Here are information relevant to answer the post-lab questions:
- Two possible sources of error in the acid-base neutralization experiment include:
- Inaccurate measurement of the volume of titrant added (due to human error or burette limitations).
- Contamination of solutions or improper rinsing of apparatus leading to dilution or unexpected concentration changes.
- Other factors: air bubbles in burette, incorrect endpoint detection (due to subjective color change), and using solutions of incorrect concentration.
- The pH at the equivalence point depends on the strengths of the acid and base involved. For a strong acid-strong base neutralization, the pH at equivalence is 7 (neutral). For a weak acid-strong base or strong acid-weak base, the pH at equivalence is not 7 but slightly basic or acidic respectively. This is due to the nature of the salt and conjugate species formed, which can hydrolyze to affect pH.
- Phosphoric acid (H3PO4) has three ionizable hydrogens but only two equivalence points observed because the third hydrogen ionizes very weakly, or the third dissociation step occurs over a very long pH range and may not be detected under typical experimental conditions [user's question implies but needs confirmation].
I will now provide a detailed answer for the user based on this information.1. Two possible sources of error in the acid-base neutralization experiment are:
- Inaccurate measurement of the volume of titrant added, which can result from human error in reading the burette or from equipment limitations. This affects the accuracy of the volume used in calculations.
- Contamination or improper rinsing of apparatus, such as using distilled water instead of the solution to rinse burettes or pipettes, which can dilute solutions slightly and change concentrations. Other errors include air bubbles in the burette, incorrect endpoint detection due to subjective color changes of indicators, or using solutions of incorrect concentration.
- The pH at the equivalence point depends on the acid-base strengths involved:
- For a strong acid-strong base reaction, the pH at equivalence point is 7 (neutral) because the salt and water formed do not affect pH.
- If the acid is weak and the base is strong, the pH will be greater than 7 at the equivalence point due to hydrolysis of the conjugate base, making the solution slightly basic.
- If the acid is strong and the base weak, the pH will be less than 7 at the equivalence point because hydrolysis of the conjugate acid makes the solution slightly acidic.
- Although phosphoric acid has three ionizable hydrogens, only two equivalence points may be observed because:
- The third proton dissociates very weakly and over a broad pH range.
- This weak third dissociation may not be detectable with typical indicators or experimental conditions used in the experiment, causing the third equivalence point to be indistinct or not observed [user's question context].
This explains the common observations and typical sources of error in acid- base titration experiments.
