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
