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Understanding buffers and weak acids.

Ascorbic acid (vitamin C) has a pKa of 4.2 at 24 degrees Celsius. State the pH of the solution when the ratio of unprotonated to protonated forms of ascorbic acid is
(a) 1:1
(b) 1:10
(c) 10:1
(d) 1:3
(e) Which form of ascorbic acid predominates at physiological pH?

Solution Preview

Ascorbic acid is a weak acid, and therefore, can serve as a buffer. When studying buffers/buffering systems, it is critical that you memorize AND understand the Henderson-Hasselbalch equation. The Henderson-Hasselbalch equation (HH) is an equation that describes the pH of a solution in terms of the pKa of a weak acid and the concentrations of the weak acid and its conjugate weak base. The HH equation is shown below.

pH = pKa + log ([A-]/[HA]), where
[A-] = the concentration of weak base (ascorbate in this case)
[HA] = the concentration of weak acid (ascorbic acid in this case)

Now that we are armed with the HH equation, we can proceed to answer all of the questions posed in the problem.

First, consider ascorbic acid. The problem states that ascorbic acid has a pKa of 4.2. This information tells us that (1) ascorbic acid has one ionizable proton (Each ionizable proton has its own pKa value. So, if there were 3 pKa values given, the weak acid would have ...