[repetelww]

Dealing with a polyprotic acid such as Phosphoric Acid is quite aggravating, but I assume it is a necessary evil.

For Part A of the particular question attached, I made an assumption that the H3PO4 form the acid and the PO4^3- phosphate ions are not particularly pertinent and the concentration of those are negligible, because of the pH range of the buffer, 7.40. The pKa of the second dissociation constant is 7.2, so the question is dealing with the H2PO4^1- form of the polyprotic acid.

Knowing that I simply plugged the pka value and the desired pH into the Henderson Hasselbach equation to calculate the ratio of the base to the acid. That being said, I did this merely on a hunch. I have little experience with polyprotic acids, but a moderate amount of experience with monoprotic acids.

I have not began work on Part B, seems like I need to understand Part A first.

[Borek]

Looks OK so far.

[repetelww]

Alright, thanks. For the final concentrations:
 
I know the concentration of H₃O⁺ and OH^- ions from the pH of the buffer.

I know that I will have a concentration of 0.010 M Phosphoric Acid at the beginning, which will correspond to needing a concentration of 0.0159 M of the salt to make the buffer. Now from here do I need to use the acid dissociation constant and the hydronium ion concentration to determine the final concentrations of the H₂PO₄^- and HPO₄^2-.

Obviously the final concentration of K+ ions will be 0.0318 M.

[Borek]

Right now it is all about a simple stoichiometry. Don't worry about equilibrium and concentrations of H⁺ or OH^-, just think - what reaction takes place when you mix H₃PO₄ and K₂HPO₄? What products and in what amounts do you need?