Question

For the titration of 0.20M formic acid with 0.36M sodium hydroxide:

a) find the pH at the equivalence point

b) find the pH at some volume past the equivalence point

c) sketch the titration curve

Answer 1

a) At the equivalence point

Suppose we start with 1 L of 0.20 M HCOOH. To reach equivalence point, we need of NaOH.

Initial concentration (M)	0.129	0	0
Change in concentration (M)	-x	x	x
Equilibrium concentration (M)	0.129-x	x	x

The equilibrium constant

Since, the value of Kb is small, we approximate 0.129-x to 0.129

b)

The pH at some volume past the equivalence point

Suppose we add 0.6 L of 0.36 M NaOH to 1 L of 0.20 M HCOOH.

Total volume

Number of moles of HCOOH

Number of moles of NaOH

Out of 0.216 moles of NaOH, 0.20 moles will neutralise 0.216 moles of HCOOH.

of NaOH will remain

c)

The titration curve is shown below.