Strong and Weak Acids and Bases
Information:
When an acid is placed in water, hydronium ions are produced according to:
HA (aq) + H2O  H3O + (aq) + A- (aq)
We write the equilibrium expression:
Kc = [H3O+] [A-]
[HA] [H2O]
In almost all cases, the [H2O] does not change upon reaction with the acid (it is really
large compared to the HA concentration) so it is incorporated into the value of Kc and
given a special name (another special K!) as Ka.
[H2O] = 1000g/L = 55 M
18g/mole
Kc x 55 = Ka = [H3O+] [A-]
[HA]
You’ll find a table of Ka values at the back of your book.
More information:
A strong acid is one that is 100% dissociated in water. In other words, if 1.0 mole of acid
is added to make a 1.0 L solution, the resulting [H3O+] is 1.0 M.
List the strong acids here: (check your book!) Memorize this list.
A weak acid is one that is significantly less than 100% dissociated in water. In weak
acid solutions made from 1.0 mole of acid to make 1.0 L of solution, the [H3O+] is much
less than 1.0 M. The relative strength of a weak acid can be determined by examining the
value of Kz. The larger the Ka, the stronger the weak acid and the higher the [H3O+].
Questions:
1. A solution of nitrous acid contains:
[HNO2] = 1.33 M
[H3O+] = 0.026 M
[NO2-] = 0.026 M
a). Write a chemical equation that shows the reaction of HNO2 with water. Label the
conjugate acid-base pairs.
b). Calculate Ka
c). Explain why [H3O+] = [NO2-] for this solution.
d. Calculate the pH of this solution.
2. Determine the following for a solution of 1.0 M HCl
a). [HCl] =
b). [H3O+] =
c). pH =
Applying ICE tables to weak acids:
1).Write the reaction equation for when HOCl is added to water.
2). Write the Ka expression.
3). Ka = 2.9 x 10-8 Complete the table for 0.30 M HOCl
HOCl
H3O+
OCl-
I
C
E
Find the pH of this solution.
Information
When a base B is added to water, hydroxide ions are produced:
B (aq) + H2O  BH+ (aq) + OH- (aq)
Based on what you know about Ka, write an expression for Kb for the above system.
Problem:
1). Determine the [OH-] in 1.0 NH3 (aq) using the fact that Kb 1.8 x 10-5.
a). Write the reaction equation for whn NH3 is added to water.
b). Write the Kb expression.
c). Construct and complete an ICE table.
More information:
Ka and Kb are related for conjugate acid base pairs.
Complete the table:
Acid
HF
Ka expression
Conjugate Base
HONO
NH3
1. How are Ka , Kb , and Kw related?
Kb expression
Ka x Kb
2. Determine the numerical value of Ka and Kb in the table above. Look up
the necessary values in your book.
3. Complete the table for conjugate acid-base pairs:
Acid
CH3COOH
H2CO3
HONO
NH4+
Ka
1.8 x 10-5
4.5 x 10-7
5.1 x 10-4
5.6 x 10-10
Base
Kb