# 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!! :)

## 1 Answer

- 9 votes

Hey. I'm doing the exam too :)

e.g. CaCl_{2}•xH_{2}O and the Relative Formula Mass (RFM), say 291.1. You find the molar mass of the anhydrous CaCl_{2} 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 CaCl_{2}•xH_{2}O to form anhydrous CaCl_{2}, weighing 3.333g. Calculate x.

We can find the molar mass of CaCl_{2} 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: CaCl_{2} : H_{2}O

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: CaCl_{2} : H_{2}O

0.03 : 0.18

Then divide both numbers by the smallest number:

molecule: CaCl_{2} : H_{2}O

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