1.5.6Principles of Chemistry
Experiment: Finding Formulae of Compounds — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 27 of 50
Part 1 of 5The idea behind the experiments
Mass changes show the ratio
- Mass changes show the ratio If a compound loses or gains a substance in a reaction, weighing before and after tells you how much of each element or component is present.
- Converting those masses to moles gives the simplest whole-number ratio of particles, which is the empirical formula.
Two common types of experiment
- Two common types of experiment Combustion adds oxygen: a metal is heated in air and gains mass as it forms an oxide.
- Heating a hydrated salt drives off water of crystallisation as steam, so the solid loses mass.
Why gases are convenient
- Why gases are convenient Experiments in which a gas is gained or lost are easy to follow because the solid left behind can simply be weighed again.
Part 2 of 5Finding x in a hydrated salt such as CuSO4·xH2O
1
Weigh a clean, dry evaporating dish and record its mass.
2
Add a known mass of hydrated copper(II) sulfate crystals and weigh the dish and crystals together.
3
Heat the dish gently over a Bunsen burner, stirring carefully, until the blue crystals have turned completely white.
4
Allow the dish to cool, then weigh the dish and the white anhydrous salt. Heating again and reweighing until the mass is constant shows that all the water has gone.
5
Calculate the mass of anhydrous salt (final mass minus dish) and the mass of water lost (hydrated mass minus anhydrous mass).
6
Take care not to overheat, because the salt can decompose and lose extra mass, which would make the water look larger than it really is.
Part 3 of 5From masses to the formula
1Find the masses
- Find the masses Mass of anhydrous salt and mass of water, both in grams.
2Convert to moles
- Convert to moles Use n = m ÷ Mr for each, using Mr of CuSO4 = 160 and Mr of H2O = 18.
3Divide by the smaller
- Divide by the smaller Divide both mole values by the smaller one so that the anhydrous salt becomes 1 and water becomes x.
4Round and write
- Round and write Round x to the nearest whole number, then write the formula as salt·xH2O.
Part 4 of 5Worked example
| Stage | Working | Result |
|---|---|---|
| Masses | Hydrated salt 2.50 g; anhydrous salt left 1.60 g | Water lost = 2.50 − 1.60 = 0.90 g |
| Moles of CuSO4 | 1.60 ÷ 160 | 0.0100 mol |
| Moles of H2O | 0.90 ÷ 18 | 0.0500 mol |
| Ratio | 0.0100 : 0.0500 divided by 0.0100 | 1 : 5 |
| Formula | x = 5 | CuSO4·5H2O |
Part 5 of 5Sources of error and good practice
Heating to constant mass
- Heating to constant mass Reheating and weighing until two readings agree proves the reaction is finished,
- so no water or oxygen is still to be lost or gained.
Overheating
- Overheating Strong heating can break the anhydrous salt down, so the mass loss is too large and x comes out too high.
Rounding
- Rounding Real results rarely give an exact whole number, so x is rounded to the nearest whole number;
- a result like 4.9 means 5.
Key terms
| Term | Meaning |
|---|---|
| Empirical formula | The simplest whole-number ratio of atoms of each element in a compound. |
| Hydrated salt | A crystalline salt that contains water of crystallisation within its structure. |
| Anhydrous | Describes a substance that contains no water; anhydrous copper(II) sulfate is white. |
| Water of crystallisation | Water molecules held in a definite ratio inside the crystals of a salt. |
| Constant mass | A mass that no longer changes after repeated heating and weighing, showing the reaction is complete. |
Avoid these mistakes
1
Using the mass of the dish as part of the salt. Always subtract the empty dish mass before doing any calculation.
2
Using the wrong Mr for water, for example 16 instead of 18. Add the masses of two hydrogens and one oxygen.
3
Writing the formula with a decimal such as CuSO4·4.8H2O. The ratio must be rounded to a whole number.
4
Stopping heating as soon as the colour looks white without checking for constant mass, so some water remains.
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