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# Water of crystallisation. URGENT! Help please!!! watch

1. Hi guys,
I am preparing for my OCR practical exam (which is tomorrow) and really struggling with identifying unknown hydrated salts.
For example XCL*2 H2O, anhydrous 5.86g, water lost 9g
or XHSO4*3 H2O, anhydrous 40.53g, water lost 16.2g
How to find out the value of X? I am so confused and not sure how to approach this problem.

Many thanks,
2. 1. Work out the relative molecular mass of the anhydrous salt.
2a. Work out how many moles the anhydrous weight corresponds to.
2b. Work out how many moles of water has been lost.
3. Compare the moles of water lost to the moles of salt you have.
3. (Original post by BJack)
1. Work out the relative molecular mass of the anhydrous salt.
2a. Work out how many moles the anhydrous weight corresponds to.
2b. Work out how many moles of water has been lost.
3. Compare the moles of water lost to the moles of salt you have.
Hi,
thanks a lot for the reply. The problem is that I can't calculate Mr of the anhydrous salt as half of the formula is unknown and I can't take the value from the Periodic Table. Is there another way to do the step 1?
Hi guys,
I am preparing for my OCR practical exam (which is tomorrow) and really struggling with identifying unknown hydrated salts.
For example XCL*2 H2O, anhydrous 5.86g, water lost 9g
or XHSO4*3 H2O, anhydrous 40.53g, water lost 16.2g
How to find out the value of X? I am so confused and not sure how to approach this problem.

Many thanks,
Work out moles of water of crystallisation.
Use the ratio given in the formula to work out moles of unknown anyhdrous salt
Use mass of residue (the anhydrous salt) and moles to get relative mass.
Hi,
thanks a lot for the reply. The problem is that I can't calculate Mr of the anhydrous salt as half of the formula is unknown and I can't take the value from the Periodic Table. Is there another way to do the step 1?
Sorry, I misread your question. You're given something like MCl•2H2O and have to work out what element M is?

In that case, you need to do the steps in the following order:

1. Work out how many moles of water have been lost.
2. Use that number and the hydrated salt formula to work out how many moles of anhydrous salt you have.
3. Use that number and the anhydrous salt mass to work out the relative molecular mass of the salt.
4. Use that RMM to work out the identity of X.
6. (Original post by charco)
Work out moles of water of crystallisation.
Use the ratio given in the formula to work out moles of unknown anyhdrous salt
Use mass of residue (the anhydrous salt) and moles to get relative mass.
That's great!!! Thank you!!!
7. (Original post by BJack)
Sorry, I misread your question. You're given something like MCl•2H2O and have to work out what element M is?

In that case, you need to do the steps in the following order:

1. Work out how many moles of water have been lost.
2. Use that number and the hydrated salt formula to work out how many moles of anhydrous salt you have.
3. Use that number and the anhydrous salt mass to work out the relative molecular mass of the salt.
4. Use that RMM to work out the identity of X.
Thanks a lot
8. (Original post by charco)
Work out moles of water of crystallisation.
Use the ratio given in the formula to work out moles of unknown anyhdrous salt
Use mass of residue (the anhydrous salt) and moles to get relative mass.
Hi, thanks a lot for your help. Do you know how to solve this problem if you don't have a ratio in the formula? Something like XCL* Y H20? I am really worried that tomorrow they going to give us a compound where we need to identify the hydrated salt and find out the formula of hydrated crystals as well.
Hi, thanks a lot for your help. Do you know how to solve this problem if you don't have a ratio in the formula? Something like XCL* Y H20? I am really worried that tomorrow they going to give us a compound where we need to identify the hydrated salt and find out the formula of hydrated crystals as well.
You will not be given a problem like that. This has two unknowns and you would need two sets of data to produce simultaneous equations to solve...

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Updated: January 11, 2015
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