OCR A-Level Chemistry AS Breadth in chemistry (01), June 2025: Question 5
1 mark · Easy difficulty · Multiple Choice
Identify which phosphorus oxide has the closest composition to 50% by mass of phosphorus.
Practise this questionQuestion
Question text
5 Phosphorus is burnt in oxygen to produce a mixture of oxides.
Which oxide has the closest to 50% by mass of phosphorus?
A PO2
B P2O3
C P2O5
D P4O9
Your answer [1]
Mark scheme
Show the mark scheme
5 A 1
How to answer it
Determining Percentage Composition by Mass of Phosphorus Oxides
This multiple-choice question assesses your ability to calculate percentage composition by mass from chemical formulae, recall relative atomic masses ( Aᵣ ) from the OCR Data Sheet ( P = 31.0 , O = 16.0 ), and apply mental arithmetic shortcuts to save time in Paper 1.
Question 5
Identifying the oxide closest to 50% phosphorus by mass [1 mark]
✅ Correct Answer
A: PO₂
💡 Key Knowledge
- Relative atomic masses:
Aᵣ(P) = 31.0 , Aᵣ(O) = 16.0 - Percentage by mass formula:
% of P = (total mass of P / Mᵣ of compound) × 100 - 50% mass threshold: If a compound is 50% phosphorus by mass, the total mass of phosphorus must equal the total mass of oxygen in the formula:
Total mass of P ≈ Total mass of O
📐 Step-by-Step Calculations
To determine which oxide is closest to 50%, calculate the percentage by mass of phosphorus in each candidate oxide using Aᵣ(P) = 31.0 and Aᵣ(O) = 16.0 :
| Option | Formula | Mass of P | Mass of O | Total Mᵣ | % P by mass | Difference from 50% |
|---|---|---|---|---|---|---|
| A | PO₂ | 1 × 31.0 = 31.0 | 2 × 16.0 = 32.0 | 63.0 | (31.0 / 63.0) × 100 = 49.21% | 0.79% (Closest) |
| B | P₂O₃ | 2 × 31.0 = 62.0 | 3 × 16.0 = 48.0 | 110.0 | (62.0 / 110.0) × 100 = 56.36% | 6.36% |
| C | P₂O₅ | 2 × 31.0 = 62.0 | 5 × 16.0 = 80.0 | 142.0 | (62.0 / 142.0) × 100 = 43.66% | 6.34% |
| D | P₄O₉ | 4 × 31.0 = 124.0 | 9 × 16.0 = 144.0 | 268.0 | (124.0 / 268.0) × 100 = 46.27% | 3.73% |
🧠 Exam Technique (Fast Shortcut)
In multiple-choice questions, avoid calculating all four percentages fully if you can spot a mass ratio:
- For an oxide to be 50% P, the mass of phosphorus must equal the mass of oxygen ( mass P : mass O = 1 : 1 ).
- Notice that Aᵣ(P) = 31.0 and 2 × Aᵣ(O) = 32.0 .
- Because 31.0 ≈ 32.0 , one P atom almost perfectly balances two O atoms in mass.
- Therefore, any oxide with a 1 : 2 atomic ratio ( PO₂ ) has an almost exact 50:50 mass split!
❌ Common Errors & Traps
- Confusing mole ratio with mass ratio: Students often mistakenly choose a 1:1 molar formula (like hypothetical PO) thinking 50% means equal numbers of moles rather than equal masses.
- Using atomic numbers instead of atomic masses: Accidentally using Z = 15 for phosphorus and Z = 8 for oxygen. Always check the OCR Periodic Table for Aᵣ values.
- Assuming real stable oxides only: Students sometimes reject PO₂ because it is an empirical/free-radical unit rather than standard phosphorus(V) oxide ( P₄O₁₀ or P₂O₅ ). The question asks mathematically which oxide among the choices has closest to 50%, regardless of typical room-temperature stability.
Topics
Module 2: Foundations in chemistry · 2.1 Atoms and reactions
Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.