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Chapter 14 | Acid-Base Equilibria 751
Example 14.10
Determination of Kb from Equilibrium Concentrations
Caffeine, C8H10N4O2 is a weak base. What is the value of Kb for caffeine if a solution at equilibrium has
[C8H10N4O2] = 0.050 M, = 5.0 10−3 M, and [OH−] = 2.5 10−3 M?
Solution
At equilibrium, the value of the equilibrium constant is equal to the reaction quotient for the reaction:
What is the equilibrium constant for the ionization of the ion, a weak base:
In a solution containing a mixture of NaH2PO4 and Na2HPO4 at equilibrium, [OH−] = 1.3 10−6 M; and
Answer: Kb for
Check Your Learning
Example 14.11
Determination of Ka or Kb from pH
The pH of a 0.0516-M solution of nitrous acid, HNO2, is 2.34. What is its Ka?
Solution
We determine an equilibrium constant starting with the initial concentrations of HNO2, and as well as one of the final concentrations, the concentration of hydronium ion at equilibrium.
(Remember that pH is simply another way to express the concentration of hydronium ion.)
We can solve this problem with the following steps in which x is a change in concentration of a species in the reaction:
