 mol H2 Answer: 0.1 mol H2

If the equation were 2Mg + 2HCl → … , the ratio would be different. Always write the equation and read the ratio.

Using the wrong ratio

Example: 2Na + 2H2O → 2NaOH + H2 If 0.2 mol Na reacts, how many moles of H2 are produced? Ratio: 2 mol Na : 1 mol H2 (not 1:1) Answer: 0.1 mol H2

Students who assume 1:1 ratios without checking get this wrong. Read the coefficients every time.

Error category 4: Rounding errors

Rounding too early

Keep full calculator precision through intermediate steps. Round only the final answer.

Wrong: moles = 5.4 / 27 = 0.2. Volume = 0.2 x 24 = 4.8 dm3. (This happens to be correct, but if the intermediate value were 0.1875, rounding to 0.2 early would give 4.8 instead of the correct 4.5.)

Right: moles = 5.4 / 27 = 0.2000. Volume = 0.2000 x 24 = 4.800 dm3. Then round the final answer.

Rounding to the wrong number of significant figures

Match the significant figures in the question data. If the data is given to 3 significant figures (e.g., 25.0 cm3), give your answer to 3 significant figures. If unsure, 3 significant figures is usually safe for 0620.

Error category 5: Calculator misuse

Not using brackets

Calculate Mr of Ca(OH)2 on a calculator. If you type 40 + 2 x 16 + 1, the calculator gives 40 + 32 + 1 = 73 (wrong, because it does not multiply 1 by 2). Type 40 + 2 x (16 + 1) = 74.

Scientific notation errors

Some calculators display small numbers in scientific notation (e.g., 2.5 x 10^-3). If you copy this as 0.025 instead of 0.0025, the answer is wrong. Read the exponent carefully.

Error category 6: Forgetting to show working

This is not a mathematical error but a mark error. If you write only the final answer and it is wrong, you score zero. If you show the formula, the substitution, and the intermediate value, you may score 2 out of 3 marks even with a wrong final answer.

Always write:

1. The formula (e.g., moles = mass / Mr)
2. The substitution (e.g., moles = 4.0 / 40)
3. The answer with units (e.g., moles = 0.10 mol)

This structure also helps you catch your own errors. If the substitution looks wrong, you can fix it before calculating.

A pre-calculation checklist

Before every calculation in an exam, run through this mental checklist:

1. Equation: Do I have a balanced equation with the correct mole ratio?
2. Units: Are my volumes in dm3? Is my mass in grams?
3. Mr: Did I calculate Mr correctly, including brackets and subscripts?
4. Formula: Am I using the right formula for what I need to find?
5. Working: Am I showing every step so method marks are available?
6. Answer: Does my answer have units? Does it make sense? (If I calculated 5000 moles from 2 grams of substance, something is wrong.)

This checklist takes 20 seconds and prevents the errors that lose marks. For a full guide to the calculation method, see how to study stoichiometry and how to show working.