1. For each part, make a titration curve by plotting pH versus volume of NaOH de

Question

1. For each part, make a titration curve by plotting pH versus volume of NaOH delivered. See the examples in the lecture tect for the basic format. Include all three titration curves in your lab report.

2. For part 1, determine the pH at the equivalence point, according to the titration curve. Is this at the expected pH for a strong acid-strong base titration? What happens to the pH as the amount of base exceeds the amount of acid? Does the titration curve have the shape expected for a strong acid-strong base titration?

3. For Part 2, determine the pH at the equivalence point according to the titration curve. Does this match up with the equivalence point for the titration of acetic acid (a weak acid) with a strong base? Is the pH 7? Should the pH be 7? Why or why not?

4. For Part 2, also determine the half equivalence point. This is the pH when exactly half of the volume of base for the equivalence point is used. Look up the significance of the pH at this point and use it to determine the pKa and Ka of acetic acid. Do these values match the literature? Why or why not?

5. Discuss the shape of the titration curve in Part 2. Is the initial pH the same or different than in part 1? Why or why not? Is the curve more sloped before the equivalence point? Why or why not?

6. For Part 3, determine the pH at all three equivalence points. Does this match with values in the text? Phosphoric acid is one of the most commonly used buffer solutions. Look up the concept of the buffering range of a weak acid and discuss why phosphoric acid is so commonly used to make buffer solutions.   

Explanation / Answer

1. In a strong acid-strong base titration , equivalence point appears at 7. In the given plot, it has appeared at 6.

After the equivalence point when all the acid has been neutralize with base, the pH will gradually increase with more and more addition of NaOH. Yes, the titration curve does have the shape expected for a strong acid-strong base titration.

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2. The pH at the equivalence point is about 8. This is the expected value for a weak acid.

Consider the equation Ch3COOh + NaOH <==> CH3COONa + H2O

at equilibrium , the salt formed CH3COONa further reacts with water :

CH3COO- + H2O <==> Ch3COOH + OH-

Due to the presence of OH- , the pH will be slightly more than 7.

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At half equivalenc point pH = pKa

equivalence point at 22.5 mL . half equivalence point V = 11.25 mL

pH at this point = 4.8

pKa of acetic acid = 4.8

The value matches with the literature value of 4.74

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Initial pH of a weak acid will be less than what is observed for a strong acid. This is due to the fact that, a strong acid is completely ionized in solution. Compared to that a weak acid is only partyly ionized. Thats why ph is higher.

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pH at the equivalence point are 5 and 9.5

Buffer is made up of a weak acid and its salt. In the graph, buffer region is the flat region just before the equivalence point.

Phosphoric acid is mostly used to prepare buffer as it has three acidic proton and depending on which proton u are removing , it can work in various buffering region.

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