Buffers
What is the pH of a buffer solution containing equimolar quantities of
(a) H _{3} PO _{4} \text { and } NaH _{2} PO _{4}?
(b) H _{2} CO _{3} \text { and } NaHCO _{3}?
Strategy
When there are equimolar quantities of a weak acid and its conjugate base in a buffer solution, the pH is always the same as the p K_{ a } of the weak acid. Look up the p K_{ a } of the weak acid. Look up the p K_{ a } of the weak acid in Table 8.3.
| TABLE 8.3 K_{ s } \text { and } p K_{ a } Values for Some Weak Acids | |||
| Formula | Name | K_{a} | p K _{a} |
| H _{3} PO _{4} | Phosphoric acid | 7.5 \times 10^{-3} | 2.12 |
| HCOOH | Formic acid | 1.8 \times 10^{-4} | 3.75 |
| CH _{3} CH ( OH ) COOH | Lactic acid | 1.4 \times 10^{-4} | 3.86 |
| CH _{3} COOH | Acetic acid | 1.8 \times 10^{-5} | 4.75 |
| H _{2} CO _{3} | Carbonic acid | 42 \times 1 n^{-7} | 6.37 |
| H _{2} PO _{4}^{-} | Dihydrogen phosphate ion | 6.2 \times 10^{-8} | 7.21 |
| H _{3} BO _{3} | Boric acid | 7.3 \times 10^{-10} | 9.14 |
| NH _{4}^{+} | Ammonium ion | 5.6 \times 10^{-10} | 9.25 |
| HCN | Hydrocyanic acid | 4.9 \times 10^{-10} | 9.31 |
| C _{6} H _{5} OH | Phenol | 1.3 \times 10^{-10} | 9.89 |
| HCO _{3}^{-} | Bicarbonate ion | 5.6 \times 10^{-11} | 10.25 |
| HPO _{4}^{2-} | Hydrogen phosphate ion | 2.2 \times 10^{-13} | 12.66 |