U2 S2 L4
Ka and Kb calculations
p 587-591: pH and Ka of a Weak Acid
p 592-594: The Base Dissociation Constant, Kb
Homework:
p 591: # 5, 6, 7, 8, 9 and 10
p 595: # 11, 12, 13, 14, 15 and 16
p 597: # 1, 2, 3, 4, 5 and 6
p 617: # 7, 8, 10 and 11
Upon completion of this lesson, you should be able to:
• calculate the value of one of Ka (or Kb) or an equilibrium
concentration given all other values for a system.
• calculate pH (or pOH) given the initial concentration of a weak
acid (or weak base) and Ka (or Kb).
• use the quadratic equation to complete Ka or Kb calculations
Using percent ionization
• Percent ionization data can only be used for
acid concentrations of 0.10 mol/L (see note on
bottom of table)
Calculate the pH of a 0.10 M solution of
methanoic acid.
[H3O ]
% ionization 
x 100%
[HA]
[H3O  ] 
%I
x [HA]
100
Finding % ionization:
•
A 0.15 M boric acid solution has a pH of 5.00, calculate the percent
ionization of boric acid.
Calculating K
• A cleaning solution is prepared by dissolving ammonia in water. The
equilibrium concentrations of ammonia and hydroxide found to be 0.35
mol/L and 0.0025 mol/L respectively. Calculate Kb
Calculation hydronium or hydroxide using K
• Propanoic acid solution has Ka = 1.3 x 10-5 and an equilibrium concentration
of 0.050 mol/L. Calculate the concentration of hydronium ions and pH at
equilibrium.
ICE tables (brrrrrr…!)
• A nitrous acid solution has an initial concentration of 0.25
mol/L. At equilibrium the hydronium concentration is 0.013
mol/L. Calculate Ka.
Calculation Ka from pH
• An aqueous methanoic acid solution has an initial molar
concentration of 0.75 M. The pH of the solution at
equilibrium is 1.95. Calculate the Ka of a methanoic acid.
• An aqueous 0.50 mol/L sodium sulfite solution is prepared. The pH at
equilibrium is 10.45. Calculate the Kb for the sulfite ion.
Quadratic equation.
• A 0.022 mol/L aspirin (acetylsalicylic acid) solution is made by dissolving
a 500.0 mg tablet in about 125 mL of water. A condensed formula for
aspirin is C8H7O2COOH. Given Ka for aspirin is 3.3 x 10-4, calculate the
pH of the aspirin solution
• Calculate the pH of a 0.50 mol/L ethanoic acid solution