Question

Lead thiocyanate, Pb(SCN)2, has a Ksp value of 2.00×10−5.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.b) Calculate the molar solubility of lead thiocyanate in 0.500 M KSCN.

Answer 1

Answer:

a) 1.71 × 10⁻³ M

b) 8.00 × 10⁻⁵ M

Explanation:

In order to calculate the solubility (S) of Pb(SCN)₂ we will use an ICE chart. We identify 3 stages (Initial, Change, Equilibrium) and complete each row with the concentration or change in the concentration.

       Pb(SCN)₂(s) ⇄ Pb²⁺(aq) + 2 SCN⁻(aq)

I                                 0                   0

C                               +S               +2S

E                                 S                  2S

The solubility product (Ksp) is:

Ksp = 2.00 × 10⁻⁵ = [Pb²⁺].[SCN⁻]² = S . (2S)² = 4S³

S = 1.71 × 10⁻³ M

b) Calculate the molar solubility of lead thiocyanate in 0.500 M KSCN.

KSCN is a strong electrolyte that dissociates to give 0.500 M K⁺ and 0.500M SCN⁻.

       Pb(SCN)₂(s) ⇄ Pb²⁺(aq) + 2 SCN⁻(aq)

I                                 0                 0.500

C                               +S                +2S

E                                 S                0.500 + 2S

Ksp = 2.00 × 10⁻⁵ = [Pb²⁺].[SCN⁻]² = S . (0.500 + 2S)²

In the term (0.500 + 2S)², 2S is negligible.

Ksp = 2.00 × 10⁻⁵ = S . (0.500)²

S = 8.00 × 10⁻⁵ M