Question

50 POINTS!! 10. 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

Answer 1

5.1 would be the clostest answer

Answer 2

Answer : Option C) 5.1

If we consider any weak acid and strong base titration. We usually observe that the pH initially rises quite fast then remains constant during the buffer region before rising rapidly to the end point.

So, if 5 mL of NaOH is considered to be the one third of the way to the endpoint, then after the initial rise which will be more than 3.4 and reaching to the buffer region before rising to the endpoint.

Now, if endpoint has pH of 8.6 a buffer region around 5 is a good but the acid could be acetic acid which has similar pH endpoints with the pKa of 4.74 - so buffer region around 5 will be the good condition.

And hence pH of 5.1 is the most probably the right answer.