1.5.6Principles of Chemistry: Part 1
Experiment: Finding Formulae of Simple Compounds — Edexcel International GCSE Science Double Award (Modular) - Chemistry Unit 1 Revision Notes & Topical Questions
5 min read 7 parts Lesson 21 of 23
Part 1 of 3Method
1
Weigh a clean, dry evaporating dish and record its mass.
2
Add a known mass of hydrated copper sulfate, roughly 2 to 3 g, and weigh the dish and salt together.
3
Heat the dish gently over a Bunsen burner, stirring carefully, until the blue crystals turn completely white.
4
Allow the dish to cool, then reweigh the dish and the white solid.
5
Heat again, cool and reweigh until the mass is constant, which shows all the water has gone.
6
Avoid overheating, which could decompose the anhydrous salt and give a mass loss that is too large.
Part 2 of 3Processing the results
| Step | What to do | Example |
|---|---|---|
| 1 | Find the mass of anhydrous salt = mass of dish and white solid − mass of dish | 1.60 g |
| 2 | Find the mass of water = mass of hydrated salt − mass of anhydrous salt | 2.50 − 1.60 = 0.90 g |
| 3 | Moles of CuSO4 = mass ÷ Mr (159.5) | 1.60 ÷ 159.5 = 0.0100 mol |
| 4 | Moles of H2O = mass ÷ Mr (18) | 0.90 ÷ 18 = 0.050 mol |
| 5 | Divide both by the smaller number to get the ratio CuSO4 : H2O | 1 : 5 |
Part 3 of 3Writing the formula
1Find the ratio
- Find the ratio The mole ratio of anhydrous salt to water is 1 : x, found by dividing by the number of moles of salt.
2Round sensibly
- Round sensibly Experimental results rarely give exact whole numbers, so round x to the nearest whole number.
3Write the formula
- Write the formula Put x in front of the water: CuSO4·5H2O in the example above.
Key terms
| Term | Meaning |
|---|---|
| Hydrated salt | A crystalline salt that contains water of crystallisation in its structure. |
| Anhydrous | Containing no water. |
| Water of crystallisation | Water molecules that are chemically part of the structure of a crystal. |
| Empirical formula | The simplest whole-number ratio of atoms of each element in a compound. |
| Heating to constant mass | Heating and reweighing until two readings agree, showing the reaction is complete. |
Avoid these mistakes
1
Heating the salt only until it looks mostly white. Heat until the colour change is complete and the mass is constant.
2
Dividing masses instead of moles to get the ratio. Each mass must first be divided by its own Mr.
3
Using the mass of the hydrated salt as the mass of CuSO4. Subtract the water mass first.
4
Giving x as a decimal in the final formula. Round it to the nearest whole number.
Last updated: