I don't understnad why I got this question worng:
The question asks
given the following solubilities, calculate the value of the Ksp for the compound:
Zinc carbonate, 3.87 x 10^-6 mol/L
My answer:
AnCO3 -----> Zn + CO3
There exists a 1:1 ratio between Zn and CO3
Ksp = (3.87 x 10^-6)(3.87 x 10^-6)
= 1.5 x 10^-11
Who said it was wrong. It looks ok to me.
I get (3.87 x 10^-6 M)^2 = 1.49766 which rounds to 1.50 x 10^-11 to three significant figures. Perhaps you got clipped for not giving three significant figures in the answer since you are allowed three. Expressing the answer as 1.5 x 10^-11 is just two s.f. Otherwise, it looks fine to me.