Let’s solve each problem step by step. We’re finding the
percent composition of each element in a compound — that means, what percent (by mass) each element contributes to the total molar mass of the compound.
We’ll use this formula for each element:
>
% of element = (mass of element in 1 mole of compound ÷ molar mass of compound) × 100%
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1. H₂O
Molar mass = 2(1.01) + 16.00 =
18.02 g/mol
- Hydrogen:
Mass from H = 2 × 1.01 = 2.02 g
% H = (2.02 ÷ 18.02) × 100 ≈
11.2%
- Oxygen:
Mass from O = 16.00 g
% O = (16.00 ÷ 18.02) × 100 ≈
88.8%
✔ Check: 11.2% + 88.8% = 100% → Good!
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2. CO₂
Molar mass = 12.01 + 2(16.00) =
44.01 g/mol
- Carbon:
Mass from C = 12.01 g
% C = (12.01 ÷ 44.01) × 100 ≈
27.3%
- Oxygen:
Mass from O = 32.00 g
% O = (32.00 ÷ 44.01) × 100 ≈
72.7%
✔ Check: 27.3% + 72.7% = 100% → Good!
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3. NaOH
Molar mass = 22.99 + 16.00 + 1.01 =
40.00 g/mol
- Sodium (Na):
% Na = (22.99 ÷ 40.00) × 100 ≈
57.5%
- Oxygen (O):
% O = (16.00 ÷ 40.00) × 100 =
40.0%
- Hydrogen (H):
% H = (1.01 ÷ 40.00) × 100 ≈
2.5%
✔ Check: 57.5% + 40.0% + 2.5% = 100% → Perfect!
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4. H₂SO₄
Molar mass = 2(1.01) + 32.07 + 4(16.00) = 2.02 + 32.07 + 64.00 =
98.09 g/mol
- Hydrogen:
% H = (2.02 ÷ 98.09) × 100 ≈
2.1%
- Sulfur:
% S = (32.07 ÷ 98.09) × 100 ≈
32.7%
- Oxygen:
% O = (64.00 ÷ 98.09) × 100 ≈
65.2%
✔ Check: 2.1% + 32.7% + 65.2% = 100% → Great!
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5. Ca(OH)₂
First, break it down:
Ca = 40.08
O = 2 × 16.00 = 32.00
H = 2 × 1.01 = 2.02
Total = 40.08 + 32.00 + 2.02 =
74.10 g/mol
- Calcium (Ca):
% Ca = (40.08 ÷ 74.10) × 100 ≈
54.1%
- Oxygen (O):
% O = (32.00 ÷ 74.10) × 100 ≈
43.2%
- Hydrogen (H):
% H = (2.02 ÷ 74.10) × 100 ≈
2.7%
✔ Check: 54.1% + 43.2% + 2.7% = 100% → Awesome!
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Final Answer:
1. H₂O: H = 11.2%, O = 88.8%
2. CO₂: C = 27.3%, O = 72.7%
3. NaOH: Na = 57.5%, O = 40.0%, H = 2.5%
4. H₂SO₄: H = 2.1%, S = 32.7%, O = 65.2%
5. Ca(OH)₂: Ca = 54.1%, O = 43.2%, H = 2.7%
Parent Tip: Review the logic above to help your child master the concept of percent composition problems worksheet.
