This exam focus covers Exam Focus within Moles & Calculations for GCSE Chemistry. Revise Moles & Calculations in Quantitative Chemistry for GCSE Chemistry with 22 exam-style questions and 20 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 13 of 15 in this topic. Treat this as a marking guide for what examiners are looking for, not just a fact list.
Topic position
Section 13 of 15
Practice
22 questions
Recall
20 flashcards
🎯 Exam Focus
Frequently Examined
Moles calculations appear in virtually every GCSE Chemistry paper. Examiners test this topic across multiple question types:
- 1-2 marks: Calculate Mr of a compound (e.g., "Calculate the Mr of CaCO₃")
- 3-4 marks: Multi-step moles calculation (mass → moles → ratio → mass)
- 2-3 marks: Percentage yield — given actual and theoretical mass, calculate %
- 2 marks: Atom economy — given equation, identify products and calculate
- 5-6 marks: Combined calculation involving conservation of mass, yield, and explanation
Command word alert: "Calculate" always requires a numerical answer with working shown. "Explain" requires you to state the conservation of mass principle. "Suggest" requires reasoning about why yield is less than 100%.
Quick Check: Calculate the Mr of H₂SO₄. (Ar: H = 1, S = 32, O = 16)
Mr = (2 × 1) + 32 + (4 × 16) = 2 + 32 + 64 = 98. Remember: add the Ar of every atom in the formula.
Quick Check: How many moles are in 9g of water? (Mr of H₂O = 18)
n = m ÷ Mr = 9 ÷ 18 = 0.5 mol. Use the moles triangle: cover n, you see m over Mr.
Quick Check: A reaction has a theoretical yield of 20g but only produces 14g. What is the percentage yield?
% Yield = (actual ÷ theoretical) × 100 = (14 ÷ 20) × 100 = 70%. The yield is below 100% because some product was lost during purification or the reaction did not go to completion.