Question

Determine the pH at the equivalence point for the titration of 40.0 mL of 0.1 M HNO2 by 0.200 M KOH. (The pK_a of nitrous acid is 3.36.)

Answer 1

HNO2 millimoles = 40 x 0.1 = 4

at equivalence point :

moles of acid = moles of base

4 = 0.2 x V

V = 20 mL

volume of base (KOH) = 20 mL

HNO2   + KOH -------------------> KNO2 + H2O

4                 4                                  0            0

0                 0                                    4           4

here salt only remained . so we have to salt hydrolysis

[salt] = salt moles / total volume

         = 4 / (40 + 20)

         = 0.067 M

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

       = 7 + 1/2 (3.36 + log 0.067)

        = 8.09

pH = 8.09