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Edexcel - Chemistry Unit 2 - 4 June 2013

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Original post by Hi, How are you ?
Isn't it 0.43 % :confused:


I got 0.86

You must have forgotten to have multiply the +or-0.05 by 2
Reply 1461
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Rhayat
Original post by Goods
Initial and final reading therefore total uncertainty is 2 times the uncertainty


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Ermm they didnt specify that 2 readings were taken. In unit 1 they specifically said that 2 readings were taken. If you look at other past papers , questions worded like this doesnt require to multiply the % error by 2.
Guys, for the reduced percentage error, did you say use a sample with less tin in it.
So that less Sn2+ ions reacted with the iodine, and so more thiosulphate ions reacted with the excess iodine, meaning more thoisulphate was needed, leading to a reduction in the error.
Bah humbug. Was hoping that periodic trends didn't feature too heavily... Turns out most of the paper is based on it?!

Thankfully no apparatus drawing questions though

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It's a burette... You take a reading at the start and a reading at the end point

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Original post by Gnome :)
It's a burette... You take a reading at the start and a reading at the end point

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You're only working out the difference between the initial and the final value of the burette once the titration is completed. That difference is = one titre value.

They said they wanted to improve the reliability, so you had to carry out more than one complete titration. Hence you had to seperate the excess iodine into seperate beakers and titrate to get more values (albeit, you would end up with smaller value, which would decrease the accuracy of the experiment, but would increase the reliability as you have to take a mean)
Recent grade boundaries:
Jan 2013- A= 65 B= 59
June 2012 - A=63 B=57
Jan 2012 - A=61 B=55
June 2011 - A=58 B=52
Original post by wildyoungheartsx
Recent grade boundaries:
Jan 2013- A= 65 B= 59
June 2012 - A=63 B=57
Jan 2012 - A=61 B=55
June 2011 - A=58 B=52


This one seemed a harder paper than Jan 13, I would say around 61 or 62 for an A.
Original post by James A
You're only working out the difference between the initial and the final value of the burette once the titration is completed. That difference is = one titre value.

They said they wanted to improve the reliability, so you had to carry out more than one complete titration. Hence you had to seperate the excess iodine into seperate beakers and titrate to get more values (albeit, you would end up with smaller value, which would decrease the accuracy of the experiment, but would increase the reliability as you have to take a mean)


I know. My message was in response to the point about why you double the error.
Original post by Gnome :)
I know. My message was in response to the point about why you double the error.


Apologies.
I got about 14.5% for the titration ???
The test to differentiate between Mg(CO3) and Ba(CO3), wasn't it a solubility test? CO3 is a 2- negatively charged anion, so the solubility of group 2 elements decreases down the group for 2- anions. Therefore with Ba(CO3) a white precipitate would form as it's insoluble, and Mg(CO3) there'd be no precipitate as it dissolves.
Reply 1472
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Goods
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Original post by Maybenexttime
The test to differentiate between Mg(CO3) and Ba(CO3), wasn't it a solubility test? CO3 is a 2- negatively charged anion, so the solubility of group 2 elements decreases down the group for 2- anions. Therefore with Ba(CO3) a white precipitate would form as it's insoluble, and Mg(CO3) there'd be no precipitate as it dissolves.


That should get the marks. The question is whether or not putting flame test results would...


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Original post by Polish master
I got about 14.5% for the titration ???


That's strange, it was 13.something%

Unless you rounded up your figures too early. I never round up until I get to the final answer. I always write the full number in the steps before the last one.
Reply 1474
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Goods
16
Original post by James A
That's strange, it was 13.something%

Unless you rounded up your figures too early. I never round up until I get to the final answer. I always write the full number in the steps before the last one.


13.8%


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Original post by James A
That's strange, it was 13.something%

Unless you rounded up your figures too early. I never round up until I get to the final answer. I always write the full number in the steps before the last one.


Maybe, I mean I got all the moles right I will get those marks.
Original post by Maybenexttime
The test to differentiate between Mg(CO3) and Ba(CO3), wasn't it a solubility test? CO3 is a 2- negatively charged anion, so the solubility of group 2 elements decreases down the group for 2- anions. Therefore with Ba(CO3) a white precipitate would form as it's insoluble, and Mg(CO3) there'd be no precipitate as it dissolves.


That is exactly what I wrote. I just hope it's accepted. Since they said no heating, I assumed a flame test is a form of heating so I didn't want to risk it.
Original post by Maybenexttime
The test to differentiate between Mg(CO3) and Ba(CO3), wasn't it a solubility test? CO3 is a 2- negatively charged anion, so the solubility of group 2 elements decreases down the group for 2- anions. Therefore with Ba(CO3) a white precipitate would form as it's insoluble, and Mg(CO3) there'd be no precipitate as it dissolves.


Well Magnesium carbonate does not in fact dissolve in water.

I reckon its a flame test (its not heating?) or a sodium hydroxide test.
It should be within the spec so I guess its flame test.
Original post by StUdEnTIGCSE
Well Magnesium carbonate does not in fact dissolve in water.

I reckon its a flame test (its not heating?) or a sodium hydroxide test.
It should be within the spec so I guess its flame test.


I put flame test, but when you react Magnesium Carbonate, with a particular sulfate, won't you get magnesium sulfate, which is then soluble in aqueous and barium sulfate, which is not soluble in aqueous? :s-smilie:
Personally, I thought they just said don't state using acid or heating to cover their backs, because they were already in the question. Flame test hadn't been in the question before, so I don't see any reason this wouldn't be accepted.

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