Question

What is the ph of a solution of 0.650 m k2hpo4, potassium hydrogen phosphate?

Answer 1

phosphate?

At 25 ∘C phosphoric acid, H3PO4, has the following equilibrium constants:

Ka1= 7.5×10^-3

Ka2= 36.2×10^−8

Ka3 = 4.2×10^−13

This is a question about the dissociation of weak polyprotic acids.

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 SUPPLIED CONCENTRATION , the answer is:

pH = (7.2076+12.3767) / 2

pH = 9.79