# Chemistry - water of crystallisation

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#1
Hi, if anyone can please explain the answer to this this in detail i would be so appreciative because im so confused at to how to get to the answer and its no where online to be found.

MCl2.2H2O is a salt with water of crystallisation, and M is a metal. the water of crystallisation makes up 22.2% by mass of the hydrated salt. What is the relative formula mass of the anhydrous salt?
0
3 years ago
#2
(Original post by ineedsomuchchem)
Hi, if anyone can please explain the answer to this this in detail i would be so appreciative because im so confused at to how to get to the answer and its no where online to be found.

MCl2.2H2O is a salt with water of crystallisation, and M is a metal. the water of crystallisation makes up 22.2% by mass of the hydrated salt. What is the relative formula mass of the anhydrous salt?
1. treat the 22.2% by mass as the actual mass of the water in grams

2. carry out a moles calculation to find the number of moles of water: Moles=22.2/18=37/30=1.23 (3 s.f.)

3. look at the molecular ratio in the formula- 1:2- therefore there are half the number of moles of the anhydrous salt compared to water=37/60=0.617 (3 s.f.)

4. As you're treating the % by mass as the actual mass, the mass of the anhydrous salt is 77.8 (100-22.2). Using this mass, you can carry out a moles calculation to find the Mr of the anhydrous salt:
Mr=mass/moles=77.8/0.617=4668/37=126 (3 s.f.)

Therefore the Mr of the anhydrous salt is 126

To find what the metal M is, subtract the Mr of Cl2 and that will be the Mr of the metal, which I think in this question is Manganese if you round to the nearest whole number

Hope this helps
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#3
I genuinely can't thank you enough for taking the time to help me!
That's much clearer and makes much more sense to me, thank youuuuuu )))
0
3 years ago
#4
Water of crystallisation problems are just empirical formulae. The maths you are doing is exactly what you learned at GCSE with Ar, but now you're using water with an Mr of 18 as one unit.
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