[sciencegirl]

Hello,

I'm stuck with the following problem:
Chromic acid is a diprotic acid with Ka values of 1.6 and 3.1e-7. Calculate
the pH of a 100 mL of 0.100 M chromic acid before and after addition of 10.0 mL of 0.100
M KOH.

What I've got so far is:

H2CrO4 <--> H2 + HCrO42-

and

HCrO4 <--> H+ + CrO4-

I'm not sure where to go from here.

[AWK]

As the first approximation (and quite good one) use only the first dissociation constant K₁ (neglect completely K₂) and quadratic equation.

[sciencegirl]

great...but how does the pH change with the addition of KOH?

[rabolisk]

KOH is a strong base. Acid and base will react.

[JustinCh3m]

calculate total moles of OH- prior to it's addition to the acid H2CrO4.  Then realize that, as it's a strong base, OH- will "rip" or deprotonate that same # moles of H+ from the H2CrO4.

Yes, just worry about the first deprotonation (K1 equilibrium only).  After taking into account the deprotonation, work with this equation using the ICE method:

H2CrO4 <-->  H+  +  CrO4-

[sciencegirl]

Great!  That helped a lot...that's what I did but wasn't sure until now.