Preparing salts: making a soluble salt from an insoluble base, the titration method for soluble salts of soluble bases, preparing insoluble salts by precipitation, and the two core practicals.

What this dot point is asking

Edexcel wants you to prepare a pure dry sample of a soluble salt from an acid and an insoluble base or carbonate (the copper sulfate core practical), prepare a soluble salt of a soluble base by titration, predict precipitates using solubility rules, and prepare an insoluble salt by precipitation. The methods are tested as ordered steps and as six-mark extended responses.

Soluble salt from an insoluble base (core practical)

When the base is insoluble (a metal oxide, hydroxide or carbonate that does not dissolve), you can add it in excess because the leftover solid is easy to filter off.

The core practical prepares pure dry copper sulfate from copper oxide and sulfuric acid:

1. Warm the dilute sulfuric acid (this speeds up the reaction).
2. Add copper oxide in excess, stirring, until some remains unreacted. This guarantees all the acid has reacted, so the solution is not acidic.
3. Filter to remove the excess copper oxide; the blue copper sulfate solution is the filtrate.
4. Evaporate some of the water by gentle heating until the solution is saturated (the point of crystallisation).
5. Cool slowly so that crystals form, then remove and dry the crystals.

Soluble salt from a soluble base (titration)

When the base is a soluble alkali (such as sodium hydroxide), you cannot use excess because you could not filter it off. Instead use a titration to add exactly the right amount of acid.

1. Use a pipette to measure a known volume of alkali into a conical flask and add an indicator.
2. Add acid from a burette until the indicator just changes colour (the end point), and record the volume.
3. Repeat without indicator, adding the same volume of acid, so the salt solution is not contaminated by indicator.
4. Crystallise the salt solution by evaporation and slow cooling.

Insoluble salt by precipitation

Use solubility rules to predict the insoluble product. The common rules: most nitrates, most chlorides (except silver and lead) and most sulfates (except barium, calcium and lead) are soluble; most carbonates and hydroxides are insoluble (except those of Group 1). To make an insoluble salt:

1. Mix the two solutions; the insoluble salt precipitates.
2. Filter to collect the precipitate as the residue.
3. Wash the residue with distilled water to remove soluble impurities.
4. Dry the precipitate.

Try this

Q1. Why is the insoluble base added in excess when preparing a soluble salt? [1 mark]

• Cue. To make sure all the acid reacts, so the solution is not acidic; the excess is filtered off.

Q2. Why is the titration repeated without indicator before crystallising? [1 mark]

• Cue. So the salt is not contaminated by the coloured indicator.

Q3. Describe how to obtain a pure dry sample of an insoluble salt after mixing the two solutions. [3 marks]

• Cue. Filter to collect the precipitate, wash it with distilled water, then dry it.