Question

36. Use the following titration curve to answer the following 14 pH 30.0 10.0 20.0 Volume of titrant added (mL) a) What type of sample is being titrated (Be specific)? b) What is the pH at the equivalence point(s) c) What is the Kai and Ka2 for this sample d) What pH region(s) would this make a suitable buffer? e) What was the initial concentration of the sample that would provide this curve? (USE Kal)

Answer 1

Solution: The given titration curve represents titration of acid against a strong base.

A) The sample is a diprotic acid which itls titrated against strong base.

B) There are two equivalence points.

First equivalence point observed at pH 4

Second equivalence point observed at pH 9.

C) The pH corresponding to half equivalence points are equal to the two pka's.

pka1 = 2.0

ka1 = 10^-pka1 = 10^-2 M

pka2 = 4.0

ka2 = 10^-pka2 = 10^-4 M

D) First buffer region = pka1 +/- 1

= pH  1.0 to 3.0

Second region = pka2 +/- 1

= pH between 3 to 5

Thus,

Overall pH rages from pH 2 to 5.

E) pH at start point = 1.5

[H+] = 10^-pH = 10^-1.5 = 0.032 M

Let acid is H2A. It dissociated as,

H2A =H+ + HA-

X == 0.032 ----0.032

Thus,

ka1 = (0.032)^2 / X

X = 0.032 x 0.032 / 10^-2

X = 0.032 x 0.032 / 0.01 = 0.10 M

Hence, concentration = 0.10 M