Why Does the Common Ion Effect Increase Ksp for AgCl in the Presence of Cl-?

[bravoghost]

I'm working Ksp problems comparing pure water solutions w/ those with an ion already present in solution. Example: AgCl and NaCl both in solution.

The pure water Ksp for AgCl is given as 1.77x10^-10. The question is what the Ksp would be with .01M of Cl- already present. After working the problem, I get an answer (verified as correct) as: 1.77x10^-8. This makes mathematic sense, but it doesn't intuitively make sense to me. The Ksp is larger (10^-8) with the extra ion than for pure water (10^-10)? Doesn't this indicate that AgCl dissociates MORE with .01M Cl- already present? How?

 

[Borek]

1.77x10^-8 is not Ksp, but concentration of Ag⁺ in the solution. Ksp is constant (as long as temperature doesn't change).