Question

Calculate the molar solubility of lead thiocyanate in 1.00 M KSCN.

Answer 1

You have to make an assumption or estimation here: (don't think about the 1.00 M KSCN)
Since 1.00M KSCN provides a common ion source of SCN-. Therefore, you will have a 1.00 M SCN-.
[Pb+2][SCN-] ~ 0.01 x 0.02^2 = 4E-6 
 Pb(SCN)2 <---> Pb++ + 2SCN- 

Bringing 1.00M SCN- back in the problem. 1.00M + 0.02M, ignoring the SCN-, and calculating the  [Pb+2]: 

Ksp = [Pb++][SCN-]^2 
2.00E-5 = [Pb++](1.00)^2 

[Pb++] = 2.00E-5 M 

The mass balance shows that: 

[Pb++] = [Pb(SCN)2] 

So [Pb(SCN)2] = 2.00E-5 M in 1.00 M KSCN.