Core Practical 9 Post Lab

• Use logarithms in relation to quantities that range over several orders of magnitude.

• Change the subject of an equation.

• Substitute numerical values into algebraic equations using appropriate units for physical quantities.

CPAC 4b – Obtains accurate, precise and sufficient data for experimental and investigative procedures and records this methodically using appropriate units and conventions.

· Record information in a table’s or appropriately

· Titre value is recorded to 0.05cm³

· pH is recorded to 2dp's

· [H+] ions is calculated to at least 4 sig fig

· Ka is calculated to 3 sig fig

The ionisation of an acid is shown by:

HA(aq) ⇌ H+ (aq) + A– (aq)

Because there is an equilibrium set up, an equilibrium constant, Ka, can be written: Ka = – + [H ][A ] / [HA]

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

The Ka of a weak acid can be measured by titrating a known volume of the acid against sodium hydroxide using phenolphthalein 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:

[H+ ] = [HA] = [A– ]

[A– ] and [HA] can be cancelled in the Ka expression

And so Ka = [H+ ]

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

CPAC 5b – Sources of information are cited demonstrating that research has taken place, supporting planning and conclusions

· Clearly identifies any material that is a direct quote as such.

· Use research to discuss how their results compare to published data:

· Use research to support any conclusions drawn

· Compiles a bibliography of texts from which material has been sourced in such a way that the reader may retrieve the information easily.