Hi.

I've been given a question on buffer solutions and i'm having major issues completing it!

I've got most of it done, just I can't work out the last bit.

The question reads:

You want to make up 100ml of a 0.25M phosphate buffer pH 7.2 and have available the following three salts: Na_{3}PO_{4}, Na_{2}HPO_{4} and NaH_{2}PO_{4}

The pKa values associated with the three dissociations of H_{3}PO_{4} are: 2.12, 6.8, 13.32

**Which two salts will you use?**

I'm pretty sure I want Na_{3}PO_{4} and Na_{2}HPO_{4} because I start with the salt closest with the pKa closest the the pH required (Na_{2}HPO_{4}) and add small amounts of the salt with the higher pKa to increase the pKa from 6.8 to 7.2

Its the next question which i'm finding difficult:

**Now calculate how much of each you need to weigh out to make this solution, show your working.**

Now using the Henderson-Hasselbalch equation i've got:

pH = pKa + log (A^{-})/(HA)

substituting in numbers, taking the negative log then taking pKa away from pH gives me:

0.4 = log (A^{-})/(HA)

multiply both sides base 10

2.512 = (A^{-})/(HA)

multiply both sides by (HA)

2.512 (HA) = (A^{-})

And thats as far as I can get. I need to use the information given to find the number of moles of both HA and H^{-} and from that work out the mass of each.

Can anyone help?