Can you help? Chemistry F321 Water of Crystallisation

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Xetter
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Stuck on the following question of the previous exam June 14 F321. We did it for our mock and I had absolutely had no idea, after being given a hard time about it in lesson I'm still stuck and was just wondering if anyone could help before I go back and ask?

Hydrated strontium chloride, SrCl2 . 6H2O, has a molar mass of 266.6 gmol^-1.
A student heats 5.332g of SrCl2 . 6H2O.
The SrCl2 . 6H2O loses SOME of its water of crystalisation forming 3.892g of a solid product.
Use the information above to dertermin the formula of the solid product.

Show your working.


If anyone could help me out with this, that would be great! Its 3 marks by the way
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TSR Jessica
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Sorry you've not had any responses about this. Are you sure you’ve posted in the right place? Posting in the specific Study Help forum should help get more responses. Hopefully someone will be able to get back to you
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rohzz345
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SrCl2 . 6H2O ------> SrCl2 . XH2O + H20
0.02mol 0.08mol
0.08/0.02 = 4 as 1:4 ratio
6-4= 2
SrCl2.2H20 hopefully thats the answer
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chuckster111
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Could you explain the ratio please?
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azo
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First work out mol of original : 3.892/266.6 = 0.02
Mol of water lost = (5.332-3.892) / 18 (Mr of H2O) = 0.08
This means that for 0.02 mol of SrCl2.6H2O that reacts, 0.08 mol of water is produced
You can either multiply both by 50, or just do 0.08/0.02 to work out for each mol of SrCl2.6H2O 4 mol of water is made.
The rest is easy, if 4 mol of water are lost, then 6-4=2 mol remain, so compound is:
SrCl2.2H2O

Hope this helps
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chuckster111
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(Original post by azo)
First work out mol of original : 3.892/266.6 = 0.02
Mol of water lost = (5.332-3.892) / 18 (Mr of H2O) = 0.08
This means that for 0.02 mol of SrCl2.6H2O that reacts, 0.08 mol of water is produced
You can either multiply both by 50, or just do 0.08/0.02 to work out for each mol of SrCl2.6H2O 4 mol of water is made.
The rest is easy, if 4 mol of water are lost, then 6-4=2 mol remain, so compound is:
SrCl2.2H2O

Hope this helps
Thanks, finally got my head around it!!
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ksf
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Hi I know this was put aggggeessss ago but I am struggling with the same question and I don't understand how you got the 0.2 for the mol of the original? I got 0.015 and have no idea how you got 0.2?
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hobinobichimchim
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(Original post by ksf)
Hi I know this was put aggggeessss ago but I am struggling with the same question and I don't understand how you got the 0.2 for the mol of the original? I got 0.015 and have no idea how you got 0.2?
firstly, i just saw this and tried it for fun to test myself and got confused by the 0.02 as well, then u replied so lol coincidence.... ok so, I think the reply by azo is a bit wrong. so i'll go through it step by step:

1) the equation for this is SrCl2.6H2O -> SrCl2.XH2O + H2O
we put X as we don't know what that the formula of the solid product is and are going to work it out
2) note down the info you have. here you have the the molar mass of SrCl2.6H2O given to you, as 266.6 gmol-1 and this is basically the Mr of this compound. the mass of it is given as 5.332g and the mass of the solid product is 3.892g
3) you can work out that the other product is H2O by the equation, and the mass of this is 5.332g-3.892g = 1.44g
4) using Mass/Mr = moles, you do 1.44/18 = 0.08 mol of H2O product.
5) do the same with the SrCl2.6H2O, so 5.332/266.6 = 0.02 mol of that.
6) 0.02:0.08 molar ratio of the 2 things can be reduced to 1:4 if you divide both sides by the smallest, 0.02. 1:4 is the molar ratio; this shows that 1 SrCl2.6H2O makes 4 H2O (you can see this from the molar ratio).
7) In the original reactant, there is 6H2O so if 4H2O is lost then the remaining has to be 2, so the formula is SrCl2.2H2O

hope this makes sense
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ksf
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Wow thank you so much for the help what a coincidence 🤣🤣
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choichi
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do you go to psc.....lmfaooo
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ksf
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yeh lmao pahahahha 🤣
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