Question

A 10.00mL solution containing 0.130M triprotic acid (H3A, pK1 = 3.05, pK2 = 7.89. pK3 = 11.06) was titrated with 0.1546M sodium hydroxide.

a) What volume (in mL) of titrant is needed to achieve a solution in which [HA2-]=[A3-]?

b) What was the pH at the final equivalence point?

Answer 1

a) H₃A ↔ H⁺ + H₂A^- ↔ H⁺ + HA^2- ↔ H⁺ + A^3-

To achieve a solution in which [HA^2-]=[A^3-], the titrant must neutralize the first and second H⁺ and half neutralize the third H⁺.

For the first equivalence point,

V₁S₁ = V₂S₂

V₁ x 0.1546 = 10.00 x 0.130

V₁ = 10.00 x 0.130/0.1546 = 8.41 mL.

The first equivalence point = 8.41 mL.

So, the second equivalence point = 2 x 8.41 mL = 16.82 mL.

The half-neutralization point of third H⁺ = 16.42 + 8.41/2 = 20.62 mL

To achieve a solution in which [HA^2-]=[A^3-], 20.62 mL of titrant is needed. (Answer)

b) At final equivalence point a salt of weak acid strong base is formed,

pH = 7 + (1/2)pKa3 + (1/2)logc ; {c = 10.0 x 0.13 /(10+8.41+8.41+8.41) = 0.0369 M}

pH = 7 + 11.06/2 + (1/2)log(0.0369)

pH = 11.81 (Answer)