Question

1) What is the pH at the equivalence point in the titration of a 21.9 mL sample of a 0.456 M aqueous hypochlorous acid solution with a 0.494 M aqueous potassium hydroxide solution?

pH =

2) When a 29.8 mL sample of a 0.430 M aqueous hydrocyanic acid solution is titrated with a 0.373 M aqueous potassium hydroxide solution, what is the pH after 51.5 mL of potassium hydroxide have been added?

pH =

Answer 1

1)

millimoles of HClO = 21.9 x 0.456 = 9.9864

At equivalence point :

mmoles of acid = mmoles of KOH

9.9864 = 0.494 x V

V = 20.22 mL

here salt only remains.

salt concentration = 9.986 / (21.9 + 20.22) = 0.237 M

pH = 7 + 1/2 (pKa + log C)

     = 7 + 1/2 (7.46 + log 0.237)

pH = 10.42

2)

mmoles of HCN = 29.8 x 0.430 = 12.814

mmoles of KOH = 51.5 x 0.373 = 19.2095

HCN   +    KOH     -------->   KCN    + H2O

12.8          19.2                      0             0

0               6.40                   12.8

here strong base reamins.

[OH-] = 6.40 / 29.8 + 51.5 = 0.0787 M

pOH = 1.10

pH = 12.90