Which of these compounds contains the greatest mass percentage of nitrogen?
Select one: a. CH3NH2 b. NaNH2 c. Al(CN)3 d. Pb(N3)2
The correct answer and explanation is:
The correct answer is: d. Pb(N₃)₂
To find which compound contains the greatest mass percentage of nitrogen, we need to calculate the mass percentage of nitrogen in each compound. This is done using the formula:
Mass % of nitrogen = (Total mass of nitrogen atoms in the compound ÷ Molar mass of the compound) × 100
a. CH₃NH₂ (Methylamine)
- C: 12.01 g/mol
- H: 5 × 1.008 = 5.04 g/mol
- N: 14.01 g/mol
- Total molar mass = 12.01 + 5.04 + 14.01 = 31.06 g/mol
- Nitrogen mass = 14.01 g
- %N = (14.01 / 31.06) × 100 ≈ 45.1%
b. NaNH₂ (Sodium amide)
- Na: 22.99 g/mol
- N: 14.01 g/mol
- H: 2 × 1.008 = 2.016 g/mol
- Total molar mass = 22.99 + 14.01 + 2.016 = 39.02 g/mol
- %N = (14.01 / 39.02) × 100 ≈ 35.9%
c. Al(CN)₃ (Aluminum cyanide)
- Al: 26.98 g/mol
- CN: 3 × (12.01 + 14.01) = 3 × 26.02 = 78.06 g/mol
- Total molar mass = 26.98 + 78.06 = 105.04 g/mol
- Nitrogen: 3 × 14.01 = 42.03 g
- %N = (42.03 / 105.04) × 100 ≈ 40.0%
d. Pb(N₃)₂ (Lead azide)
- Pb: 207.2 g/mol
- N₃: 2 × (3 × 14.01) = 2 × 42.03 = 84.06 g/mol
- Total molar mass = 207.2 + 84.06 = 291.26 g/mol
- %N = (84.06 / 291.26) × 100 ≈ 28.9%
Although CH₃NH₂ has a high percentage, it is actually CH₃NH₂ that has the highest nitrogen mass percentage, not Pb(N₃)₂.
Correction:
Let us compare the results again:
- a. CH₃NH₂ → 45.1%
- b. NaNH₂ → 35.9%
- c. Al(CN)₃ → 40.0%
- d. Pb(N₃)₂ → 28.9%
Correct Answer: a. CH₃NH₂
Final Answer: a. CH₃NH₂
Explanation:
CH₃NH₂ (methylamine) has the highest nitrogen mass percentage because its molecular mass is relatively low while it contains one nitrogen atom. When calculating mass percentages, compounds with small molar masses and a significant portion made up of nitrogen tend to have higher nitrogen percentages. Even though Pb(N₃)₂ contains more nitrogen atoms, its very heavy lead atom reduces the overall percentage contribution from nitrogen. Therefore, CH₃NH₂ contains the greatest mass percentage of nitrogen.