AQA A-Level Chemistry Paper 3, 2024: Question 8

Overall yield across a two-step synthesis

What this question tests

Understanding that overall yield for sequential steps is found by multiplying step yields (as fractions/decimals).
Converting percentages to decimals correctly and giving the final answer as a percentage.
AO2 skill: applying a known rule to a simple synthetic route.

Yield Multi-step synthesis % ↔ decimal AO2 application

Part (a): Overall yield of R

Given yields: P → Q = 50%, Q → R = 30%

✅ Correct answer (from mark scheme)

A: 15%

Mark breakdown: 1 mark (AO2) for selecting 15%.

💡 Key knowledge

In a sequence, each step only acts on what was successfully made in the previous step.
Therefore, overall yield = (yield of step 1) × (yield of step 2) × … (as decimals).
Percentages must be converted to fractions/decimals before multiplying (e.g. 50% = 0.50).

📐 Calculation (step-by-step)

Convert to decimals: 50% = 0.50 , 30% = 0.30
Multiply for sequential steps: overall yield = 0.50 × 0.30 = 0.15
Convert back to a percentage: 0.15 × 100 = 15%

🧠 Exam technique (how to secure the mark fast)

For “two-stage synthesis” questions, think multiply, not add.
Use a quick “100 units” check:
- Start with 100 mol of P → 50 mol of Q (50%).
- Then 50 mol of Q → 15 mol of R (30% of 50).
- So 15 out of 100 = 15% overall.
MCQ tip: if one option equals 0.5 × 0.3 (i.e. 0.15), that’s your pick.

❌ Common errors (and why they lose marks)

Adding yields: 50% + 30% = 80% (this is option D). This is wrong because yields compound step-by-step; the second yield is applied to a reduced amount.
Not converting % before multiplying: e.g. 50 × 30 = 1500 (nonsense unless you divide by 100 twice).
Only giving the second yield (30%): ignores losses in step 1.

Examiner-style insight: the mark scheme awards the single mark for the correct overall yield (15%). The most common slip is choosing 80% by incorrectly adding the two step yields.

Mini-check: Can you explain it in one sentence?

🧠 One-sentence full-mark explanation (useful even for MCQ confidence)

“Overall yield for consecutive steps is found by multiplying the step yields as decimals: 0.50 × 0.30 = 0.15 , so the overall yield is 15%.”