BR, Gen Chem Section III, Passage X, Question 65

[sillyjoe]

The question ask "In which solution is Ag2S MOST soluble?"
A. 0.10 M Ag+
B. 0.01M Ag+
C. 0.10M S2-
D. 0.01M S2-

Ksp Ag2S = 1.6*10^-49 and the Molar Solubility of Ag2S = 3.4*10^-17

Choice D is the correct answer but I do not understand why based off the explanation given in the book. Can anyone please help?

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[SuperSneaky24]

I believe it's due to the common ion effect. Each molecule of Ag2S dissolves into 2 Ag+ ions and 1 S2- ion.

Therefore, when you place Ag2S into a solution with any common ions, it's solubility won't be as high as if it was dissolved in a pure water solution (Le Chatlier's principle). Therefore, your answer choices should be crossed out down to B and D, as they have the lowest concentration of ions. Then, you ask yourself, which ion would dissolve more, Ag+ or S2-? Since 1 molecule of Ag2S dissolves into 2 Ag+ ions and only 1 S2- ion, then the S2- ion would make the most sense.