Here is the gathered information to answer the post-lab questions:
- Two possible sources of error in the experiment:
- Inaccurate measurement of volume of titrant added (due to human error or limitations of burette).
- Contamination or improper cleaning of equipment affecting solution concentrations.
- Incorrect determination of endpoint due to subjective color change of indicator.
- Air bubbles in burette causing volume reading errors.
These errors affect the accuracy of titration results by distorting volume and concentration measurements.
- pH at equivalence point in acid-base neutralization:
- For strong acid-strong base titration, pH should be 7 (neutral).
- For strong acid-weak base, pH is less than 7 due to acidic conjugate ions.
- For weak acid-strong base, pH is greater than 7 due to basic conjugate ions.
- If the reaction in Part I involves a weak base or weak acid, pH at equivalence point will deviate from 7. This happens because the conjugate acid or base formed hydrolyzes to affect pH.
- Phosphoric acid has 3 ionizable hydrogens but only 2 equivalence points observed:
- The third equivalence point is often not observed because the third dissociation happens at a much higher pH, where the change is very gradual and difficult to distinguish.
- The difference between the second and third pKa values is small, so the titration curve does not show a distinct third endpoint [inferred chemistry knowledge].
I will now provide a detailed answer based on this information.1. Two possible sources of error in the experiment are:
- Inaccurate measurement of volume: This can result from limitations of the burette or human error while reading the volume, causing errors in determining how much titrant was added. Such inaccuracies affect the molarity calculations and final results.
- Endpoint determination errors: The endpoint is detected by a color change in an indicator, which can be subjective or gradual. If the endpoint is overshot or detected late, the titration results will be inaccurate. Other potential errors include contamination of solutions and air bubbles in the burette, which affect volumes and concentrations.
- The pH at the equivalence point depends on the strengths of the acid and base involved:
- If a strong acid reacts with a strong base, the equivalence point pH will be 7 (neutral).
- If a strong acid reacts with a weak base (or vice versa), the pH at equivalence will deviate from 7. For example, a strong acid with a weak base typically results in a pH less than 7 due to acidic conjugate ions formed.
- In the reaction from Part I, if the pH at equivalence is not 7, this means one of the reactants was a weak acid or weak base whose conjugate affects the pH.
- Phosphoric acid has three ionizable hydrogens but only two equivalence points were observed because:
- The third equivalence point corresponds to the third dissociation step, which has a pKa very different from the second; the change in pH is very gradual and subtle.
- This gradual change makes the third equivalence point difficult to observe distinctly during titration, so it often appears as if only two equivalence points occur [inferred chemical principles].
This explains the common experimental observations related to titration errors, pH at equivalence, and phosphoric acid titration behavior.
