30.0 mL of 0.10 M Ca(NO3)2 and 15.0 mL of 0.20 M Na3PO4 solutions are mixed. After the reaction is complete, which of these ions has the lowest concentration in the final solution?

a) Na +
b) NO3 -
c) Ca +2
d) PO4 -3

The answer is c) but I don't understand completely why, could someone please explain? Thank you!

1 Answer
SCooke
Jan 7, 2018

It's all about the solubilities of the products.

Explanation:

Only the calcium ion will form a precipitate. ALL off the nitrate and sodium ions remain in solution.

Whatever the actual solubility product for calcium phosphate is, at the most all of the calcium precipitating would still leave significant phosphate ions in solution (too many to combine completely with the calcium: #(Ca^(+2))_3(PO_4^(-3))_2#), so the ion minimized in the solution would be the calcium.

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