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5 Stoichiometry Mistakes That Cost IGCSE Chemistry Students Marks
Most students who struggle with stoichiometry in Cambridge IGCSE Chemistry don’t struggle because they can’t calculate. They can. Given a quiet room and no time pressure, most of them can work through a moles question just fine.
They lose marks in the actual IGCSE Chemistry exam for a smaller, more specific reason: five predictable slips that show up in Cambridge’s own examiner reports, series after series. None of them are about ability. All of them are fixable in an afternoon, once a student knows to look for them.
This matters more than it might seem, because stoichiometry isn’t really “one topic” in IGCSE Chemistry 0620. It’s the mathematical spine of the whole Extended syllabus. Electrolysis calculations, energetics, titrations, and organic yield questions are all the same four moves wearing different clothes. Get the moles logic solid once, and a large share of the numerical exam gets easier at the same time.
The IGCSE Chemistry slip that costs the most marks: assuming a 1:1 ratio
This is the single highest-frequency error in stoichiometry, and it’s almost never about the maths. A student correctly finds the moles of the substance they’re given, then carries that number straight across to the substance they need to find, without checking the balanced equation’s coefficients. A 2:1 or 3:2 ratio gets silently treated as 1:1.
The fix is a habit, not a formula: read the ratio off the equation explicitly, in writing, before calculating anything. Not mentally. Written down.
Confusing dm³ and cm³ at the very last step
Cambridge’s June 2024 examiner report describes candidates who correctly found the moles of one reactant, correctly applied the equation’s ratio to find the moles of the next substance, genuinely strong work, and then lost the mark on the final line by multiplying by 24 instead of 24,000, giving an answer in dm³ when the question asked for cm³.
This is worth sitting with, because it isn’t a conceptual gap. The student understood the chemistry. What was missing was one habit: checking what unit the final answer needs to be in before doing the last multiplication, not after.
Rounding an empirical formula ratio instead of clearing it
When a mole ratio comes out as something like 1 : 2.5, the correct move is to multiply every part of the ratio by whatever number clears the fraction: ×2 for .5, ×3 for .33, ×4 for .25. Examiner reports show candidates instead rounding 2.5 down to 2, turning a correct ratio into an incorrect formula.
A related version of the same mistake: calculating a ratio of 5, then writing the answer as “(CH₂)₅” instead of expanding it to C₅H₁₀. A molecular formula has to be written out in full, the shorthand isn’t accepted, even when the underlying number is right.
Skipping the equation on percentage questions
For percentage yield and percentage composition questions, examiners note that weaker candidates often “made no attempt to use the equation or the moles of reactants and products”, in other words, they tried to shortcut a question that required the full mole-ratio method. This looks like a knowledge gap. It’s usually a confidence gap: the student recognised the question as “the hard kind” and disengaged from method rather than working through it.
The percentage itself is simple arithmetic. The theoretical yield it depends on isn’t a guess, it has to come from a proper stoichiometric calculation based on the limiting reactant.
Not showing the working
A large share of the marks in this topic are method marks, not answer marks. On “Calculate” and “Show that” questions especially, a numerically correct answer with no visible working can lose marks it would otherwise earn, and a wrong answer with clearly structured working often picks up partial credit it wouldn’t get if left blank. In stoichiometry, the working is the evidence, not just a formality.
The four-step process underneath all of it
Once these five slips are named, the actual method is short. Every IGCSE Chemistry stoichiometry question, from a straightforward reacting-mass calculation to a multi-step titration, runs on the same sequence:
- Get to moles. From mass (moles = mass ÷ Mr), from gas volume at r.t.p. (moles = volume in dm³ ÷ 24), or from concentration (moles = concentration × volume).
- Read the ratio. Off the balanced equation’s coefficients, written down, never assumed.
- Apply the ratio to get the moles of whatever the question is actually asking for.
- Convert back out to mass, volume, or concentration, whichever the question needs, in the unit it needs.
Multi-step questions that feel unfamiliar, e.g. mass of one reactant to mass of a completely different product aren’t a new type of question. They’re this same sequence run twice, joined by the ratio step in the middle. Once a student sees that, “hard” multi-step problems stop feeling like a different subject.
Quick-reference formula sheet
| 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 in grams |
| Moles from gas volume | moles = volume (dm³) ÷ 24 | r.t.p. only; ÷1000 to convert cm³ → dm³ |
| Moles from concentration | moles = concentration (mol/dm³) × volume (dm³) | ÷1000 to convert cm³ → dm³ |
| Percentage yield | (actual yield ÷ theoretical yield) × 100 | Theoretical yield needs a full stoichiometric calculation |
| Empirical formula ratio | moles of each element ÷ smallest moles value | Never round, multiply to clear fractions |
| Mole ratio between substances | Read from the balanced equation’s coefficients | Never assume 1:1 |
The takeaway
None of this is about a student being “bad at chemistry.” Cambridge’s own examiner reports show the same five slips recurring across series, in candidates who otherwise understand the method. The gap is precision and process, not raw ability, which is also why it’s one of the more coachable parts of the IGCSE exam.
If your child is doing well on the concepts but the marks aren’t showing it, that’s usually where the problem actually is. It’s also exactly what we work on in our weekly IGCSE chemistry class, not re-teaching the mole concept from scratch, but finding the specific step where marks are quietly leaking, and closing it.
Continue your IGCSE stoichiometry revision
- IGCSE Moles and Stoichiometry Made Simple: The Complete Notes
- Empirical Formula Questions and Answers for IGCSE Chemistry
- IGCSE Stoichiometry Worksheet with Answers (Free Practice)
- How to Calculate Empirical Formula: IGCSE Chemistry Explained
- The Ultimate IGCSE Stoichiometry Formula Sheet
- The Mole Formula Explained: A Step-by-Step IGCSE Guide
- IGCSE Stoichiometry Notes: Everything You Need Before the Exam
- IGCSE Stoichiometry Past Paper Questions and Full Solutions
- 20 IGCSE Moles Questions to Test Your Understanding
- Mastering Mole Calculations for IGCSE Chemistry