Water Of Crystallisation, step by step

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  • Created by: Hummi C
  • Created on: 21-12-12 18:26
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e.g. CaCl2·xH2O and the Relative Formula Mass (RFM), say 291.1. You find the molar mass of
the anhydrous CaCl2 using the periodic table, so 40.1+35.5+35.5= 111.1. Then subtract that
from the overall RFM, so 291.1111.1=180. Then find the molar mass of water, which is 18.
Then, to solve for x divide 180 by 18. 180/18=10. So the number of water of crystallisation is
10.
Alternatively, if we were given the mass first we would go through a different process.
Example:
A student carries out an experiment. They heat 6.573g of CaCl2·xH2O to form anhydrous
CaCl2, weighing 3.333g. Calculate x.
We can find the molar mass of CaCl2 using the periodic table, which is
40.1+35.5+35.5=111.1. Then work out the mass of the water evaporated, which is
6.5733.333=3.240. We can then plot a ratio table to look like this:
molecule: CaCl2 : H2O
molar mass: 111.1 : 18
mass: 3.333 : 3.240
Then divide the bottom number on either column by the top number (3.333/111.1 &
3.240/18) So you have
molecule: CaCl2 : H2O
0.03 : 0.18
Then divide both numbers by the smallest number:
molecule: CaCl2 : H2O
1 : 6
So the final answer is 6.
NOTE: Hypothetically speaking, if you come to a final answer like 7.0245325 or something
like that, you can round to 7.

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