The question
Name: ______________________ Date: ____________ Identifying an unknown weak acid by titration (10 points)
Graph
Titration of 25.00 mL of an unknown monoprotic acid with 0.150 M NaOH
A student pipets 25.00 mL of a solution of an unknown monoprotic weak acid, HA, into a beaker and titrates it with 0.150 M NaOH from a buret, measuring the pH with a calibrated pH meter. The data are plotted below. The steep rise is centered at 18.00 mL of NaOH, and the pH at 9.00 mL is 4.20.
Indicators available: methyl red (pKa 5.0), bromothymol blue (pKa 7.1), thymol blue (pKa 8.9).
Data table
| Volume of NaOH added (mL) | pH |
|---|---|
| 0 | 2.59 |
| 3 | 3.51 |
| 6 | 3.9 |
| 9 | 4.2 |
| 12 | 4.5 |
| 15 | 4.9 |
| 16 | 5.1 |
| 17 | 5.43 |
| 17.5 | 5.74 |
| 18 | 8.5 |
| 18.5 | 11.24 |
| 19 | 11.53 |
| 20 | 11.82 |
| 22 | 12.11 |
| 24 | 12.26 |
(a) Calculate the concentration of HA in the original 25.00 mL sample. [1 point]
(b) Determine the value of Ka for HA. Justify your choice of point on the curve. [2 points]
(c) Calculate the pH of the original HA solution, before any NaOH is added. Use your values from parts (a) and (b). [1 point]
(d) Draw a box to represent a small volume of the solution in the beaker after 9.00 mL of NaOH has been added, and put 4 particles of HA in it. Draw the other solute particles that should be present in the same volume. Represent the conjugate base as A⁻ and sodium ions as Na⁺; do not draw water. [1 point]
(e) Calculate the pH at the equivalence point. (Use Ka = 6.3 × 10⁻⁵.) [2 points]
(f) Identify the best indicator for this titration from the list. Justify your choice. [1 point]
(g) Between 3 mL and 15 mL of NaOH, the pH changes by less than 1.5 units. Explain why, in terms of the species present. [1 point]
(h) A second student does not rinse the buret with the NaOH solution after rinsing it with distilled water. Would this student's calculated concentration of HA be greater than, less than, or equal to the value in part (a)? Justify your answer. [1 point]