Consider the titration of 100.0 mL of 0.10 M phosphoric acid with 0.10 M NaOH.
a. Determine the pH at the third half- equivalence point by assuming it is a €œspecial point€ (see Fig.).
b. Calculate the pH at the third equivalence point.
c. Why must the answer to part a be incorrect? Why can€™t we use the €œspecial point€ on the graph? (Explain the assumption made in using the special point and why it is not valid in this case.)
d. Calculate the pH at the third half-equivalence point.
Figure

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Excess OH HA At H2A HA2. pH Use Ppk HA Use M HjA HA Use Ka Volume of NaOH