Mass percentage of nitrogen calculated from molar masses

Steps:

• Identify molecular formulas: A (C₂H₈N₂), B (N₂H₄), C (NH₃O), D (CH₅N).
• Compute molar masses: A=60 g/mol, B=32 g/mol, C=33 g/mol, D=31 g/mol.
• Calculate nitrogen mass: A=28 g, B=28 g, C=14 g, D=14 g.
• Determine %N = (N mass / molar mass) × 100 and compare to 45%.

Why D is correct:

• Methylamine (CH₃NH₂) has molar mass 31 g/mol with 14 g N, so %N = (14/31) × 100 ≈ 45.2%, matching the given value.

Why the others are wrong:

• A: %N = (28/60) × 100 ≈ 46.7%, slightly higher than 45%.
• B: %N = (28/32) × 100 = 87.5%, much higher than 45%.
• C: %N = (14/33) × 100 ≈ 42.4%, lower than 45%.

Final answer: D