You have been given a dilute solution of an unknown acid. Devise and carry out an experimental proce — Acids and Bases / Titration Chemistry Question

Equipment to be used in the experiment should be articulated. Strategy must indicate that the student understands that this is a titration experiment. Plan must state that increments of base will be added to a measured volume of acid and student will look for color changes.

Example response (volumes, etc. may vary; dilution not required): I will measure out 10 mL of the unknown acid and place it in the beaker. I will add 3-5 drops of Universal Indicator Solution and approximately 30 mL of distilled water. I will add NaOH solution 0.5 mL at a time, stir, and record the color (pH) of the solution after each addition. I will keep adding NaOH solution until all of the acid has reacted (the solution is strongly basic due to excess NaOH). I will repeat this titration, adding smaller amounts of NaOH at a time if it didn’t take very many additions to yield an excess of NaOH.

Students must report their data in the form of a legible data table. Each trial should indicate that a sufficient number of data points were collected (small increments of NaOH added). Correct color sequence must be evidenced. Data from two or more trials should be presented.

Example response (colors may vary due to different formulations of Universal Indicator Solution): The initial color of the mixture of unknown acid, distilled water and Universal Indicator Solution is red.

mL of NaOH added | Trial #1 Color and pH | mL of NaOH added | Trial #2 Color and pH
0.5 | The table should show all | 0.5 |
1.0 | volume additions & the | 1.0 | Smaller incremental
1.5 | color and pH after each, | 1.5 | volumes of NaOH may
2.0 | for at least 2 trials. | 2.0 | have been added during
2.5 | | 2.5 | the second trial.
3.0 | | 3.0 |
3.5 | | 3.5 |
4.0 | …and so on | 4.0 | …and so on

The acid that I was given is c. [3 points; 2 points if diprotic and data supports]

Graph or illustration must reflect the data that the student actually collected. Interpretation must show an understanding of titration chemistry. For full credit, the student must identify the equivalence points and explain the significance of the equivalence and half-equivalence points such that the claim of triprotic, diprotic or monoprotic was justified. [Graph not to scale; also, student graph would not be expected to be as neat as the example, but should be similar in form.]

A rough graph of my data shows that there are three regions where there is a significant (readily observable) increase in pH with the addition of a small amount of NaOH solution. I’ve labeled these equivalence points as EP₁, EP₂ and EP₃. Each represents the pH of this acid solution upon reaction with enough NaOH to remove one available proton. There are three equivalence points (although the third is not as distinct as the first two), which indicates that the unknown acid is triprotic. The EP/2 (half-equivalence points or midpoints) indicate where the pH = the pKa for the reaction of each of the three available protons in a triprotic acid with NaOH.