Question

± Common-Ion Effect on Solubility for Lead Thiocyanate Lead thiocyanate, Pb(SCN)2, has a Ksp value of...

± Common-Ion Effect on Solubility for Lead Thiocyanate

Lead thiocyanate, Pb(SCN)2, has a Ksp value of 2.00×10−5.

Part A

Calculate the molar solubility of lead thiocyanate in pure water. The molar solubility is the maximum amount of lead thiocyanate the solution can hold.

Common-Ion Effect

Consider the dissolution of AB(s) :

AB(s)⇌A+(aq)+B−(aq)

Le Châtelier's principle tells us that an increase in either [A+] or [B−] will shift this equilibrium to the left, reducing the solubility of AB. In other words, AB is more soluble in pure water than in a solution that already contains A+ or B− ions. This is an example of the common-ion effect.

Part B

Calculate the molar solubility of lead thiocyanate in 0.800 M KSCN.

A)

At equilibrium:

Pb(SCN)2 <----> Pb2+ + 2 SCN-

s 2s

Ksp = [Pb2+][SCN-]^2

2*10^-5=(s)*(2s)^2

2*10^-5= 4(s)^3

s = 1.71*10^-2 M

B)

KSCN here is Strong electrolyte

It will dissociate completely to give [SCN-] = 0.8 M

At equilibrium:

Pb(SCN)2 <----> Pb2+ + 2 SCN-

s 0.8 + 2s

Ksp = [Pb2+][SCN-]^2

2*10^-5=(s)*(0.8+ 2s)^2

Since Ksp is small, s can be ignored as compared to 0.8

Above expression thus becomes:

2*10^-5=(s)*(0.8)^2

2*10^-5= (s) * 0.64

s = 3.125*10^-5 M

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