Question

Consider the titration of a 27.8 −mL sample of 0.125 M RbOH with 0.110 M HCl. Determine each of the following. A) the initial pH B)the volume of added acid required to reach the equivalence point C) the pH at 6.0 mL of added acid D) the pH at the equivalence point E) the pH after adding 4.0 mL of acid beyond the equivalence point

Answer 1

A) Initial pH

RbOH is a strong base

pOH = -log[OH-] = -log(0.125) = 0.90

pH = 14 - pOH = 13.1

B) Volume of acid needed to reach equivalence point

= (0.125 M x 27.8 ml)/0.110 M = 31.59 ml

C) When 6.0 ml of HCl added

excess [RbOH] = (0.125 M x 27.8 ml - 0.110 M x 6 ml)/(27.8 + 6) = 0.081 M

pOH = -log(0.081) = 1.09

pH = 14 - pOH = 12.91

D) Equivalence point

moles of RbOH = moles of HCl

This is a titration of a strong base versus strong acid. At equivalence point, equal amounts of H+ and OH- are present in solution.

pH = 7.0 (neutral)

E) pH after 4.0 ml of excess acid is added beyond equivalence point

pH = -log[H+] = -log[(0.110 M x 4.0 ml)/(27.8 + 35.59) ml] = 2.16