Unit 8: Topic 10 - Buffer Capacity Flashcards
A buffer made with 1 mole NH3 / 1 mole NH4+ has the same pH as a buffer made with 2 mole NH3 / 2 mole NH4+, however, why does the latter buffer have a higher buffering capacity?
Consider adding 0.2 moles of strong acid to each buffer solution, and assume 1L of water. The conjugate base in the buffer will react with the strong acid. In the first solution, 0.2 moles of base is reacted and 0.2 moles of acid is formed, so the new [A-]/[HA] ratio is 0.8/1.2, whereas the new ratio in the second buffer solution is 1.8/2.2. Since the magnitude of log(0.8/1.2) is greater than that of log(1.8/2.2), the pH of the second buffer solution is more resistant than the pH of the first solution. The ratio changes slightly less if there are more components of both acid and base.
Consider two buffer solutions: one with 1mol HA and 2mol A-, and one with 2mol HA and 1mol A-. Why does the first solution have a greater capacity for added acid?
Consider a buffer solution to be depleted if at least one of the weak acid or weak base is fully depleted. In the first solution, there is 2 moles of conjugate base. This requires 2 moles of acid to fully deplete the base. However, in the second solution, with only 1 mole of conjugate base present, only 1 mole of acid is required to deplete the base. Since there is more base in the first buffer solution, it has a higher capacity to neutralize added acid, since more acid is needed to fully kill the buffer.
