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?

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...e ionizable protons.) While the problem does not specifically ask you to write a chemical equation, it is a good idea to write the chemical equation when you are starting to do buffer calculations. Writing the chemical equation helps you understand what is happening.

The pKa for ascorbic acid is a characteristic of the following equilibrium reaction:
Ascorbic acid <-> ascorbate(-) + H(+)

Now, parts (a) through (e) of the problem ask for the pH of the solution where the ratio of unprotonated to protonated forms of ascorbic acid is a certain value. Which form is unprotonated? The weak base (A-).
Which form is protonated? The weak acid (HA).

(a) pH when unprotonated:protonated = 1:1 ? (or A-:HA = 1:1)

pH = pKa + log ([A-]/[HA])
pH = 4.2 + log (1:1)
pH = 4.2

The above calculation demonstrates an important principle. Simply stated, when the pH of a buffer is equal to the pKa of the weak acid, the ratio of weak acid to weak base is 1:1. In addition, ...