question about ksp and solubility

[SN1]

Lead iodide has a Ksp = 7 x 10^-9 at 25 degree celsius. Which statement is true if some potasium iodide is added to a saturated solution of PbI2

a) solubility product constant decreases, while molar solubility increases
b) solubility product constant increases, while the molar solubility decreases
c) solubility product constant increases, while the molar solubility increases.
d) solubility product remains the same, while the molar solubility increases.
e) none of the above


the answer is "e" and when I looked at the solution in the back. It said that I should have automatically eliminated choices a,b, and c.

I know that when I add some potassium iodide, that's gonna dissociate and increase the iodide concentration and shift the equilibrium to the left. This means I can eliminate choices b and c immediately. However, for choice A isn't it true that the solubility product decreases? When you add iodide you are shifting the equilibrium to the left and therefore you make more reactant and decrease the solubility correct? is SOLUBILITY PRODUCT CONSTANT and SOLUBILITY two DIFFERENT things? the answer key said that only temperature can affect equilibrium constant so I guess they are indeed two different things.

And I know that the molar solubility is supposed to decrease. I worked out the math and it made sense because I would be dividing by a bigger number with more iodide.

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[Ari Rezaei]

Be careful with k (Ksp, Kw, Kp, Ketc.) values, like in Ksp = [Pb]^2

The solubility product CONSTANT is Ksp. It doesn't change if the concentrations change, otherwise it wouldn't be a constant! The only thing that can change a solubility product CONSTANT is temperature. Not concentrations, acids, bases, salts, radiation, or divine intervention.

Molar solubility can change, along with Q if you're familiar with the product quotient. For this, you're correct when you say the reaction would shift to the left, or to more PbI2, based on Le Chatlier's principle. Hence, the molar solubility decreases. No need for math even.

 

[SN1]

Be careful with k (Ksp, Kw, Kp, Ketc.) values, like in Ksp = [Pb]^2

The solubility product CONSTANT is Ksp. It doesn't change if the concentrations change, otherwise it wouldn't be a constant! The only thing that can change a solubility product CONSTANT is temperature. Not concentrations, acids, bases, salts, radiation, or divine intervention.

Molar solubility can change, along with Q if you're familiar with the product quotient. For this, you're correct when you say the reaction would shift to the left, or to more PbI2, based on Le Chatlier's principle. Hence, the molar solubility decreases. No need for math even.

Would this be the same for kw, ka, kb, and all the other k's? Shifting to the left decreases solubility but will NOT change the value of the constant correct?

 

[premac]

Would this be the same for kw, ka, kb, and all the other k's? Shifting to the left decreases solubility but will NOT change the value of the constant correct?

Correct.