[Fzang]

Calculate the molarity of all species in a 0.200 M H₂SO₄ solution.

And I'm given the K_a for HSO₄^-, which is 1.20*10^-2

This wouldn't be a problem with a monobasic acid. Now I have a polybasic acid, so what do I do?

[Borek]

Assume first dissociation step went to completion. That just means you have some initial concentration of H⁺ (before HSO₄^- starts to dissociate).

[Fzang]

So with an acid that dissociates completely, say 1 mole acid, I'd just act as if I started by having 1 mole protons and 1 mole conjugate base, in total?

[Borek]

Yes. If the acid first dissociation step is strong enough.

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

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

[Fzang]

Thanks.