A bottle of acetic acid of unknown molarity is found in a chemical storeroom. The determination of t — Acid-Base Titration Chemistry Question
Problem Context
A bottle of acetic acid of unknown molarity is found in a chemical storeroom. The determination of the concentration of this acid has been assigned to you. You find some standardized sodium hydroxide solution, but there are no indicators in the storeroom. Just before giving up, you remember that you brought grape juice to drink with your lunch today. In addition to a number of other organic compounds, red and purple grapes contain multiple anthocyanins, naturally occurring compounds which can act as acid-base indicators.
Devise and carry out a procedure to determine the concentration of the acetic acid. You should keep detailed notes of your data and observations, and show all your calculations.
Give a brief description of your experimental plan.
Model Answer
a. Statement that this was a titration task.
b. Plan to use volumes of acetic acid solution, standardized NaOH solution, and grape juice indicator that seem reasonable for a titration analysis.
c. Specific equipment used is properly described (e.g.: a 10 mL graduated cylinder).
Record your data and observations.
Model Answer
a. The observation of the color of the grape juice in acidic and basic solution.
b. At least two titration trials performed, with the final result for the molarity of the acetic acid determined by averaging the results of multiple trials.
c. If drops were counted as part of the titration analysis, a description of how the drop count was converted into volume (if needed for the calculations) was provided.
d. A data table, rather than a series of sentences that included data, was created that was easy to read and included units.
e. All data required for the calculations was neatly and clearly provided. This likely included the volume of acetic acid solution used, the volume of NaOH solution used, the number of drops of grape juice used as indicator.
Show all calculations.
Model Answer
a. Student clearly indicates, either in words or by a balanced chemical equation, or as shown in the calculations, that there is a 1:1 mole:mole ratio between the moles of acetic acid and moles of sodium hydroxide reacting.
b. Calculation of acetic acid concentration from each trial and then averaging, OR calculation of concentration from average volume of titrant used. For example: MAVA = MBVB (since acetic acid and NaOH react in a 1:1 mole ratio)
MA = MBVB/VA (volumes need not be converted to L; just must both be expressed in the same unit, e.g., mL)
Alternatively, something similar to the following (based on avg. volume; will vary based on volume of acetic acid used):
8.4 mL NaOH x (1 L NaOH / 1000 mL NaOH) x 0.501 mol NaOH = 4.2 x 10 -3 mol NaOH
4.2 x 10 -3 mol NaOH x (1 mol HOAc / 1 mol NaOH) = 4.2 x 10 -3 mol HOAc (may not be shown explicitly)
4.2 x 10 -3 mol HOAc / 0.00500 L HOAc used = 0.84 M (steps may be shown & calc. as one sequence)
c. The calculated value for the molarity of acetic acid fell within an acceptable range that was established by the grading team.
d. The calculations were neatly done, easy to follow, and included appropriate units.
e. The student chose to include a statement or statements of likely sources of error in the titration work.
The concentration of the acetic acid = _________M.