Water of crystallisation

So these crystals are little balls of issues which require a lot of tissues! They are blue crystals when hydrated and a white powder when anhydrous. Here take a look for yourself at its hydrated form:

Hydrated - solid crystals contain water of crystallisation

Anhydrous - contains no water of crystallisation

Dehydration - removal of water of crystallisation

Degree of hydration - the number of moles of water of crystallisation chemically bonded in 1 mole of the given compound

Now this is the part where you begin to hate on these little crystals as once again we are forced with maths, joy! So lets look at an example of this:

Determine the formula of hydrated magnesium sulphate:

Mass of hydrated salt MgSO₄ . 'X'H₂O = 4.312g

Mass of anhydrous salt MgSO₄ = 2.107g

Mass of H₂O = 2.205g

First calculate the amount of moles in the anhydrous salt (If you don't know how to do this check out my post on amount of substance to learn all about it)

Molar mass

Mg - 24.3

S - 32.1

O₄ - 64 (One oxygen is 16 but we have 4 so 4 x 16 = 64)

Molar mass = 24.3 + 32.1 + 64 = 120.4g mol^-1

n = 2.107 / 120.4 = 0.0175 mol

2.107 is found as mass of anhydrous salt.

Then calculate the amount, in mol, of water;

Molar mass

H₂- 2 (One hydrogen is 1 but we have 2 so 1 x 2 = 2)

O-16

molar mass = 2 + 16 = 18

n = 2.205 / 18 = 0.1225

2.205 is found as the mass of water.

To determine the formula of the hydrated salt you find out the molar ratio. To do this you divide both moles by the smaller moles (anhydrous-0.0175)

0.0175 / 0.0175 = 1

0.1225 / 0.0127 = 7

This makes seven lots of water to every MgSo4, this make the formula:

MgSO₄.7H₂O

Just apply this to your question and you got it down!

Did you find this helpful? Feel free to drop a comment and let me know what you liked and if I have missed something!

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