📏 15.4 what is % composition?
Percentage composition = the mass of each element in 100 g of compound.
Formula: % element = (mass of element in compound / formula mass) × 100
always divide by TOTAL formula mass, then ×100. The sum of all percentages ≈ 100% .
🧪 example 15.9 · Ca(OH)₂
✦ Formula mass = 40 (Ca) + 2×16 (O) + 2×1 (H) = 40 + 32 + 2 = 74
✦ % Ca = (40 ÷ 74) × 100 = 54.05%
✦ % O = (32 ÷ 74) × 100 = 43.24%
✦ % H = (2 ÷ 74) × 100 = 2.70%
✦ Sum = 54.05 + 43.24 + 2.70 = 99.99% ✔
Ca = 40, two O = 32, two H = 2. total 74. easy!
🔬 example 15.10 · from masses
8.657 g compound → 5.217 g C , 0.962 g H , 2.478 g O
% C = (5.217 ÷ 8.657) × 100 = 60.26%
% H = (0.962 ÷ 8.657) × 100 = 11.11%
% O = (2.478 ÷ 8.657) × 100 = 28.62%
→ 100g compound contains 60.26g C , 11.11g H , 28.62g O
just divide element mass by total mass, then ×100. super simple.
⚡ quick check 15.4 · % sulphate in Al₂(SO₄)₃
✦ Molar mass Al₂(SO₄)₃ = 2×27 + 3×(32+64) = 54 + 3×96 = 54 + 288 = 342 g/mol
✦ Mass of sulphate (SO₄) in compound = 3 × 96 = 288
✦ % SO₄ = (288 ÷ 342) × 100 = 84.21%
SO₄ mass = 96. three SO₄ groups → 288. formula mass 342. 288/342 = 0.8421 → 84.21% .
💧 % composition helps to check purity of compounds — if experimental % matches theoretical, compound is pure.
📋 10 mcqs · % composition
🔑 answer key & result (after submit)
⏳ 35‑min lesson planner · % composition
0-5′
🧃 what is %?
5-12′
📐 formula & Ca(OH)₂
12-19′
⚖️ ex 15.10 (experiment)
19-26′
✏️ quick check Al₂(SO₄)₃
26-35′
🎮 quiz + real life use
🎯 activities (easy for class 10):
• “percent bingo” – teacher calls element, students calculate %.
• “purity detective” – compare lab % with theoretical.
• match compound cards with correct % (like Ca(OH)₂, Al₂(SO₄)₃).
✅ strictly based on file: 15.4, 15.9, 15.10, quick check 15.4. No empirical/molecular.