CHEM - Water of Crystallization
Chemistry, water of crystallization, hydrrated, anhydrous - IGCSE | IBDP | DSE | GCE | IAL | AP Chemistry
Some compounds require water molecules to form crystals. Without water, they will not form crystals and exist in powder form. The water needed to form crystals is called water of crystallization.
☝️ Blue crystal is hydrated copper sulfate and white powder is anhydrous copper sulfate ☝️
Chemical Equation for Water of Crystallization
Hydrated compounds contain a fixed number of water molecules and the chemical formula is written as salt·xH2O.
For example, 5 moles of water is required for every mole of copper sulfate to form a crystal, so the chemical formula of hydrated copper sulfate is CuSO4·H2O.
The chemical equation to form hydrated copper sulfate from anhydrous copper sulfate is:
CuSO4 + 5H2O → CuSO4·5H2O
Anhydrous copper sulfate can be obtained by heating hydrated copper sulfate:
CuSO4·5H2O → CuSO4 + 5H2O
For IGCSE Chemistry, it is important to know how to determine the water of crystallization!
Steps to determine the water of crystallization:
👇 Let's try some examples from IGCSE Chemistry 👇
A hydrated salt has the formula AB2·xH2O. When the hydrated salt is heated, this reaction occurs.
AB2·xH2O → AB2 + xH2O
A scientist heats a sample of the hydrated salt until all the water has been lost. She records the mass of the salt before and after heating.
Mass of hydrated salt = 6.1 g
Mass of salt after heating = 5.2 g
(a) Describe how the scientist could make sure that all the water has been lost.
(b) Use the scientist’s results to find the value of x in AB2·xH2O. [Mr of AB2 = 208, Mr of H2O = 18]
The mineral rozenite contains crystals of hydrated iron(II) sulfate, FeSO4·xH2O. A student wants to find the value of x. She uses this apparatus to remove and collect the water of crystallisation from a sample of iron(II) sulfate crystals.
She uses this method.
When iron(II) sulfate crystals are heated gently, they decompose according to this equation.
FeSO4·xH2O → FeSO4 + xH2O
These are the student’s results.
(a) Calculate the mass of anhydrous FeSO4 formed after heating to constant mass
mass of anhydrous iron(II) sulfate = 15.76 - 11.96 = 3.80 g
(b) Calculate the mass of water collected in tube B after heating to constant mass.
mass of water = 17.56 - 15.76 = 1.80 g
(c) Find the chemical formula of hydrated iron(II) sulfate.
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