Bio Chem

The pKa of propanoic acid (propionic acid), CH3CH2COOH, is 4.87. Consider an equilibrium mixture of propanoic acid and its conjugate base with a pH of 4.87. Draw the structure of the form (acid or conjugate base) that predominates after a decrease in [H3O ]. Include all hydrogen atoms and any appropriate formal charges.

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  1. pH = pKa + log (base)/(acid)
    pH = 4.87
    pKa = 4.87
    Therefore,
    4.87 = 4.87 + log (base)/(acid)
    and (base)/(acid) = 1 which means
    (base) = (acid)

    If you lower the (H3O^+), that raises the pH so let's substitute a higher number for pH, say 5.87 and see what happens.
    5.87 = 4.87 + log (base)/(acid)
    1 = log (base)/(acid) and
    (base)/(acid) = 10 or
    (base) = 10(acid)
    which means base predominates by a factor of 10 over the acid.
    The base is CH3CH2COO^-. You must draw the structures and any charges since we can't do that on this forum.

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