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Acids and Bases 225
Quick Tip
Quick Tip
The [HA] is the equilibrium molar concentration of the undissociated weak acid, not
its initial concentration. The exact expression would then be [HA] Minitially
[H ], where Minitially is the initial concentration of the weak acid. This is true
because for every H that is formed an HA must have dissociated. However,
many times if the Ka is small, you can approximate the equilibrium concentration
of the weak acid by its initial concentration, [HA] < Minitially. If we know the initial molarity and K a of the weak acid, we can easily calculate
the [H ] or [A ]. If we know the initial molarity and [H ], it is possible to calculate
the K a.
Different sources may give different values for K a, always use the values given to
you by your instructor or textbook.
For polyprotic acids, acids that can donate more than one proton, the K a for the
first dissociation is much larger than the K a for the second dissociation. If there
is a third K a, it is much smaller still. For most practical purposes, you can simply
use the first K a.
K b—the Base Dissociation Constant
Weak bases (B), when placed into water, also establish an equilibrium system
much like weak acids:
B(aq) H 2O(l) K HB (aq) OH (aq)
The equilibrium constant expression is the weak base dissociation constant, K b,
and has the form:
K b [HB ][OH ]
[HB]