Question

The following values were experimentally determined for the titration of .145g of a weak acid with .100 M NaOH. vol. 0 - 2.88 vol. 5 - 4.15 vol. 10 - 4.58 12.5 - 4.76 15.0 - 4.93 20.0 - 5.36 24.0 - 6.14 24.9 - 7.15 25.0 - 8.73 26.0 - 11.29 30.0 - 11.96 What is the required volume to reach eqivalence point?

Answer 1

I believe you are considering the pH vs volume curve. Calculate the ΔV and ΔpH values (where V denotes volume).

V (mL)	ΔV (mL) = (Vdown – Vup)	pH	ΔpH = (pHdown – pHup)
0.0	-	2.88	-
5.0	5.0	4.15	1.27
10.0	5.0	4.58	0.43
12.5	2.5	4.76	0.18
15.0	2.5	4.93	0.17
20.0	5.0	5.36	0.43
24.0	4.0	6.14	0.78
24.9	0.9	7.15	1.01
25.0	0.1	8.73	1.58
26.0	1.0	11.29	2.56
30.0	4.0	11.96	0.67

The sharpest change in pH occurs when the pH jumps from 8.73 to 11.29. This signifies the change in pH at the equivalence point. The pH at the equivalence point is the mid-point of 8.73 and 11.29. The volume change is 1.0; the equivalence point is the midpoint of 25.0 mL and 26.0 mL, i.e, 25.5 mL (ans).