Question #c1b5b

1 Answer
Nov 21, 2016

The pH of a buffer solution depends on the ratio of concentrations of an acid and its conjugate base, using the Henderson-Hasselbalch equation.
pH=pK_a-log_10(([HA])/([A^-]))

Explanation:

In this example, the acid is NH_4^+, which has a pK_a=9.24

Total volume of solution = 30mL + 15 mL = 45 mL = 0.45 L

Acid concentration: (0.1M times 0.015L)/(0.045L) = 0.033M

Base concentration: (0.2M times 0.030L)/(0.045L) = 0.133M

pH=pK_a-log_10(([HA])/([A^-]))=9.24-log_10(0.033/0.133)=9.84

Sanity Check:
If the acid and base concentrations were equal, the pH would be equal to the pK_a, but here the base concentration is 4X the acid concentration, so the solution is a little bit more basic and the pH is a little bit higher than pK_a.