Approximate nutrient analysis: what is the nitrogen content (by mass) of ammonium sulphate fertiliser, (NH4)2SO4, expressed as a percentage?

Introduction / Context:
Quick estimation of nutrient percentages is common in agronomy and plant design. Ammonium sulphate is a straight nitrogenous fertiliser widely used for basal application and as a source of both NH4+-N and sulphur (as sulphate). Its nitrogen content is about one-fifth by mass, a figure frequently needed for dosage calculations.

Given Data / Assumptions:

• Chemical formula: (NH4)2SO4.
• Atomic masses (approx.): N = 14, H = 1, S = 32, O = 16.

Concept / Approach:
Compute mass fraction of nitrogen in the salt: two nitrogen atoms contribute 28 units to the total formula mass. The total formula mass is 2*(14+41) + 32 + 416 = 2*18 + 32 + 64 = 132. The nitrogen percentage ≈ 28/132 ≈ 0.212 ≈ 21%. Many handbooks round this to about 20–21% for practical agronomic use.

Step-by-Step Solution:

Verification / Alternative check:
Fertiliser specification sheets commonly list ammonium sulphate as 21-0-0-24S, aligning with the computed value.

Why Other Options Are Wrong:

• 5%: far below the calculated value.
• 50% or 65%: unrealistically high for this salt.
• 35%: not supported by stoichiometry.

Common Pitfalls:
Neglecting hydrogen’s contribution or miscounting oxygen, which skews the denominator.

Final Answer:
20