Question

When 1 Liter volumes of the below solutions are mixed, which will result in the precipitation of PbCl2? (Ksp = 1.7 * 10^-5)

a.) 0.04M Pb(NO3)2 and 0.08M KCl

b.) 0.001M Pb(NO3)2 and 0.05M KCl

c.) 0.002M Pb(NO3)2 and 0.02M KCl

d.) 0.01M Pb(NO3)2 and 0.005M KCl

Answer 1

PbCl2(s) <-> Pb2+(aq) + 2 Cl-(aq)

Qsp = [Pb2+][Cl-]^2

a)

[Pb2+] = 0.04 M

[Cl-] = 0.08 M

Use:

Qsp = [Pb2+][Cl-]^2

= (0.04)*(0.08)^2

= 2.56*10^-4

Since Qsp is greater than Ksp, precipitate will form

b)

Use:

Qsp = [Pb2+][Cl-]^2

= (0.001)*(0.05)^2

= 2.5*10^-6

Since Qsp is less than Ksp, precipitate will not form

c)

Use:

Qsp = [Pb2+][Cl-]^2

= (0.002)*(0.02)^2

= 8*10^-7

Since Qsp is less than Ksp, precipitate will not form

d)

Use:

Qsp = [Pb2+][Cl-]^2

= (0.01)*(0.005)^2

= 2.5*10^-7

Since Qsp is less than Ksp, precipitate will not form

Answer: a