Consider the titration of 20.00 mL of 0.1728 M Ascorbic acid (Ka = 7.9 x 10-5) with 0.4329 M NaOH.

Match the following regions in the titration curve with the appropriate pH range.

1.Initial pH


2.Before equivalence point


3.At equivalence point


4.After equivalence point

A.
8 - 9

B.
9 - 14

C.
0 - 3

D.
3 - 8

This is involved and I could write a book on it. Tell me what you don't understand about the problem and we can go from there.

How would you approach this question in the first place? How would one think to get this answer, can someone tell me their thought process on this question im really confused.

To determine the appropriate pH range for each region of the titration curve, we need to consider the acid-base reactions that occur during the titration.

First, let's understand the reaction between ascorbic acid (H2A) and sodium hydroxide (NaOH):
H2A + NaOH -> NaA + H2O

In this reaction, one mole of ascorbic acid reacts with one mole of sodium hydroxide to form one mole of sodium ascorbate and one mole of water.

Now, let's analyze each region of the titration curve:

1. Initial pH: Before any NaOH is added, only ascorbic acid is present. Therefore, the initial pH will be determined by the concentration of ascorbic acid. Given that the pKa value of ascorbic acid is 7.9 x 10^(-5), the pH will be lower than 7. The appropriate pH range would be (0 - 3). Therefore, option C is correct.

2. Before equivalence point: As we gradually add NaOH, it will react with ascorbic acid converting it to sodium ascorbate. However, there will still be some ascorbic acid present in the solution. Therefore, the pH will be determined by the concentration of both ascorbic acid and sodium ascorbate. Since sodium ascorbate is the conjugate base of a weak acid, it will result in a basic solution. Thus, the pH range would be (8 - 9). Therefore, option A is correct.

3. At equivalence point: This is the point at which the amount of NaOH added is stoichiometrically equivalent to the amount of ascorbic acid initially present. At the equivalence point, all ascorbic acid has been converted to sodium ascorbate. However, sodium ascorbate is still a weak base, so the solution will be slightly basic. Thus, the pH range would be (8 - 9). Therefore, option A is correct.

4. After equivalence point: Beyond the equivalence point, the excess NaOH that has been added will determine the pH of the solution. Since NaOH is a strong base, it will result in a highly basic solution. Therefore, the pH range would be (9 - 14). Therefore, option B is correct.

In summary, the correct matching would be:
1. Initial pH: 0 - 3 (Option C)
2. Before equivalence point: 8 - 9 (Option A)
3. At equivalence point: 8 - 9 (Option A)
4. After equivalence point: 9 - 14 (Option B)