Core Practical 9: Determine the Ka for a weak acid

Overview:

Procedure:

Reminder:

• when using a burette, volumes should be recorded to 2 decimal places with the second figure being 0 or 5 only

• titrations should always be repeated until concordant results are obtained

• the K_a value is an indication of acid strength. The larger the value of the K_a, the stronger the acid

• the K_a of a weak acid can be measured be titrating a known volume of the acid against sodium hydroxide using phenolpthalein as an indicator. A further equal volume of acid is then added, and the pH of the resulting solution is measured. Because effectively half of the acid has been titrated:

• [Ha]=[A^-] so [A^-] and [HA] can be cancelled in the K_a expression

• so K_a = [H⁺]

• the pH value of the combined solutions can be converted to [H⁺] to give a K_a value

Use the pH of your solution to calculate [H⁺]

10^-4.66=[H⁺]

[H⁺]=2.19×10^-5 moldm^-3

Calculate the value K_a for the ethanoic acid

At half equivalence:

pH = pK_a

4.66=pK_a

K_a=2.19×10^-5moldm^-3

What are some sources of uncertainty in this experiment? What can you do to overcome them?

• it is difficult to determine the end point of the titration

• → use white tile to see the colour change clearly

• inaccuracies when reading burette volumes

• → ensure that you always read from the bottom of the meniscus

The following graph is obtained for the same reaction, measuring the pH after 1cm³ additions of the alkali. Show by annotation and explanation how this graph can also be used to determine the K_a of the weak acid.