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Preparing inorganic compounds and finding water of crystallisation

Edexcel IAL Chemistry · WCH13 Practical Skills in Chemistry I

Unit 3 can ask about making a salt, finding the formula of a hydrated salt, or preparing a substance from a gas. Each step has a reason you should be able to give.

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Making a soluble salt from an insoluble base

1. Warm the acid and add the insoluble base or carbonate in excess, so all the acid reacts.

2. Filter off the unreacted solid.

3. Evaporate the filtrate until crystals start to form on a cold glass rod (the crystallisation point), then leave to cool and crystallise.

4. Filter off the crystals and dry them between filter papers or in a warm place.

Do not evaporate to dryness: a hydrated salt would lose its water of crystallisation, and some salts decompose.

For a soluble base (an alkali), use a titration to find the exact volumes, then repeat without indicator.

Water of crystallisation

Heat a weighed sample of hydrated salt in a crucible, cool, weigh and repeat to constant mass.

n(anhydrous salt) = mass of residue / M
n(water) = mass lost / 18.0
x = n(water) / n(salt)

If heating is incomplete, x comes out too low. If the salt decomposes further, or the residue is weighed hot, x may be wrong. Cool the anhydrous salt in a desiccator so it does not absorb water from the air.

Preparations involving gases

Example: iron(III) chloride from iron and chlorine (June 2025).

Chlorine is made by oxidising concentrated HCl (with potassium manganate(VII)) and is dried by passing through anhydrous calcium chloride (a drying agent), held in place with ceramic wool.
Chlorine is allowed to flush air out before heating the iron, so the iron does not react with oxygen.
Excess chlorine is absorbed by sodium hydroxide solution because it is toxic.
The product is collected dry. Iron(III) chloride absorbs water from the air, so a mass higher than expected means it has become hydrated.

Worked example

2.46 g of MgSO4·xH2O is heated to constant mass, leaving 1.20 g of MgSO4. Find x.

n(MgSO4) = 1.20 / 120.4 = 0.00997 mol
n(H2O) = (2.46 - 1.20) / 18.0 = 0.0700 mol
x = 0.0700 / 0.00997 = 7.02

Answer: x = 7

Key idea: Excess insoluble base, filter, evaporate to the crystallisation point, cool, filter, dry. x = n(water) / n(anhydrous salt) after heating to constant mass. Dry gases with a drying agent, flush out air, absorb toxic excess.

Check yourself

2.50 g of CuSO4·xH2O is heated to constant mass, leaving 1.60 g of CuSO4 (M = 159.6). Find x.
Show answer
5. n(CuSO4) = 1.60 / 159.6 = 0.01003; n(H2O) = 0.90 / 18.0 = 0.0500; x = 4.99, so 5.
Copper(II) oxide (M = 79.5) reacts with excess sulfuric acid, and the solution is crystallised to give CuSO4·5H2O (M = 249.6). Calculate the maximum mass of crystals from 2.00 g of copper(II) oxide.
Show answer
6.28 g. n(CuO) = 2.00 / 79.5 = 0.02516 mol; mass = 0.02516 x 249.6 = 6.28 g.
In the preparation above, 5.10 g of crystals are obtained from 2.00 g of copper(II) oxide. Calculate the percentage yield.
Show answer
81.2 %. 5.10 / 6.28 x 100 = 81.2%.
Why is the insoluble base added in excess when making a salt?
- To make the crystals larger
- To speed up filtration
- So that all the acid reacts and the product is not contaminated with acid
- To lower the yield
Show answer
So that all the acid reacts and the product is not contaminated with acid. The excess solid is easy to filter off; leftover acid is not.
A student's value of x for a hydrated salt is lower than expected. What is the most likely reason?
- The salt was not heated to constant mass, so some water remained
- The salt was heated too long
- The crucible lid was used
- The balance read to 3 decimal places
Show answer
The salt was not heated to constant mass, so some water remained. Less mass lost means less water counted.
Why should a solution of a hydrated salt not be evaporated to dryness?
- The yield would be too high
- The solution would boil over
- The crystals would lose their water of crystallisation or decompose
- It would take too long
Show answer
The crystals would lose their water of crystallisation or decompose. Gentle crystallisation keeps the hydrated form.
Iron(III) chloride prepared from iron and chlorine weighs more than the theoretical mass. What is the most likely reason?
- It absorbed water from the air and became hydrated
- Some iron did not react
- The iron was oxidised by oxygen
- Excess chlorine escaped
Show answer
It absorbed water from the air and became hydrated. This was the hardest question on the June 2025 paper: the anhydrous chloride takes up water from the air.

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Last updated 8 November 2026. Spotted a mistake? Tell us.