Question

A solution is 0.1 M in Cl^– and 0.001 M in CrO^2-₄ . Solid AgNO₃ is gradually added to it Assuming that the addition does not change in volume and K_sp(AgCl) = 1.7 x 10^-10 M² and  K_sp(Ag₂CrO₄) = 1.9 x 10^-12 M³

Select correct statement from the following :

(1) AgCl precipitates first because its K_sp is high. 

(2) Ag₂CrO₄ precipitates first as its K_sp is low. 

(3) Ag₂CrO₄ precipitates first because the amount of Ag⁺ needed is low. 

(4) AgCl will precipitate first as the amount of Ag⁺ needed to precipitate is low.

Answer 1

Answer is (4)

(i) [Ag⁺] required to ppt AgCl(s)

K_sp = IP = [Ag⁺] [Cl^–] = 1.7 x 10^–10 

[Ag⁺] = 1.7 x 10^–9 

(ii) [Ag⁺] required to ppt Ag₂CrO₄(s) 

Ksp = IP = [Ag⁺]² [CrO₄ ^–2] = 1.9 x 10^–12 

[Ag⁺] = 4.3 x 10^–5 

[Ag⁺] required to ppt AgCl is low so AgCl will ppt 1^st .