IGCSE Stoichiometry: Why You Keep Losing Marks on Questions You Actually Know

This is a condensed run-through for the day or two before the exam. You already understand stoichiometry, the goal here is making sure nothing on this list has quietly gone soft. Read it end to end once, then use it as a checklist against your own working on a few practice questions.

The full syllabus scope, in one place

Cambridge IGCSE Chemistry 0620 (Extended tier) examines the following under stoichiometry:

Stating formulae of elements and compounds, and constructing word and symbol equations with state symbols
Deducing formulae of ionic compounds from charges, and constructing ionic equations
Relative atomic mass (Ar) and relative formula mass (Mr)
Calculating reacting masses in simple proportions
The mole as a unit of amount of substance, and the Avogadro constant (6.02 × 10²³)
moles = mass ÷ molar mass, and using it to find mass, molar mass, or number of particles
The molar gas volume: 24 dm³ at r.t.p.
Stoichiometric calculations involving reacting masses, limiting reactants, gas volumes, and solution concentrations (g/dm³ and mol/dm³)
Titration calculations
Empirical and molecular formula from experimental data
Percentage yield, percentage composition, and percentage purity

If any item on this list feels unfamiliar rather than just “a bit rusty,” that’s worth flagging now, not discovering mid-exam.

Every formula in one table

Find this	Formula	Watch for
Relative formula mass, Mr	Sum of (Ar × number of each atom)	Use Ar, not atomic number
Moles from mass	moles = mass (g) ÷ Mr	Mass always in grams
Mass from moles	mass = moles × Mr
Moles from gas volume	moles = volume (dm³) ÷ 24	r.t.p. only; convert cm³ → dm³ by ÷1000
Gas volume from moles	volume (dm³) = moles × 24	Convert to cm³ by ×1000 if needed
Moles from concentration	moles = concentration (mol/dm³) × volume (dm³)	Convert cm³ → dm³ by ÷1000
Concentration from moles	concentration = moles ÷ volume (dm³)
Mole ratio between substances	Read from the balanced equation’s coefficients	Never assume 1:1
Percentage yield	(actual yield ÷ theoretical yield) × 100	Theoretical yield needs a full calculation
Percentage composition/purity	(mass of component ÷ total mass) × 100
Empirical formula ratio	moles of each element ÷ smallest moles value	Never round; multiply to clear fractions
Number of particles	moles × 6.02 × 10²³	Avogadro constant

Command words to recognise on sight

Calculate: a numerical process is required, shown clearly, not just a final number.
Determine: often an unstructured, multi-step problem; you have to plan the route yourself.
State: precise wording, no working needed, but exact phrasing matters (“mol” is accepted, “moles” as a plural is not, for example).
Show that: the answer is given to you; marks come from the method, not from confirming the number works. Full working is non-negotiable here.
Deduce: conclude from the information given, common in empirical formula and ionic equation questions.

The five checks to run before submitting any stoichiometry answer

1. Did you use Ar (the larger periodic table number), not the atomic number?
2. Did you convert every volume to dm³ before substituting into a formula?
3. Did you read the mole ratio off the balanced equation, rather than assuming 1:1?
4. If a mole ratio wasn’t a whole number, did you multiply to clear the fraction, rather than round?
5. Is your working visible, and does your final answer include the correct unit?

Running through these five checks on a few past questions tonight is a better use of time than doing ten more fresh questions. It targets exactly the places marks are lost, rather than practising what’s already solid.

One last thing worth remembering

Stoichiometry marks are disproportionately method marks. A wrong answer with clear, structured working often scores more than a right answer with none. If time is short, prioritise writing every step down over chasing speed. A slower, fully-shown answer beats a fast, unshown one almost every time this topic is assessed.

If any part of this list still feels shaky the night before, that’s still fixable. It’s exactly the kind of targeted, last-mile gap we help close in Nowa Link’s IGCSE chemistry coaching.

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