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UPSC Combined Geo-Scientist

Chemist

  • 3 descriptive papers
  • 200 marks each
  • 3 h per paper

Mains Paper I covers inorganic chemistry.

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

Q1(c). Phosphate buffer at pH 7.40

  • 5 marks
  • Topic: Acid–base equilibria
  • Answered
Question

Phosphate ions are abundant in cells. The typical total phosphate concentration [HPO₄²⁻]+[H₂PO₄⁻] is 2.0×10⁻² M. What are the concentrations of HPO₄²⁻ and H₂PO₄⁻ at pH 7.40? (Given: pKₐ for H₂PO₄⁻ ion is 7.20.)

Answer in brief

[HPO₄²⁻]=1.23×10⁻² M and [H₂PO₄⁻]=7.7×10⁻³ M.

Key points

  • the cell works as a buffer because pH 7.40 is within one unit of pKₐ 7.20; the buffer capacity peaks at pH = pKₐ.

The full model answer

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

H₂PO₄⁻ + H₂O ⇌ H₃O⁺ + HPO₄²⁻

pH=pKₐ+log([HPO₄²⁻])/([H₂PO₄⁻]) ⇒ log([HPO₄²⁻])/([H₂PO₄⁻])=7.40-7.20=0.20 ⇒ ratio=10^(0.20)=1.585

With [HPO₄²⁻]+[H₂PO₄⁻]=2.0×10⁻² M:

[H₂PO₄⁻]=(0.020)/(1+1.585)=7.74×10⁻³ M, [HPO₄²⁻]=0.020-0.00774=1.23×10⁻² M

the cell works as a buffer because pH 7.40 is within one unit of pKₐ 7.20; the buffer capacity peaks at pH = pKₐ.

Model answer to Q1(c), 2026 Mains Paper I: solution page 3
Solution page 3 of 26
Read the whole 2026 solution2026 question paper

CoversbufferPhosphate buffer at pH 7.40

All parts of Question 1