Question

What is the solubility of MX₂?

MX₂ M²⁺ + 2X^1- (Ksp = 8.32e-07)

a) in water
solubility = _____M

b) in 0.23 M KX
solubility = ______ M

c) in 0.18 M M(NO3)2
solubility = ___________ M

Answer 1

a)

The salt dissolves as:

MX2 <----> M2+ + 2 X-

   s 2s

Ksp = [M2+][X-]^2

8.3*10^-7=(s)*(2s)^2

8.3*10^-7= 4(s)^3

s = 5.92*10^-3 M

Answer: 5.92*10^-3 M

b)

KX here is Strong electrolyte

It will dissociate completely to give [X-] = 0.23 M

At equilibrium:

MX2 <----> M2+ + 2 X-

   s 0.23 + 2s

Ksp = [M2+][X-]^2

8.3*10^-7=(s)*(0.23+ 2s)^2

Since Ksp is small, s can be ignored as compared to 0.23

Above expression thus becomes:

8.3*10^-7=(s)*(0.23)^2

8.3*10^-7= (s) * 5.29*10^-2

s = 1.57*10^-5 M

Answer: 1.57*10^-5 M

c)

M(NO3)2 here is Strong electrolyte

It will dissociate completely to give [M2+] = 0.18 M

At equilibrium:

MX2 <----> M2+ + 2 X-

   0.18 +s 2s

Ksp = [M2+][X-]^2

8.3*10^-7=(0.18 + s)*(2s)^2

Since Ksp is small, s can be ignored as compared to 0.18

Above expression thus becomes:

8.3*10^-7=(0.18)*(2s)^2

8.3*10^-7= 0.18 * 4(s)^2

s = 1.07*10^-3 M

Answer: 1.07*10^-3 M