How do you work out Water of Crystallisation? (AS Chemistry OCR)

  • 0 votes

Could someone please give me a step by step way of working out the Water of Crystallisation, I'd be most grateful! I'm really struggling and it's the biggest problem for me for the upcoming OCR Chemistry A exam on Atoms, Bonds and Groups!

Please help!! :)

Posted Thu 20th December, 2012 @ 16:59 by hols_95

1 Answer

  • 9 votes

Hey. I'm doing the exam too :)

e.g. CaCl­•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 291.1-111.1=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 6.573-3.333=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.

Hope this helps! x

Answered Fri 21st December, 2012 @ 18:18 by Hummi C
Edited by Hummi C on Fri 21st December, 2012 @ 18:18