Acids, Bases and Salts: Question 5

Worked solution

Part (a): Choosing and describing the preparation method

Why not titration? Titration is used when both the acid and the base are soluble (an acid and an alkali), because a precise, measured volume of the alkali is added to the acid using an indicator to find the exact neutralisation point. Magnesium oxide is an insoluble base, so it cannot be measured out this way in solution. Instead, an excess of the solid can simply be added and any unreacted solid removed afterwards.

Method (excess insoluble base):

1. Warm a measured volume of dilute sulfuric acid gently.
2. Add magnesium oxide powder to the warm acid a little at a time, stirring, until some solid remains unreacted (i.e. an excess. This shows all the acid has been neutralised).
3. Filter the mixture to remove the excess, unreacted magnesium oxide, keeping the filtrate, a solution of magnesium sulfate.
4. Evaporate the filtrate gently to drive off some of the water, until the solution is hot and saturated.
5. Leave the saturated solution to cool slowly, so that magnesium sulfate crystals form.
6. Filter off the crystals and pat/leave them to dry (e.g. between filter paper), giving pure, dry crystals.

Part (b): Hydrated and anhydrous salts

• A hydrated substance is one that is chemically combined with a fixed amount of water within its solid structure. Magnesium sulfate heptahydrate, $\text{MgSO}_4 \cdot 7\text{H}_2\text{O}$, is hydrated. Each formula unit has 7 water molecules bound into its crystals.
• An anhydrous substance contains no water at all.

On strong heating: the water within the crystal structure is driven off as steam, and the crystals lose their water:

$\text{MgSO}_4 \cdot 7\text{H}_2\text{O(s)} \rightarrow \text{MgSO}_4\text{(s)} + 7\text{H}_2\text{O(g)}$

This leaves behind a white, anhydrous magnesium sulfate powder in place of the original crystals.

Final answers

• (a) The excess-insoluble-base method is used because magnesium oxide is insoluble (titration needs a soluble alkali); react in excess with warm acid, filter off the excess solid, evaporate the filtrate to saturation, cool to crystallise, then filter and dry the crystals.
• (b) Hydrated = combined with water in the solid structure; anhydrous = contains no water. Strong heating drives off the water as steam, leaving white anhydrous $\text{MgSO}_4$ powder.