Question

What is the pH of a solution of 0.600 M K2HPO4, potassium hydrogen phosphate?

Answer 1

When pKas of polyprotic intermediates have a difference of 2 or more you just average them using the equation: pH = (pKa2 + pKa3) / 2  
pKa2 = -log(Ka2) ; pKa3 = -log(Ka3) 
so, for this problem, REGARDLESS OF THE CONCENTRATION GIVEN, the answer is:  
pH = (7.2076+12.3767) / 2  
pH = 9.79

Answer 2

Answer:

9.8

Explanation:

The salt K₂HPO₄ will dissociate to form the ions K⁺ and HPO₄⁻², and because of the stoichiometry of the dissociation (1:2:1) [HPO₄⁻²] = 0.600 M. So, two reactions may happen:

HPO₄⁻² + H₂O ⇄ H₂PO₄⁻ + OH⁻

HPO₄⁻² + H₂O ⇄ PO₄⁻³ + OH⁻

H₃PO₄ is a polyprotic acid and have the values of pKa:

pKa1 = 2.15

pKa2 = 7.21

pKa3 = 12.4

In this case, the second and the third equilibrium are presented, thus, the pH depends only on the pKa values, and it will be:

pH = (1/2)*(pKa2 + pKa3)

pH = (1/2)*(7.21 + 12.4)

pH = 9.8