Acids & Bases Problem Set

Question 11: pH and buffering capacity of a mixed solution

If equal volumes of 0.05 M NaH₂PO₄ and 0.05 M H₃PO₄ are mixed, which of the following best describes the resulting solution? (pKa's for phosphoric acid are 2.0, 6.8 and 12.0)

A.	pH 2 and poorly buffered.
B.	pH 2 and well buffered. Equal amounts of phosphoric acid (H3PO4) and monosodium phosphate (NaH2PO4) will be present at a pH of 2.0, which is the pKa for phosphoric acid. Because both components of the mixture have 50mM phosphate and the solution is poised at the pKa of the first ionization, the solution will be able to absorb as much as 25mM of either acid or base before its buffering capacity is exhausted. The solution is thus judged to be "well buffered."
C.	pH 6.8 and well buffered.
D.	pH 12 and well buffered.
E.	pH 6.8 and poorly buffered.