Question

The half-equivalence point of a titration occurs half way to the end point, where half of the analyte has reacted to form its conjugate, and the other half still remains unreacted. if 0.500 moles of a monoprotic weak acid (ka = 5.0 × 10-5) is titrated with naoh, what is the ph of the solution at the half-equivalence point?

Answer 1

Answer is: pH of the solution at the half-equivalence point is 4,30.
Chemical reaction: HA + NaOH → NaA + H₂O.
n₀(HA) = 0,500 mol.
Ka = 5,0·10⁻⁵.
Half of the analyte has reacted, so n(HA) = 0,5 mol ÷ 2 = 0,25 mol.
From chemical reaction: n(HA) : n(NaOH) = 1 : 1.
n(NaOH) = n(NaA) = 0,25 mol.
pH = pKa + log(n(NaA)/n(HA).
pH = -log(5,0·10⁻⁵) + log (0,25 mol/0,25 mol).
pH = 4,30.