UPSC Combined Geo-Scientist, Mains · 2026 · Mains Paper I

Q1(j). Urea required to precipitate Fe³⁺ as Fe(OH)₃

Question

Calculate the minimum moles of urea required for complete precipitation of 0.02 mol of Fe³⁺ as Fe(OH)₃.

0.03 mol of urea.

this is precipitation from homogeneous solution: the slow rise in pH gives dense, filterable Fe(OH)₃ or basic salts, with little co-precipitation.

The full model answer

Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.

Urea hydrolyses slowly on boiling and releases hydroxide:

(NH₂)₂CO + 3H₂O →CO₂ + 2NH₄⁺ + 2OH⁻

Each mole of urea gives 2 mol OH⁻.

Fe³⁺ + 3OH⁻ → Fe(OH)₃ ↓ ⇒ n(OH⁻)=3×0.02=0.06 mol

n(urea)=(0.06)/(2)=0.03 mol (=1.8 g, M=60 g mol⁻¹)

this is precipitation from homogeneous solution: the slow rise in pH gives dense, filterable Fe(OH)₃ or basic salts, with little co-precipitation.

Covershomogeneous precipitationUrea required to precipitate Fe³⁺ as Fe(OH)₃