Q.

If equal volumes of AB2 and XY (both are salts) aqueous solutions are mixed, which of the following combination will give a precipitate of AY2 at 300 K

(Given Ksp (at 300 K) for AY2=5.2×10-7)                                    [2025]
 

1 3.6×10-3M AB2, 5.0×10-4M XY  
2 2.0×10-4M AB2, 0.8×10-3M XY  
3 2.0×10-2M AB2, 2.0×10-2M XY  
4 1.5×10-4M AB2, 1.5×10-3M XY  

Ans.

(3)

AY2(s)  A2+(aq)+2Y-(aq)

Qsp(AY2)=[A2+][Y-]2

By mixing equal volumes of two solutions, concentration of each is reduced to half.

Precipitation occurs when Qsp(AY2)>Ksp(AY2)

(1)  Qsp=[A2+][Y-]2

               =(3.6×10-32)×(5.0×10-42)2

               =1.125×10-10<Ksp(5.2×10-7) (no ppt)

(2)  Qsp=[A2+][Y-]2

               =(2.0×10-42)×(0.8×10-32)2

                =1.6×10-11<Ksp(5.2×10-7) (no ppt)

(3)  Qsp=[A2+][Y-]2

               =(2.0×10-22)×(2.0×10-22)2

                =1.0×10-6>Ksp(5.2×10-7) (ppt formed)

(4)  Qsp=[A2+][Y-]2

               =(1.5×10-42)×(1.5×10-32)2

               =4.21875×10-11<Ksp(5.2×10-7) (no ppt)