[LHM]

Hg₂CrO₄ just begins to precipitate when equal volumes of 0.0004 M Hg₂(NO₃)₂ and 0.00002 M K₂CrO₄ are combined. What is the approximate K_sp value of Hg₂CrO₄?

A) 1 x 10^-18 M
B) 8 x 10^-9 M
C) 2 x 10^-9 M
D) 4 x 10^-9 M

I know how to do typical Ksp problems, it's just that the Hg₂²⁺ is throwing me off. I tried thinking of it as just one ion, instead of two, but I got B, which is the wrong answer. In other words, I tried multiplying 0.0004 by 0.00002 because I thought that K_sp = [Hg₂²⁺][CrO₄^2-] in this case, but I guess it's not.

When I tried assuming that it was two ions, I had that K_sp=(0.0008)²(0.00002)=1.28*10^-11, which didn't get me any of the answers. So how are you supposed to do this?

[rabolisk]

Ksp = [Hg2₂²⁺][CrO₄^2-] is true.
The values you used were wrong.

Hg2CrO4 just begins to precipitate when equal volumes of 0.0004 M Hg2(NO3)2 and 0.00002 M K2CrO4 are combined. Read it carefully again.

[AWK]

Recalculate concentrations after mixing