UPSC Combined Geo-Scientist, Mains · 2026 · Mains Paper II
Q1(j). pH when a weak base is 65% neutralised
Question
Calculate the pH of an aqueous solution of a weak monoacidic base at 298 K which has been neutralised by a strong monobasic acid to the extent of 65%. Given K_b and K_w are 2.0×10⁻⁵ and 1.0×10⁻¹⁴ respectively at 298 K.
Answer in brief
pH =9.03.
The full model answer
Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.
After 65% neutralisation: [BH⁺]:[B]=65:35, a basic buffer.
pK_b=-log(2.0×10⁻⁵)=4.70=pK_b+log([BH⁺])/([B])=4.70+log(65)/(35)=4.70+0.27=4.97
pH=14-4.97=9.03
Coversbuffer of a weak basepH when a weak base is 65% neutralised
All parts of Question 1
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- 1(b) Andrew's isotherms and the critical-point conditions5 marks
- 1(c) First-order Bragg angle for the (110) planes5 marks
- 1(d) Intrinsic semiconductor and its conversion to extrinsic5 marks
- 1(e) Fugacity against pressure of a non-ideal gas5 marks
- 1(f) Kₚ and K_c for A+B⇌ C5 marks
- 1(g) Half-life of a nuclide with two parallel first-order decays5 marks
- 1(h) Soap and detergent5 marks
- 1(i) Galvanic cell for 2 Cu⁺leftharpoonsCu²⁺+Cu5 marks
- 1(j) pH when a weak base is 65% neutralised5 marks
- 1(k) Uncertainty in an energy eigenstate5 marks
- 1(l) Expectation value and eigenvalue5 marks
- 1(m) Normal modes of vibration5 marks
- 1(n) Raman activity and polarisability ellipsoids of CO₂5 marks
- 1(o) The incorrect statement on electronic spectra5 marks
- 1(p) Non-radiative decay competing with fluorescence5 marks