Question

a.) What is the pH at the equivalence point when a HF solution is titrated with NaOH? Choose from: less than 7, 7, or greater than 7.

b.)Calculate the pH at the equivalence point when a 200. ml 0.050M HF solution that is titrated with 0.040M NaOH solution? (K_a HF = 7.2 x 10^-4)

Answer 1

a) The pH at the equivalence point in the titration of a weak acid with a base would be greater then 7. the conjugate base of a weak acid is a strong base. The conjugate base would pick up a proton from water, forming OH- in solution. Thus the solution would be basic.

b) moles of HF = moles of salt = 0.05 M x 0.2 L = 0.01 mols

Volume of NaOH added = 0.01/0.04 = 0.25 L

molarity of salt in solution = 0.01/0.45 = 0.022 M

salt hydrolyzes as,

F- + H2O <==> HF + OH-

Kb = [HF][OH-]/[F-]

let x amount has hydrolyzed,

Kb = Kw/Ka = 1 x 10^-14/7.2 x 10^-4 = x^2/0.022

x = [OH-] = 5.53 x 10^-7 M

pOH = 6.26

pH = 14 - pOH = 7.74