Question

You are asked to create 1L of a buffer solution that has a total of 1M of phosphates. The final pH of the solution should be 7.2. How much (in g) of each of NaH₂PO₄ and Na₂HPO₄ should be added. The 25^oC dissociation constants are pK₁=2.12, pK₂=7.21, and pK₃=12.67.

Don't know how to do question, if anyone could help that would be great.

Answer 1

Its a question of buffer so the pH has to be calculated using

in the above case A^- = conjugate base = Na₂HPO₄

                 HA = acid = NaH₂PO^₄

PK2= has to be taken because we are trying to pH 7.21

so 7.2 = 7.21 +log(Na₂HPO₄ /NaH₂PO^₄₎

solving ,   -.01 = log(Na₂HPO₄ /NaH₂PO^₄₎

on taking antilog we can get the desired ratio.

Multiply each by molecular mass to make 1 mole of phosphate