Question

Given the Ksp values for the following salts, determine the order that they would precipitate as AgNO3 is gradually added to a solution containing 0.01 M each of NaBr, Na2CrO4 and Na3PO4.

Salt                              Ksp

AgBr                     5.0 x 10-13

Ag2CrO4                9.0 x 10-12

Ag3PO4                 1.8 x 10-18

First: Second: Last:

Answer 1

Find concentrations required for precipitation

a)

Ksp = [Ag+][Br-]

5*10^-13 = (Ag+)(0.01)

[Ag+] = 5*10^-13/0.01 = 5*10^-11 M

b)

Ksp = [Ag+]^2[CrO4-2]

9*10^-12 = (Ag+)^2(0.01)

[Ag+] = ((9*10^-12)/(0.01))^0.5 = 0.00003 3*10^-5

c)

Ksp = [Ag+]^3[PO4-3]

1.8*10^-18 = (Ag+)^3(0.01)

[Ag+] = ((1.8*10^-18)/(0.01))^(1/3) = 0.0000056 = 5.6*10^-6

then...

first:

AgBr                     

second:

Ag3PO4                 

third

Ag2CrO4