CHEM1002: Worksheet 8: Introduction to Acids and Bases
Model 1: pH
Water is able to act as both an acid and a base and it is possible for water to react with itself in an acid-base
reaction called the autoprotolysis or autoionization of water:
H2O(l) + H2O(l) H3O+(aq) + OH-(aq)
The equilibrium constant for this reaction Kw = [H3O+(aq)][OH-(aq)]. At 25 °C, Kw = 1.0 × 10-14. Several
definitions have proven to be useful:
pH = -log10[H3O+(aq)], pOH = -log10[OH-(aq)] pKw = -log10Kw
Critical thinking questions
1. During the course of a titration, a student measures the pH several times. What is [H3O+(aq)] for each
pH value below? Use scientific notation in your answers.
pH#
0.50#
1.50#
2.50#
3.50#
4.50#
5.50#
5.75#
[H3O+(aq)]#
#
#
#
#
#
#
#
2. Look at the numbers you calculated in Q1. What do you notice about how the part of the number before
and after the decimal point in pH affects the value of [H3O+(aq)]?
3. What is the numerical value of pKw at 25 °C?
Model 2: Strong and Weak Acids
A strong acid is one that is essentially 100% dissociated in water: if 0.1 mole of the acid is added to enough
water to make a 1.0 L solution, the solution will have [H3O+(aq)] = 0.1 M and will be pH = 1.
A weak acid is one that is significantly less than 100% dissociated in water: if 0.1 mole of the acid is added to
enough water to make a 1.0 L solution, the solution will have [H3O+(aq)] < 0.1 M and will be pH > 1.
When an acid HA is placed in water, H3O+(aq) ions are produced according to the reaction:
HA(aq) + H2O(l) H3O+(aq) + A−(aq)
Ka =
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