Question

calculate the% of water of crystallization in sodium thiosulphate crystals​

Answer 1

Explanation:

14.4 Water of crystallisation in a crystallised salt

What is water of crystallisation?

Water of crystallization are the molecules of water that are incorporated into some salt crystals when they are crystallised out of water.

e.g. when blue crystals of copper(II) sulfate are crystallized out of water the actual formula of the crystals is ...

NOT simply CuSO4, but on crystallisation CuSO4.5H2O is formed,

because five water molecules are associated with each 'CuSO4'. in its 'hydrated' crystalline form.

Example 14.4.1: Reminders on calculating formula mass e.g. with MgSO4.7H2O

Relative atomic masses: Mg = 24, S = 32, O = 16 and H = 1

You need to add together the formula mass of MgSO4 plus the relative mass of seven water molecules.

Relative formula mass of water = (2 x 1) + 16 = 18

Relative formula mass of MgSO4 = 24 + 32 + (4 x 16) = 120

Relative mass of seven water molecules = 7 x 18 = 126

Relative formula mass of crystals = MgSO4 + (7 x H2O) = 120 + 126 = 246

Example 14.4.2 How to calculate the theoretical % of water in a hydrated salt

eg magnesium sulphate MgSO4.7H2O 'hydrated' salt crystals

Relative atomic masses: Mg = 24, S = 32, O = 16 and H = 1

Relative formula mass of crystals = 24 + 32 + (4 x 16) + {7 x (1 + 1 + 16)} = 246

Relative mass of seven water molecules = 7 x 18 = 126

so % water = 126 x 100 / 246 = 51.2%