SCHE 2060: Principles of Organic Chemistry
Vermont Tech
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CHE 2060: Guide to using the Henderson-Hasselbalch equation
Biological organism and solutions are buffered, meaning that they are able to maintain a
stable pH when challenged by addition of acid or base. When added to unbuffered solutions,
acid would cause the pH to drop while base would cause the pH to increase. Buffered
solutions contain a weak acid and a salt of that weak acid. The acid provides protons to
counter any base added to the buffered solution. The salt of the weak acid is a metal cation
ionically bound to the anion of the weak salt and that anion counters (or absorbs) the
protons of any acid added to the buffered solution. Weak acids create good buffers for pH
values close to their pKa values.
The Henderson-Hasselbalch equation describes the relationship between:
• The pH of the buffered solution;
• The pKa value of the weak acid; and
• The ratio of the weak acid to its conjugate base (aka its salt).
pH = pKa + log [conjugate base]
[weak acid]
The Henderson-Hasselbalch equation can be rearranged to solve for any of its components.
Here are a few reminders or pointers.
• To solve for the log of a ratio, take the anti-log of both sides of the equation.
• To take an antilog use the appropriate button on your calculator or raise 10 to the
power of the value to calculate that value’s antilog.
Functional groups are the parts of organic molecules that do work by participating in
chemical reactions. For many functional groups, their protonation – and thus reactivity – is
dependent on the pH of their environment, or solution. For example, carboxylic acid groups
are protonated and act as acids only at pH values below their pKa of 4.7ish. Far above that pH
carboxylic acids become deprotonated carboxylate ions and act as bases.
• When the pH of a functional group’s environment is close to that group’s pKa roughly
half of the functional groups are protonated, and half are unprotonated.
See examples and solutions on the next page……………
