Question

What is the pH of a phosphate solution made by the addition of 6 equivalents of NaOH to an initial solution of phosphoric acid? The pKa values are pK1 = 12, pK2 = 20, pK3 = 40 a.)7.4 b.)10.6 c.)12.6 d.)8.8

Answer 1

First of all,

the pKa values given for H3PO4 buffer solution are incorrect, the correct values are as follows

pK1 = 2.11

pK2 = 7.2

pK3 = 12.7

now for calcculate pH at each of equilibrium point, we use this formula

pH = pK1 + log [salt]/[Acid]

now in the above question we need to know the initial concentration of both Phosphoric acid and NaOH, in order to put value and apply the above formula.

Since it is mentioned 6 equivalent of NaOH are used to the reaction will go will the end

and we will use pH = pK3 + log [salt]/[acid]