Mass Percentages of Phosphorus and Nitrogen in Fertilizers

Steps:

• Calculate molar mass of Na₃PO₄: 3(23) + 31 + 4(16) = 164 g/mol; %P = (31/164) × 100 ≈ 19%.
• Calculate molar mass of NH₄NO₃: 2(14) + 4(1) + 3(16) = 80 g/mol; %N = (28/80) × 100 = 35%.
• Identify key elements: P from Na₃PO₄ and N from NH₄NO₃ improve plant growth.
• Match to options: 19% P and 35% N corresponds to choice B.

Why B is correct:

• B matches calculated %P (19%) in Na₃PO₄ and %N (35%) in NH₄NO₃ using the mass percentage formula (% element = (atomic mass × 100) / molar mass).

Why the others are wrong:

• A: 18% understates P in Na₃PO₄ (actual 19%); ignores correct N.
• C: 42% overstates both; confuses with other compounds like KNO₃.
• D: 42% wrong for N (actual 35%); 18% wrong for P.

Final answer: B