new pharmacy student needs help!

[Jodes]

Hi, im am having difficulty with a problem in my study guide

could anyone explain it to me?

Q: calculate the molar concentrations of components needed to prepare 500ml of a buffer with a capacity of 0.1 and a pH of 6.5. The molecular weights of phosphoric acid (98g/mole), sodium dihydrogen phosphate (138g/mole), disodium hydrogen phosphate (142g/mole) and trisodium phosphate (164g/mole) are given.



I am studying this subject by correspondence and haven't studies chemistry before.

I'm thinking to produce a buffer with a pH of 6.5 i would use a combination of sodium dihydrogen phosphate & disodium phosphate, but how do i work out pKa and amounts of each component?

cheers

[sash563]

wow i have just qualified as a pharm and i cant memb learning any of this but i am sure there is a formula to work it out i think