A weak acid with an initial pH of 3.2 was titrated with a strong base. 15 mL of 0.1 M NaOH was added to the acid to reach the equivalence point at a pH of 8.6. What would you expect the approximate pH of the analyte to be after the first 5 mL of 0.1 M NaOH was added?

Question

A weak acid with an initial pH of 3.2 was titrated with a strong base. 15 mL of 0.1 M NaOH was added to the acid to reach the equivalence point at a pH of 8.6. What would you expect the approximate pH of the analyte to be after the first 5 mL of 0.1 M NaOH was added?

8.6

3.4

5.1

7.2

I would choose 5.1. If one third of the base is added (in which the whole 15ml was utilized to bring to a basic state of 8.6), it must be 5.1, as the other answers are mathematically impossible. Is this correct??

Answer 1

The answer is 4.1

Answer 2

Your explanation is incorrect. Let me guide you through the correct way to approach this question.

To determine the approximate pH of the analyte after the first 5 mL of 0.1 M NaOH was added, we need to understand the concept of titration and the calculations involved.

During titration, a strong base (NaOH) is added to a weak acid until the equivalence point is reached. At this point, the moles of acid and base are in a 1:1 ratio, resulting in a neutral solution.

The initial pH of the weak acid is 3.2, which indicates it is acidic. As the strong base is added, it will react with the weak acid to form a salt and water. The pH will increase as the solution becomes more basic.

Given that the equivalence point is reached at a pH of 8.6 when 15 mL of 0.1 M NaOH is added, we can calculate the number of moles of NaOH added.

moles NaOH = volume (in L) * concentration (in mol/L)
moles NaOH = 0.015 L * 0.1 mol/L
moles NaOH = 0.0015 mol

Since the moles of acid and base are in a 1:1 ratio, we have also added 0.0015 mol of weak acid.

Now, to approximate the pH after the first 5 mL (0.005 L) of NaOH is added, we need to determine the remaining moles of weak acid.

moles weak acid remaining = initial moles - moles NaOH added
moles weak acid remaining = 0.0015 mol - 0.0015 mol
moles weak acid remaining = 0 mol

Since all the weak acid has reacted with the NaOH, we are left with a solution of the salt formed and excess base. This solution will have a pH greater than 7 but less than 8.6, indicating it is still basic.

Therefore, the correct answer is 7.2, as it falls within this range and is the nearest option to the expected pH after the first 5 mL of NaOH was added.