OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2017: Question 4

Determining Percentage by Mass in Calcium Compounds

What this question tests

This question assesses your ability to calculate the percentage mass of a specific element within a given chemical compound (Foundation Tier quantitative chemistry skills). It tests formula recall, molar mass calculation, and comparative fraction analysis.

Question 4

Exam Breakdown & Worked Solution

✅ Correct Answer

C (calcium hydroxide)

Awarded 1 mark for selecting C.

💡 Key Knowledge

Formula for percentage mass: (Mass of element / Mr of compound) × 100
Every compound in this question contains exactly one calcium atom (Ca, Ar = 40.1).
Therefore, the compound with the smallest total molar mass (Mr) will yield the greatest percentage by mass of calcium.

🧠 Exam Technique

Don't waste time calculating the exact percentages for all four options! Recognise the mathematical shortcut: because the numerator (mass of Ca) is constant across all options, you only need to compare the total molar masses (Mr) of the compounds.

❌ Common Errors

Forgetting to multiply the subscript numbers outside brackets (e.g., in Ca(OH)₂ , remembering there are 2 O and 2 H atoms).
Guessing based on the size or complexity of the anion rather than calculating/comparing the total Mr.

📐 Step-by-Step Molar Mass (Mr) Comparison

Using atomic masses: Ca = 40.1, C = 12.0, O = 16.0, N = 14.0, H = 1.0, S = 32.1

A. Calcium carbonate (CaCO₃): 40.1 + 12.0 + (3 × 16.0) = 100.1 g mol⁻¹
B. Calcium nitrate (Ca(NO₃)₂): 40.1 + (2 × 14.0) + (6 × 16.0) = 164.1 g mol⁻¹
C. Calcium hydroxide (Ca(OH)₂): 40.1 + (2 × 16.0) + (2 × 1.0) = 74.1 g mol⁻¹ (Lowest Mr ➔ Greatest % Ca)
D. Calcium sulfate (CaSO₄): 40.1 + 32.1 + (4 × 16.0) = 136.2 g mol⁻¹

OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2017: Question 4

Question

Mark scheme

How to answer it