Question

Calculate the pH at the equivalence point for the titration of 1.288 M HA (a weak acid with pKa = 2.16) with 1.288 M NaOH.

Answer 1

At equivalence point the moles of weak acid equals the moles of NaOH added. Hence, the following reaction takes place.

Note: In the above OH^- is from NaOH.

Hence, we can create the following ICE table for the reaction.

Initial, M	1.288	1.288	0
Change, M	-1.228	-1.228	+1.228
Equilibrium, M	0	0	1.228

Note that the final situation will be of 1.228 M of conjugate base of the weak acid.

Now, the conjugate base will dissociate partially in water as follows:

First we need to calculate the K_b of the conjugate base from pK_a of the acid.

We know pK_a is defined as negative logarithm of K_a.

Hence,

Now, K_a and K_b are related as follows:

Now, we can create an ICE table with initial to find the equilibrium OH- concentration.

Initial, M	1.288	0	0
Change, M	-x	+x	+x
Equilibrium, M	1.288-x	x	x

Hence, we can write the expression of K_b from the ICE table as follows:

Hence, the equilibrium OH^- concentration is

Now, we can determine the pOH of the solution as follows:

Now, pOH and pH are related as

Hence, the pH of the solution at the equivalence point is approximately 8.135.