The pH of acids at different concentrations
Aims
1
2
To compare a strong and a weak acid
To examine the effect of dilution on a weak acid
Introduction
Hydrochloric acid is a strong acid which ionises completely in water:
HCl(g)+ H2O(l)
+
H3O
(aq)
+ Cl
--
(aq)
Ethanoic acid is a weak acid which ionises only partially in water:
CH3COOH(l) +
H2O(l) =
H3O+(aq) +
CH3COO-(aq)
The extent of ionisation depends on the initial concentration of the ethanoic acid. By measuring
the pH at different concentrations, you can see the effect of dilution. These results can be
generalised for any weak acid.
Requirements
pH meter (already calibrated), distilled water, 2 x 10 cm3 pipettes, pipette filler, 2 x 100 cm3
volumetric flasks, 8 x 100 cm3 beakers, 0.10M hydrochloric acid, 0.10M ethanoic acid
Procedure
1 Pipette 10 cm3 of 0.10M hydrochloric acid into a 100 cm3 volumetric flask. Make up to
the mark with distilled water, insert the stopper and shake well for at least a minute.
Transfer this solution into a clean, labelled beaker. This solution has a concentration of
0.010M. In a similar way, use the method of successive dilutions to prepare 0.0010M
and 0.00010M solutions of hydrochloric acid. Starting with the least concentrated
solution, find the pH of each solution using a pH meter.
2 Repeat step 1 starting with 0.10M ethanoic acid.
3 Calculate the expected pH values of each solution of hydrochloric acid
Results
Concentration of
acid/mol dm-3
Calculated pH of
solutions of
hydrochloric acid
Measured pH of
solutions of
hydrochloric acid
Measured pH of
solutions of ethanoic
acid
0.00010
0.0010
0.010
0.10
Questions
1 Compare the pH of ethanoic acid with hydrochloric acid at each concentration.
(i)
In which of the two acids is the concentration of hydrogen ions greater?
(ii)
What does this tell you about the extent of ionisation of ethanoic acid
compared with hydrochloric acid?
2 (i) What happens to the difference between the pH of the two acids as concentration
decreases? What does this tell you about the effect of dilution on ionisation?
(ii) Use Le Chatelier’s Principle to explain the effect of dilution on the extent of ionisation of
ethanoic acid.
MARKING
Results table completed (1)
pH decreases as acid concentration decreases (1)
Q1 (i) [H+] greater in hydrochloric acid[1]
(ii) Extent of ionization less in ethanoic acid [1]
Q2 (i) pH decreases as acid concentration decreased [1]
Degree of ionization of ethanoic acid increases on dilution
[1]
(ii) CH3COOH
+
H2O
=>
CH3COO+
H3O+
+
Ka = [CH3COO ][ H3O ]/[ CH3COOH]
Addition of water causes [CH3COO-], [ H3O+] and [ CH3COOH] to all decrease. Because
[CH3COO-] and [ H3O+] are on the rhs and only [CH3COOH] is on the lhs, a shift to the right
goes further towards restoring the original concentrations than a shift to the left. [4]
Total 10 marks