Let’s solve each problem step by step. We’re finding the
percent composition of each element in a compound — that means: what percent of the total mass of the compound comes from each element?
To do this, we need:
1. The atomic masses of each element (from the periodic table).
2. The formula of the compound.
3. Calculate the molar mass of the whole compound.
4. For each element: (total mass of that element in one mole of compound ÷ molar mass of compound) × 100% = percent composition.
We’ll use these approximate atomic masses:
- Cu = 63.55 g/mol
- Br = 79.90 g/mol
- Na = 22.99 g/mol
- O = 16.00 g/mol
- H = 1.008 g/mol
- N = 14.01 g/mol
- S = 32.07 g/mol
---
Problem 1: CuBr₂
Step 1: Molar mass of CuBr₂
= 1×Cu + 2×Br
= 63.55 + 2×79.90
= 63.55 + 159.80
=
223.35 g/mol
Step 2: Percent Cu
= (63.55 / 223.35) × 100% ≈
28.45% → rounds to
28.4%
Step 3: Percent Br
= (159.80 / 223.35) × 100% ≈
71.55% → rounds to
71.6%
✔ Matches given answer.
---
Problem 2: NaOH
Step 1: Molar mass of NaOH
= Na + O + H
= 22.99 + 16.00 + 1.008
=
39.998 g/mol ≈
40.00 g/mol
Step 2: Percent Na
= (22.99 / 40.00) × 100% =
57.475% → rounds to
57.5%
Step 3: Percent O
= (16.00 / 40.00) × 100% =
40.0%
Step 4: Percent H
= (1.008 / 40.00) × 100% =
2.52% → rounds to
2.5%
✔ Matches given answer.
---
Problem 3: (NH₄)₂S
This means: 2 nitrogen atoms, 8 hydrogen atoms (because NH₄ has 4 H, and there are two of them), and 1 sulfur atom.
Step 1: Molar mass
= 2×N + 8×H + 1×S
= 2×14.01 + 8×1.008 + 32.07
= 28.02 + 8.064 + 32.07
=
68.154 g/mol
Step 2: Percent N
= (28.02 / 68.154) × 100% ≈
41.11% →
41.1%
Step 3: Percent H
= (8.064 / 68.154) × 100% ≈
11.83% →
11.8%
Step 4: Percent S
= (32.07 / 68.154) × 100% ≈
47.06% →
47.1%
✔ Matches given answer.
---
Problem 4: N₂S₂
Step 1: Molar mass
= 2×N + 2×S
= 2×14.01 + 2×32.07
= 28.02 + 64.14
=
92.16 g/mol
Step 2: Percent N
= (28.02 / 92.16) × 100% ≈
30.40% →
30.4%
Step 3: Percent S
= (64.14 / 92.16) × 100% ≈
69.60% →
69.6%
✔ Matches given answer.
---
Final Answer:
All percent compositions provided in the worksheet are correct as calculated:
1) CuBr₂: Cu = 28.4%, Br = 71.6%
2) NaOH: Na = 57.5%, O = 40.0%, H = 2.5%
3) (NH₄)₂S: N = 41.1%, H = 11.8%, S = 47.1%
4) N₂S₂: N = 30.4%, S = 69.6%
Parent Tip: Review the logic above to help your child master the concept of percent composition problems worksheet.
