Learning Strategies in Biochemistry: Water and Biomolecules
Essential Review or Remedial Work
From Your Full-Year General Chemistry Course
Review equilibrium contstant expressions, including Ka expressions for weak acids. Also
review pH, pKa, titration curves, and buffers.
From Your Full-Year Organic Chemistry Course
Review Lewis diagrams, formal charge, and resonance. If you are given a partial or
shorthand structure of an organic molecule, you should be able to complete the structure,
indicating the location of all multiple bonds and unshared electron pairs, and drawing
complete sets of resonance structures where applicable.
Essential Memory Work: Derive the Henderson-Hasselbalch Equation
Learn to derive the Henderson-Hasselbalch equation from the Ka expression for a
weak acid. Learning this derivation will help you to understand ionization and its
relation to pH. You must call on the Henderson-Hasselbalch equation repeatedly in
assessing the ionization state of complex molecules.
Estimating Percent Ionization
Once you have learned to derive the equation, solve it for the ratio ([A-]/[HA]),
and remember this formula or how to derive it. You will see from this relationship that
the unprotonated form of a weak acid/base pair predominates 10-fold for each unit by which
the pH exceeds the Pka (there's a mouthful!).Examples will help:
If the pKa of a weak acid HA is 5.0, then A- predominates over HA by 10-fold at pH 6.0 (one unit higher than the pKa, so 101-fold excess), 1000-fold at pH 8.0 (3 units higher, so 103-fold excess), and so forth. In like manner, the protonated form predominates by 10-fold at pH 4.0, and 1000-fold at pH 2.0. Knowing this rule of thumb makes it easy to estimate the relative amounts of both forms of a conjugate acid-base pair from the pH, if you know the pKa.
To figure out the ionization state and charge of any complex molecule at any pH, just
figure out the charge on the predominant form of each ionizable group at the pH of
interest (you'll need a table of pKa values), and then add up the charges to
get the net charge.
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