concentrations of pure solids. As another example, K for the reaction
NH 4 Cl(s) NH 3 (g) + HCl(g)is
K = [NH 3 ][HCl]Acid Ionization Constants
When a weak acid does not ionize completely in a solution, an equilibrium is
reached between the acid molecule and its ions. The mass action expression can
be used to derive an equilibrium constant, called the acid dissociation constant,
for this condition. For example, an acetic acid solution ionizing is shown as
TIP
Ka incorporates the concentration of water.The concentration of water in moles/liter is found by dividing the mass of 1
liter of water (which is 1,000 g at 4°C) by its gram-molecular mass, 18 grams,
giving H 2 O a value of 55.6 moles/liter. Because this number is so large compared
with the other numbers involved in the equilibrium constant, it is practically
constant and is incorporated into a new equilibrium constant, designated as Ka.
The new expression is
Ionization constants have been found experimentally for many substances and
are listed in chemical tables. The ionization constants of ammonia and acetic acid
are about 1.8 × 10−5. For boric acid Ka = 5.8 × 10−10, and for carbonic acid Ka =
4.3 × 10−7.
If the concentrations of the ions present in the solution of a weak electrolyte
are known, the value of the ionization constant can be calculated. Also, if the
value of Ka is known, the concentrations of the ions can be calculated.
A small value for Ka means that the concentration of the un-ionized moleculemust be relatively large compared with the ion concentrations. Conversely, a