TPR-H SW Gen Chem #57

[circulus vitios]

The [Pb+2] of a solution is 1.25E-5 M. Calculate the [SO4-2] that must be exceeded before PbSO4 can precipitate. (The solubility product of PbSO4 at 25C is 1.6E-8.)

The solution is:

Ksp=[Pb+2][SO4-2], so [SO4-2]=Ksp/[Pb+2]=(1.6E-8)/(1.25E-5)=1.3E-5M.

Why isn't finding the molar solubility of [SO4-2] the correct approach?

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[circulus vitios]

Anyone?