[vanderwaals]

Which response has both answers correct? Will a precipitate form when 250 mL of 0.33

M Na₂CrO₄ are added to 250 mL of 0.12 M AgNO₃? Ksp(Ag₂CrO₄) = 1.1 x 10–12

What is the concentration of the Ag+ ion remaining in solution?

A. Yes, [Ag+] = 1.0 x 10–11 M.

B. Yes, [Ag+] = 0.06 M.

C. Yes, [Ag+] = 1.0 x 10–6 M.

D. No, [Ag+] = 0.060 M.

E. No, [Ag+] = 0.105 M.

The correct answer is A. I understand why a precipitate forms, as Q > K. However, when I calculate the concentration of Ag+ remaining, I get 2.85 x 10-6 M, which is not an answer choice. To calculate this, I first forced the reaction to completion, so the concentration of Ag+ would be 0 M and the concentration of CrO₄2- would be 0.135 M. I then called the equilibrium concentration of Ag+ x and the equilibrium concentration of CrO₄2- (0.135 + x), plugged it into the Ksp expression, and solved for x. Could someone tell me what I am doing wrong? Or am I misinterpreting the problem?

[AWK]

Your answer is almost correct (2 significant digits).

[Borek]

the concentration of CrO₄2- would be 0.135 M

Show how you got that number.

IMHO there is no correct answer between those given (and as AWK stated, you are pretty close).

[vanderwaals]

All the Ag+ (0.06 M) must react to form Ag₂CrO₄, so the CrO₄2- concentration would be 0.165 M - half of that, so 0.165 M - 0.03 M = 0.135 M.

I'm still getting 2.9 x 10-6...Was it just my sig figs that were off or is my answer itself wrong?

[Borek]

OK, I made a typo in my calculations, you are right.

Still, the problem is there is no correct answer between those given.