Full Length 5 CP Q15

NS_Tutor_Sophia
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Joined: Tue Aug 01, 2017 11:49 am

Full Length 5 CP Q15

A student recently asked us why the correct answer should have approximately equal proportions of a weak acid and its conjugate base for the following question: "Which of the following solutions would create an effective buffer?"

The answer to this question is that a buffer is more effective (has more "buffer room") when a weak acid and its conjugate base, or a weak base and its conjugate base, are present in approximately equal proportions. This is so that the capacity to buffer against added acid or acid base is approximately equal. For example, if base is added to a buffer solution, we want there to be a large quantity of weak acid present to "soak up" and neutralize the added base. Likewise, if acid is added to the solution, we want there to be a large quantity of conjugate base present to neutralize the added acid.

More technically, note the Henderson-Hasselbalch equation: pH = pKa + log [A-]/[HA]. The closer the ratio of [A-]/[HA] is to 1, the closer the pKa will be to the pH of the solution. For an effective buffer, the weak acid-conjugate base pair should have a pKa closest to the desired pH of the solution, and thus the ratio of weak acid : conjugate base should be closest to 1.
Sophia Stone
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