[kct]

This problem is driving me nuts because it seems so simple, yet I am not getting the right answer.
Here it is:

A 0.125 M soln of Mn(NO3)2 is saturated with H2S ([H2S] = 0.10). Calculate the maximum pH (minimum H3O+) required to prevente the precipitation of MnS.

I wrote the equation:

MnS(s) + 2H3O+ <---> Mn2+ + H2O + 2H2S
                 x               0.125                0.1

Kspa = Ksp/(1.0 x 10^-21) = 3 x 10^7

3 x 10^7 = (0.125)(0.1)^2
                        x^2

[H3O+] = 8.1 x 10^-6
So that the pH is 5.19!! The answer is 4.70 . Does anyone see my problem?!?! I appreciate your help

[Borek]

Hmm... What values do you use for Ksp, Ka1 and Ka2?

MnS - pKsp = 9.6. That means that maximum non-precipitating concentration of S^2- is 2*10^-9.

Take a look here:

http://www.chembuddy.com?left=pH-calculation&right=pH-polyprotic-acid-base

Knowing concentration of S^2- to find out pH you may use one of two equations - 9.8 or 9.13 (after some simple rearranging). 9.8 is less precise, as you have to assume [H₂S] is 0.1M, but that's very good assumption in case of so weak acid.

Using K_a1 = 7 and K_a2 = 12 (overall dissociation constant K_a12 = 19) and equation 9.8 I have got pH=5.65. I was too lazy to rearrange 9.13, but when pH 5.65 is put into 9.13 [S^2- ] is calculated as 1.9*10^-9 - pretty close to 2*10^-9.

[kct]

Well, the Ksp I used was 3 x 10^-14

Because this is a diprotic acid I use the Kspa = 3 x 10^-14                                                                           kakw

which is 3 x 10^7