After a student determined the pK values of a H_3 PO_4 by titration, she realize

Question

After a student determined the pK values of a H_3 PO_4 by titration, she realized that she had recorded on her data sheet that the concentration of NaOH used in the titration was 0.25 M, but her partner had filled the buret with 0.20 M NaOH. Would you expect the calculated pK values to be correct or incorrect? Explain your reasoning. If you had made the same mistake as the student in question 1 above, what part of your laboratory report, if any, would you expect to be incorrect? Explain your reasoning. The indicator phenolphthalein was added to the phosphoric acid solution prior to the titration. A good indicator is one that causes a color change near the equivalence point. Was phenolphthalein a good choice in this case? Explain why or why not. The volumes between successive equivalence points in the titration of a polyprotic acid are theoretically equal. Consider again the titration of citric acid (H_3 C_6 H_5 O_7) as in question 1 of the Pre-laboratory Questions where a titration required 15.3 mL of 0.180 M NaOH to reach the first equivalence point. How many mL of this base would be required so that the titration solution was essentially a solution of sodium citrate (Na_3 C_6 H_5 O_7)?

Explanation / Answer

Q1.

Clearly, you should expect the pKa vlues to be incorrect, since the concentration of NaOH is used to relate to OH- moles

so

moles of OH- = mol of H+

Each point will have its stoichiometric ratio

so

final concentration of H+ = 1/3*concentration of acid

since pKa depends in concetnration, then pKa will be shifted

Q2.

expect the total amount of OH- moles to be incorrect

meaning that the volume used was different from the theoretical value if it was correct

Q3.

The indicator is fine for the 2nd ionization point, i.e. pKa = 7.2 approx

but for the third equivalence point, it will not work

it will also not work for the initial pKa value which is about pKa1 < pH < pKa2

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