[jnh33]

Hi,

How can I calculate the pH of zinc nitrate, Zn(NO3)2 once it dissolves in water?

Since nitric acid is strong, I thought I can just use the Kb of the conjugate base of Zn(OH)2 as a starting point. But Zn(OH)2 is insoluble and so doesn't have a Kb. What can I do?

[opsomath]

It would be very incorrect to use zinc hydroxide in this calculation, since you have no hydroxide added.

Let me ask you in response, what is the most important acid-base reaction going on in this system?

[jnh33]

How about this?

The most important acid base reaction is Zn+H2O -> Zn(OH)2+H+

So, if I assume my zinc nitrate fully dissociates, and the dissociated zinc fully reacts with water to form Zn(OH)2, then I'll have two mols of H+ for each mol of zinc nitrate added. So, 5 mM of Zn(NO3) would result in 10 mM of H+, or pH 2.

I guess a more accurate solution would be to find the Ksp of Zn(OH2), but that would probably be very small so can be ignored.

[opsomath]

It's not accurate to assume that the reaction you drew goes to completion; that's basically assuming you have a strong acid.

The most direct solution is to look up the pKa of the hydrated Zn(II) cation.