Common Mistakes in A Level Chemistry Calculations (and How to Avoid Them)
By Naeem Bukhari
Not Writing Down Units Until the End
A surprising number of calculation errors come from losing track of units partway through a multi-step problem — using a volume in cm³ where the formula needs dm³, or a concentration that was never actually converted. Writing the unit next to every value from the first line, not just the final answer, catches most of these before they become a wrong answer.
Rounding Too Early
Rounding an intermediate value in a multi-step calculation — moles, then concentration, then a final answer — compounds small errors into a noticeably wrong final result. The safer habit is to carry full, unrounded values through every step and round only the final answer to an appropriate number of significant figures.
Treating Equilibrium Calculations as Purely Mechanical
Kc and Kp questions are usually answered correctly in terms of the arithmetic, but marks are often lost on the reasoning around them — explaining what happens to the position of equilibrium when a condition changes, without confusing that with what happens to the value of K itself. K only changes with temperature; concentration and pressure changes shift the position of equilibrium, not K. Mixing these two ideas up is one of the most common conceptual slips at A Level.
Forgetting That Mark Schemes Reward Working
A correct final answer with no visible working can lose most of the marks available for a question, while an answer with a small arithmetic slip but clearly correct method usually keeps the majority of them. Examiners mark the reasoning, not just the number — which is exactly why practising the full method, not just checking a final answer against a key, is what actually improves a grade.
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