What is the solubility of ZnCO3 at 25C in a buffer solution with a pH of 10.21?
ZnCO3 <----> Zn++ + CO3 – -
CO3-- + H2O ----> HCO3- + OH-
pOH = 14 – 10.21 = 3.79
[OH-] = 1.62x10^-4 M = [CO3--] = [Zn++]
Ksp = [Zn++][CO3-] = (1.62x10^-4)( 1.62x10^-4) =
2.63x10^-8
when i input my answer in my homework it says that its incorrect
Your calculations are correct, but it seems that you made a mistake in the result. The correct answer should be:
Ksp = [Zn++][CO3-] = (1.62x10^-4)(1.62x10^-4) = 2.63x10^-8
So the solubility of ZnCO3 in a buffer solution with a pH of 10.21 at 25°C is approximately 2.63x10^-8 M.
To calculate the solubility of ZnCO3 at 25°C in a buffer solution with a pH of 10.21, you followed the correct steps up until finding the concentration of hydroxide ions ([OH-]).
However, it seems that you made a mistake when calculating the solubility product constant (Ksp) for ZnCO3. The correct calculation is:
Ksp = [Zn++][CO3-] = (1.62x10^-4)(1.62x10^-4) = 2.63x10^-8
Since you obtained the same answer as in your calculation, your Ksp value is correct. Therefore, the solubility of ZnCO3 in the given buffer solution is 2.63x10^-8 M.
If your answer is being marked as incorrect in your homework, it's possible that there could be a mistake in the way you are entering your answer. Make sure that you are providing the correct number of significant figures and entering the value in the correct format specified by your homework system. Double-check your answer and its format, and if you are still having trouble, it may be helpful to reach out to your instructor or teaching assistant for clarification.
I think the trouble is two-fold.
1. I believe you are trying to calculate Ksp and not solubility or S.
2. You have not taken into account the increased solubility due to the acidity.
Your equations look good. But notice the following.
..........ZnCO3 ==> Zn^2+ + CO3^2-
and CO3^2- + H^+ ==> HCO3^-
and HCO3^- + H^+ ==> H2CO3
As the acidity increases it reduces the CO3^2- according to Le Chatelier's Principle (the CO3^2- is shifted to the right) and the HCO3^- is shifted to the right. That makes CO3^2- from ZnCO3 too small so the solubility of ZnCO3 increases to make up the difference.
I think the easiest way to do these is as follows:
I can't write Greek symbols so let's let a2 = alpha 2 = (CO3^2-).
Then a2 = (CO3^2-) = k1k2/D where D is the denominator and =
(H^+)^2 + k1(H^+) + k1k2
So plug in H^+, k1 and k2 for H2CO3 and solve for the k1k2/D term. Then
...........ZnCO3 ==> Zn^2+ + CO3^2-
I..........solid.......0.......0
C..........solid.......S......S*a2
E.........solid........S......S*a2
and solve for S.
If you need further assistance, please post your work and especially show what you are using for Ksp, k1 and k2.