Question

Hydrofluoric acid, HF, is a weak acid with an acid-dissociation constant (K_a) of 6.3 ✕ 10⁻⁴. If 0.023 L of 0.17 M HF is titrated with 0.16 M NaOH, a strong base, determine the pH at the equivalence point. (The ion-product constant of water isK_w = 1.01 ✕ 10⁻¹⁴ at 25°C.)

Answer 1

at equivalence point : millimoles of acid = millimoles of base

0.023 x 0.17 =   0.16 x V

V = 0.024 L

total volume = 0.023 + 0.024 = 0.047 L

at equivalence point only salt is remains

salt concentration = 0.023 x 0.17 / total volume

                              = 0.083 M

pKa = -log Ka

pKa = 3.20

pH = 7 + 1/2 [pKa + logC]\

pH = 7 + 1/2 [3.20 + log 0.083]

pH = 8.06