Buffer Solutions

[mojoro86]

Hi,
I need some help with my chemistry homework. Let's take it one step at a time. I have 20.0 mL of 0.100M KH2PO4 and 20.0mL of 0.100M NaOH. I need to find the concentration of the conjugate base (KHPO4-). How do I do that? I know that NaOH dissociates completely, but that's about as far as I've gotten. What do I do?

[sdekivit]

you make the reaction equation for the reaction between H2PO4(-) and OH(-) and then calculate the amount of mols H2PO4(-) that are converted by OH(-) yielding your product.

[mojoro86]

But I don't know how to find how many moles are converted, because I don't know what the Ka of KH2PO4 is. The only thing I could work from would be the Ka of C3PO4, but i'm not sure how to find the concentrations of H2PO4-, H+, and remaining KH2PO4 at equilibrium with that information. And on top of that, I don't know what part the NaOH plays in all of this. I would appreciate if anyone could help me out on this.

Thanks

[Borek]

Assume stoichiometric reaction between strong base NaOH and weak acid H2PO4(-). Hydrolysis is too weak for significant changes of the HPO4(2-) concentration (pKa2 = 7.2).