Making sodium, potassium and ammonium salts
4 marks · NormalA student tries to make sodium nitrate by mixing sodium hydroxide and nitric acid using the traditional 'add excess solid, then filter' method. The table shows the solubility of sodium nitrate, sodium sulfate and copper(II) sulfate in water:
| Salt | Solubility in water at room temperature (g/100g water) |
|---|---|
| Sodium nitrate | 92 |
| Sodium sulfate | 21 |
| Copper(II) sulfate | 23 |
Use the data to explain why the excess solid method does not work for making sodium nitrate, and how this relates to the need for titration.
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Also accept: Because sodium nitrate is very soluble, any excess would dissolve and cannot be filtered off
Common mistakes: Just restates high solubility without linking to method; Describes copper(II) sulfate method
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Marking points
B1 · 1 mark · Identifies that sodium nitrate is very soluble in water.
B2 · 1 mark · Explains excess solid would dissolve so cannot be filtered off.
B3 · 1 mark · Links the need for titration to determining the exact amount required.
B4 · 1 mark · Uses data to compare solubility with another salt and/or justifies method selection.
Worked solution
Sodium nitrate has a high solubility (92 g/100g water), much greater than sodium sulfate (21 g/100g) or copper(II) sulfate (23 g/100g). If you add excess sodium hydroxide or nitric acid, the excess would dissolve rather than stay as a solid. This means you cannot filter off any unreacted solid to remove it, unlike with less soluble salts such as copper(II) sulfate. To make sure all acid and alkali react exactly, titration is used so only the necessary amounts are mixed, allowing for preparation of a pure salt.