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

Q1(d). Onset of AgCl precipitation

Question

Find the concentration of added Ag⁺ to just start precipitation of AgCl in a 1.0×10⁻³ M solution of NaCl, if Kₛₚ of AgCl at 25°C is 1.0×10⁻¹⁰.

[Ag⁺]=1.0×10⁻⁷ M.

If [Ag⁺][Cl⁻]<Kₛₚ: no precipitate. If >Kₛₚ: AgCl separates.
NaCl is strong, so [Cl⁻]=[NaCl]=1.0×10⁻³ M; the volume change on adding silver is neglected.

The full model answer

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

Precipitation starts when the ionic product just equals Kₛₚ:

AgCl(s) ⇌ Ag⁺ + Cl⁻ Kₛₚ=[Ag⁺][Cl⁻]

[Ag⁺]=Kₛₚ[Cl⁻]=1.0×10⁻¹⁰1.0×10⁻³=1.0×10⁻⁷ M

If [Ag⁺][Cl⁻]<Kₛₚ: no precipitate. If >Kₛₚ: AgCl separates.
NaCl is strong, so [Cl⁻]=[NaCl]=1.0×10⁻³ M; the volume change on adding silver is neglected.

this is the basis of the Mohr method: Cl⁻ is titrated first and Ag₂CrO₄ forms only after the chloride is used up.

CoversKₛₚOnset of AgCl precipitation