[looboy57]

I've been struggling with this question for awhile. I can't seem to get my answer to be any of the options listed, and I also am not sure how to deal with polyprotic buffers.

I would think it'd be 5 mM because the pH = pKa, but apparently that is not the case.


How much KOH or HCl must you add to one litre of 10 mM phosphoric acid H3PO4 (pKas of 2.15, 7.20, and 12.38) to make a phosphate buffer at pH 7.20?

A) You can't make a buffer by adding HCl or KOH
B) 15 millimoles of KOH
C) 20 millimoles of KOH
D) 25 millimoles of HCl
E) 15 millimoles of HCl

Thanks a lot guys

[macman104]

What is the definition of a buffer?

[looboy57]

A solution of a  weak acid and it's salt that resists pH change. Usually pH of the solution is ~ pKa.

[macman104]

So where do you see your required components in what you just combined?

[looboy57]

Sorry, I'm not exactly sure what you're asking, but:

I see my [H3PO4] having to be 5 mM and H2PO4 being 5 mM.

[Borek]

Which pKa will you use then?

[looboy57]

That's where I get confused.

I'd use pka2 = 7.20. But I read somewhere that you're supposed to use the first pka. When I use both I still don't get the right answer.