Gen Chem multiple choice Ksp

[kov82]

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The Ksp of PbCrO4 is 1.0 x 10^-16 , what is the molar solubility of PbCrO4 in a solution w/ pH 4?

a) 10^-4
b) 10^-8
c) 10^-16
d) 10^-20
e) 10^-22

I said "a" the book says "b" and totally ignores the pH, at low pH wouldn't the conjugate base of a weak acid react w/ the H+'s in the acid? greatly increasing the solubility of PbCrO4? x^2 = 10^-16 does not take pH into account, which is the only reason I said "a", I guessed in the absence on the acid the answer would be "b" but since its in an acid, solubility must increase more and the only option was "a" possibly by dividing (10^-8)/(10^-4 from pH) giving 10^-4 am I wrong? is it book error?

 

[Sublimation]

The Ksp of PbCrO4 is 1.0 x 10^-16 , what is the molar solubility of PbCrO4 in a solution w/ pH 4?

a) 10^-4
b) 10^-8
c) 10^-16
d) 10^-20
e) 10^-22

I said "a" the book says "b" and totally ignores the pH, at low pH wouldn't the conjugate base of a weak acid react w/ the H+'s in the acid? greatly increasing the solubility of PbCrO4? x^2 = 10^-16 does not take pH into account, which is the only reason I said "a", I guessed in the absence on the acid the answer would be "b" but since its in an acid, solubility must increase more and the only option was "a" possibly by dividing (10^-8)/(10^-4 from pH) giving 10^-4 am I wrong? is it book error?

they put the pH in there to reassure u that it will dissociate completely. Now given the nature of this problem all u have to do is set it up.

X^2=1E-16
X= 1E-8

Which is the answer.

 

[kov82]

thanks, I see it now, but I should have guessed since my textbook that gave me the explanation of how pH causes the solubility to increase didn't put a calculation example or problem in there...... even the calculation I did to get "a" was just a guess since 8-4= 4 , thanks again...

 

[doc toothache]

In the presence of H+ one would expect to see H2CrO4, in which case the concentration of [CrO4]-2 would be 10^-4.

 

[kov82]

so your saying the answer is 10^-4?

 

[doc toothache]

so your saying the answer is 10^-4?

In the absence of another choice.. yes.