Question

Calculate the pH of:

1) .10 M Na2HPO4

2) .10 M NaH2PO4

Teacher's hints:

The pH of any intermediate salt of a polyprotic acid is independent of concentration. It will equal the average of the pKa's, according to the formula pH=1/2 (pKa1 + pKa2).

I'm not sure what pKas to use.

Answer 1

The formula is correct. All you have to do is apply the equation with the correct values of pKa.

For Na₂HPO₄ and NaH₂PO₄ these are salts from the phosphoric acid and the pKa of the phosphoric acid are 2.15, 6.86 and 12.38.

For the case of Na₂HPO₄:

NaH₂HPO₄ came from the phosphoric acid:

H₃PO₄ + Na -----------> H₂PO₄^- + H⁺   pKa1 = 2.15

H₂PO₄^- ----------> HPO₄^2- + H⁺   pKa2 = 6.86

HPO4^2- ---------> PO₄^3- + H⁺ pKa3 = 12.38

pH1 = 2.15 + 6.86 / 2 = 4.51 for NaH₂PO₄

pH2 = 6.86 + 12.38 / 2 = 9.62 for Na₂HPO₄

Hope this helps.