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: Na3PO4, Na2HPO4 and NaH2PO4
The pKa values associated with the three dissociations of H3PO4 are: 2.12, 6.8, 13.32
Which two salts will you use?
I'm pretty sure I want Na3PO4 and Na2HPO4 because I start with the salt closest with the pKa closest the the pH required (Na2HPO4) 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?