Question

a)If the acetate to acetic acid ratio of a solution is five, what is its pH? The pKa of acetic acid is 4.75.

b)What is the dominant ionic form of phosphoric acid in a solution at pH 8.2?

Answer 1

pH of Buffer solution is calculated by using Henderson's equation as

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

pH = pKa + log [Acetate]  / [Acetic acid]

pH = 4.75 + log 5

pH = 4.75 + 0.6990

pH = 5.45

ANSWER : pH = 5.45

Consider dissociation of Phosphoric acid in water.

H₃PO₄ (aq) + H₂O (l) H₃O ⁺ (aq) + H₂PO₄^- (aq)

For above reaction, K1 = [H₃O ⁺] [H₂PO₄^-] / [H₃PO₄ ]

We have, pK1 = - log K1 = 2.148

H₂PO₄^- (aq) + H₂O (l) H₃O ⁺ (aq) + HPO₄^2- (aq)

For above reaction, K2 = [H₃O ⁺] [HPO₄^2-] / [H₂PO₄^- ]

We have, pK2 = - log K 2 = 7.198

HPO₄^2- (aq) +  H₂O (l) H₃O ⁺ (aq) + PO₄^3- (aq)

For above reaction, K2 = [H₃O ⁺] [PO₄^3-] / [HPO₄^2-]

We have, pK3 = - log K 3 = 12.375

We know that, 1) if pH < pKa then acidic form predominates and if pH > pKa basic form predominates.

pH of solution is 8.2 . It is in between pKa values 7.198 and 12.375. Hence, predominant form of phosphoric acid is HPO₄^2-.