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# A-level Chemistry Revision Squad! watch

1. (Original post by Gwenog_quidditch)
So I've been stuck on this AS past paper chem question for too long and I would be very, very grateful if anyone could please help me with it, thanks. Here it is:
Anhydrous calcium chloride, CaCl2, can be used to dry some organic liquids. During this process, hydrated calcium chloride, CaCl2.2H20 is formed.
CaCl2 + 2H20 --> CaCl2.2H20
Mr 111
In a drying process, 5.55g of anhydrous calcium chloride, CaCl2, is used. Calculate the amount of water that can be removed from the organic liquid. (2 marks)

Thank you!!
What do you mean by the amount of water like the volume or the number of moles of water that can be removed ? If the number of moles, then it is 0.1 moles, because 5.55g of anhydrous CaCl2 is used, it Mr is 111, the number of moles of CaCl2 is 0.05, the moles ratio of the CaCl2 to water is 1:2 so the number of moles of water used 0.1 which is the number of moles that will be removed from the organic liquid
2. (Original post by haj101)
Hi- I've tried to give it a shot, forgive me if there are any mistakes...

Okay so first I calculated the moles of NaOH: which is conc x (vol/1000) to get me 2.496 x 10-3 moles
We know its a 1:2 ratio, for every one mole of acid, two moles of NaOH is reacted, so we divide the moles of NaOH we have calculated by 2, to get 1.248 x 10-3 moles of the acid (in 25cm3)
Now we have the moles of the acid in 25cm3 we need to calculate moles in 250cm3 so we multiply the moles by 10 to get 0.01248 moles of the acid
Now we have the moles of the acid in 250cm3. We can now divide the mass of the acid crystals by the moles to give us the Mr which is 1.575/0.01248= 126

Now we have the Mr of H2C2O4.nH2O we then first find out the Mr of the first part: H2C2O4= which is 90. 126 - 90 is 36 so H20 is 36. The Mr of H20 is 18, so we divide 36/18 to get us n which is 2.
So n is 2. I think.
I got the same answer
3. (Original post by PlayerBB)
I got the same answer
Yay!! Thank god!
4. (Original post by PlayerBB)
What do you mean by the amount of water like the volume or the number of moles of water that can be removed ? If the number of moles, then it is 0.1 moles, because 5.55g of anhydrous CaCl2 is used, it Mr is 111, the number of moles of CaCl2 is 0.05, the moles ratio of the CaCl2 to water is 1:2 so the number of moles of water used 0.1 which is the number of moles that will be removed from the organic liquid
It just says 'amount of water'... but it's okay I think I've got it now thanks
5. (Original post by PlayerBB)
What do you mean by the amount of water like the volume or the number of moles of water that can be removed ? If the number of moles, then it is 0.1 moles, because 5.55g of anhydrous CaCl2 is used, it Mr is 111, the number of moles of CaCl2 is 0.05, the moles ratio of the CaCl2 to water is 1:2 so the number of moles of water used 0.1 which is the number of moles that will be removed from the organic liquid
It's almost certainly moles as it's a bit of a hassle to convert into a volume, and also moles is the SI unit for the amount of substance
6. (Original post by samb1234)
It's almost certainly moles as it's a bit of a hassle to convert into a volume, and also moles is the SI unit for the amount of substance
Yeah, that's what I thought!!

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7. (Original post by kiiten)
Please could someone explain this too thanks

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8. 0.9/3 not 0.9/2.1 . It's like if you had 10 sweets and eat 5, the percentage isn't 5/5 =100% it's 5/10 =50% so you need to divide by how much there is at the start
9. (Original post by samb1234)
0.9/3 not 0.9/2.1 . It's like if you had 10 sweets and eat 5, the percentage isn't 5/5 =100% it's 5/10 =50% so you need to divide by how much there is at the start
Right, thanks
1. has anyone done the enthalpy change isa for AS Chem AQA?
10. (Original post by K3001N)
1. has anyone done the enthalpy change isa for AS Chem AQA?
Do you mean the required prac? If so then yeah

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11. With the calcium sulfide lattice structure, why are the sulfide ions larger than the calcium ions when calcium has a larger relative atomic mass? Please help
12. (Original post by jack.lp.thompson)
With the calcium sulfide lattice structure, why are the sulfide ions larger than the calcium ions when calcium has a larger relative atomic mass? Please help
Think about the number of electrons and the charge.
13. (Original post by kiiten)
Think about the number of electrons and the charge.
Wouldn't the number of electrons be the same? (same as argon) I don't understand why the sulfide ion would be larger. Please explain
14. (Original post by jack.lp.thompson)
Wouldn't the number of electrons be the same? (same as argon) I don't understand why the sulfide ion would be larger. Please explain
For calcium yes. Can you use the periodic table to find out what charge the sulfide and calcium ions will have? - (sorry im new to this 'helping people' thing so im trying to help without telling you the answer )

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15. Haha don't worry about it. And well as it is Ca2+ and S2-, wouldn't two electrons be lost/gained from each element causing them both have the same electron configuration as argon? (20-2=18) (16+2=18)??? So confused! haha . Please could just tell me why the sulfide ion is bigger?? :O aha
16. (Original post by jack.lp.thompson)
Haha don't worry about it. And well as it is Ca2+ and S2-, wouldn't two electrons be lost/gained from each element causing them both have the same electron configuration as argon? (20-2=18) (16+2=18)??? So confused! haha . Please could just tell me why the sulfide ion is bigger?? :O aha
Well, sulfur is more electronegative than calcium due to trends in electronegativity increasing across the period and up the group. If you have studied electronegativity you will know that this is from nuclear charge, atomic radius etc.

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17. Ah ok. That makes sense now. Thanks
18. What's the name of this compound?
- Just for reassurance

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19. (Original post by PlayerBB)
What's the name of this compound?
- Just for reassurance
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ermmm.. I get N-methylethylphenylamine?? probs wrong :/

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