Base/Weak Acid Titration with pH Probes
Recall that the equivalence point (E.P.) occurs when the acid has been neutralized. When titrating a weak acid with a strong base, the pH at the E.P. will generally be above 7. The E.P. on the titration curve can be located at the mid point of the steepest section.
The half equivalence point (H.E.P.) can be easily found on the titration curve. As the name implies, it is located "half-way" to the E.P. This point is significant in that at the HE.P., half of the weak acid (HQ)
has been converted into its conjugate base (Q). That is another way of saying [HQ] = [Q] at the
H.E.P. Consider the Henderson-Hasselbach equation.
pH = pKa + 10g ( [Q] / [HQ] )
If [HQ] = [0], then [Q) [HQ) = 1.
Since the log of 1 = 0, at the H.E.P. pH = pKa
For both titrations, go to Data Collection and set it to Events with Entry. Make the entry volume with units mL. Use the keep button to take measurements every 0.5 ml. After reaching the end of the titration, use the graph to get accurate values for the equivalence and half-equivalence points.
Titration 1:
Buret: 0.1 M NaOH (strong base)
100-mL beaker: 0,1 M CH,COOH (weak acid)
Trial 1:
Equivalence point: volume=11 pH=8.55
Half-Equivalence point: volume=6.5 pH=4.79

What is the Ка of CH,COOH (weak acid)?