To make a phosphate buffer at pH 6.82 starting with one liter of 10 mM phosphoric acid you could add...?

Question

(pKs are of 2.15, 6.82, and 12.38), you could add

A) 25 millimoles of HCl.

B) You can't make a buffer by adding HCl or KOH.

C) 5 millimoles of HCl.

D) 20 millimoles of K+.

E) 15 millimoles of KOH.

Answer 1

E) 15 millimoles of KOH.

the first 10  mmol of KOH transform the H3PO4 --> H2PO4-

the next 5 millimoles of KOH take H2PO4-  half way through its neutralization, at which point the pH = PKa 2  of 6.82

it is a good buffer, because it is 50% weak acid H2PO4-  & 50 % its conjugate base (HPO4)-2  right in the middle of its buffering region