4.3: Titration of a Diprotic Acid - Data and Report
- Page ID
- 544322
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Name: ____________________ Partner(s): ____________________ Date: ____________________
DATA AND OBSERVATIONS
Drop counter calibration data: ______________________ mL/drop
| First Titration | Second Titration | |||
|---|---|---|---|---|
| Buret reading (mL) | pH | Buret reading (mL) | pH | |
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POST-LAB QUESTIONS
- For each titration, create a titration curve using a spreadsheet or data analysis software. Insert your graphs here and comment on the overall appearance of the graphs. What major features are there? How repeatable was the experiment?
- Estimate the equivalence points, \( V_{\text{eq}}^{(1)} \) and \( V_{\text{eq}}^{(2)} \), from the titration curves. These are most easily estimated by looking for the largest slope of the titration curve. From these, calculate the half-equivalence points, \( V_{1/2}^{(1)} \) and \( V_{1/2}^{(2)} \), estimate the pH values (=pKa1 and pKa2) at the half-equivalence points, and calculate Ka1 and Ka2 for each curve. Enter all of these values into the Results Table below.
| Calculation |
Trial 1 |
Trial 2 |
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|---|---|---|---|
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Volume at the first equivalence point [mL] |
\( V_{\text{eq}}^{(1)} \) |
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Volume at the first half-equivalence point [mL] |
\( V_{1/2}^{(1)} = \frac{1}{2}V_{\text{eq}}^{(1)}\) |
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| pKa1 | pKa1 = pH @ \( V_{1/2}^{(1)}\) |
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| Ka1 | Ka1 = 10-pKa1 |
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Volume at the second equivalence point [mL] |
\( V_{\text{eq}}^{(2)} \) |
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Volume at the second half-equivalence point [mL] |
\( V_{1/2}^{(2)} = \frac{V_{\text{eq}}^{(1)}+V_{\text{eq}}^{(2)}}{2}\) |
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| pKa2 | pKa2 = pH @ \( V_{1/2}^{(2)}\) |
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| Ka2 | Ka2 = 10-pKa2 |
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Calculate the concentration of the unknown acid from the equivalence volumes. You will get four values: one from the first and second equivalence points on each graph. Report the individual values and the average, and comment on the precision. Show your calculations.
You calculated the concentration of acid four different ways (using \(V_{\text{eq1}}\) and \(V_{\text{eq2}}\) for two trials). Theoretically, all four should be identical.
- The Check: Calculate the Percent Difference between the concentration derived from \(V_{\text{eq1}}\) and the concentration derived from \(V_{\text{eq2}}\).
- Analysis: If the difference is > 5%, which equivalence point do you trust more? (Usually \(V_{\text{eq2}}\) is steeper and easier to read). Justify your choice in your conclusion.
- From the estimate of the value of pKa1 and pKa2 from each of the graphs, use a table of acid ionization constants to propose a candidate for the identity of the unknown acid.