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Some barium chloride is added to a solution that contains both $\ce{K2SO4}$ ($\pu{0.050 M}$) and $\ce{Na3PO4}$ ($\pu{0.020 M}$).
(a) Which begins to precipitate first: the barium sulfate or the barium phosphate?
(b) The concentration of the first anion species to precipitate, either the sulfate or phosphate, decreases as the precipitate forms. What is the concentration of the first species when the second begins to precipitate?

I've figured out the first part of this problem, that barium phosphate is the first to precipitate (due to lower $K_\mathrm{sp}$), but I can't seem to figure out the other part. I even read about selective precipitation and I can't get the answer that was in the back of the book (namely, $\pu{1.1 \times 10^{-6} M}$).
Granted, sometimes I'm pretty there are errors in the books I use to teach myself, but this is not one of those instances where I am so sure.

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