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Edexcel Chemistry A2 Unit 5 ~ Wednesday 19th June 2013 (Now Closed)

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Sorry im quite confused, does that mean that:
- the unreacted I2 = unreacted Cr2O7^2- ?
- because the Cr2O7- produces 3 moles of I2 as KI is added, so the total moles of I2 present/3 = total moles of leftover Cr2O7^2-
so if i use the moles of excess Cr2O7^2- to subtract this leftover of Cr2O7^2- this will give me the reacted Cr2O7^2-

if this is right then I get it now :biggrin:

DDDDD
THANK YOU SO MUCH!!!!!

Hmm, I think so, I'll show you what I did.
You are given:
- Some dichromate is left to react with the ethanol in the 5cm^3 of the drink, when it has reacted, the leftover dichromate is reacted with Iodine and then titrated with thiosulphate.
- Total volume of dichromate is 10cm^3
- Concentration of initial total volume pf dichromate is 0.08 moldm^-3
- Volume of S2O3^2- = 34.4cm^3
- Concentration of S2O3^2- = 0.025 moldm^-3
- Ratio of ethanol to dichromate ions is 3:2
- Cr2O7^2- + 14H^+ + 6I^- --> 2Cr^3+ + 3I2 + 7H2O [1]
- 2S2O3^2- + I2 --> S4O6^2- + 2I^- [2]

So start by working out this:
Moles of S2O3^2- = (0.025 x 34.4) / 1000 = 8.6 x 10^-4 moles
Ratio of S2O3^2- to I2 is 2:1 from equation [2] so moles of I2= (8.64 x 10^-4) / 2 = 4.3 x 10^-4
Ratio of I2 to leftover (unreacted) Cr2O7^2- is 2:3 from equation [1] so moles of unreacted Cr2O7^2- = 1.43 x 10^-4
Total moles of Cr2O7^2- = (10 x 0.08) / 1000 = 8 x 10^-4
So moles of reacted Cr2O7^2- is (8 x 10^-4) - (1.43 x 10^-4 ) = 6.557 x 10^-4
Ratio of Cr2O7^2- to ethanol is 2:3 (given) so moles of ethanol = 9.84 x 10^-4
Therefore Concentration of ethanol = (1000 x 9.84 x 10^-4) / 5 = 0.197 moldm^-3

Reply 1021

F1's Finest

The hardest part of the above question is to realise that you want to work out the value for the unreacted dichromate ions. Apart from that, it's just how a normal titration calculation should be attempted.

Reply 1022

AtomicMan

Original post by SKK94

Thanks!!

Yeah the right answer is C...that means this element has an electronic configuration of [Ar]-4s2-3d3 right?

But how do we eliminate the other options?

- It cannot be A because though there are jumps b/w 1st and 2nd i.e, there are irregular jumps b/w the rest - so it doesn't agree with the electronic config. of a transition metal.

- D has a jump b/w 1st and 2nd i.e, but the 1st i.e seems comparatively too high for a 4s2 electron (?)

- B looks like it has only one jump (b/w 1st and 2nd) and no others. Doesn't this imply there are 6 electrons in the d-orbital, making it a possible transition metal( [Ar]-4s2-3d6 ) ?

You are right with B. I honestly dont know, the only thing Id think is due to the fact that you have a full 3d sub-orbital, so when you take the first electron in the full 3d sub-orbital, youd expect a lower ionisation energy, as the repulsion between the electrons in the sub-orbital means it is a less stable sub-orbital, so less energy would be required to remove that first electron.

Edit: this expected dip/lesser increase would be at the 3rd ionisation energy, which B doesnt really represent.

(edited 10 years ago)

Reply 1023

GeorgeL3

Original post by ayeswary

Hey. Was just wondering anyone taking unit 4 chem. I am a bit new to this page and a bit worried of exams after having a week of tough practical papers. Could someone explain to me the formula for the buffer solution. Still don't get the formula. :smile:

Do you mean
pH = -log(Ka) - log([Acid] / [Base])
If so that's basically it for what you need to know about it. (Remember [] means concentration of).
If you haven't done maths and aren't comfortable using logs, read through this: http://www.mathsisfun.com/algebra/logarithms.html

The other thing you need to remember is that when the solution is half ionized, there are equal concentrations of acid and base so it just becomes pH = -log(Ka)

If you want a set of revision notes for unit 4 I've got some here:

Chemistry Unit 4 revision cards - George Lewis.docx190.0KB

Reply 1024

ayeswary

Thank you so much George. I think kind if got it. About the log part I am use to it as I am doing Maths as well. :smile:

thanks a lot. :smile:

.

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Reply 1025

ayeswary

Am also having slight problem on the NMR part when questions ask you to determine the structure where they give you the m/e and ask you to figure out the structure. Anyone knows how to get the structure. :smile:

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Reply 1026

posthumus

Original post by ayeswary

Posted from TSR Mobile

Would you be able to post a question ? :smile:

Also they must give you what elements are in the molecule also ?

Reply 1027

ayeswary

ImageUploadedByStudent Room1368925332.538657.jpg

How do you this question? :smile:

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Reply 1028

posthumus

Original post by ayeswary

How do you this question? :smile:

Posted from TSR Mobile

Hi I'm just curious to know if this is from Unit 5 paper ? :smile:

Well anyway we know it's going to be an aldehyde or ketone due to the point made in a.

b. suggests this compound can undergo iodoform reaction ... which suggests its either a methylketone or ethanal

Okay firstly it can't be ethanal, because the peak 86 is too high

CH3CHO

This gives mass of: 12+3+12+1+16= 44

Okay so we know it's going to be a methylketone !

now methyl ketones have this group:

COCH3

12+16+12+3 = 43 ... this is great because it's one of the peaks, so we're doing well :biggrin:

Now lets complete the structure by adding CH2s' & a CH3

86-43 = 43 ... okay so we have 43 atomic mass stuff left :smile:

So I think it would be sensible to assume that the rest of the structure contains CH3CH2CH2 .... 12+3+12+2+12+2= 43

So the structure is CH3CH2CH2COCH3 :biggrin:

Hope that helps :smile:

Reply 1029

Preziosavita

I found the Hodder book here: I've never heard of it before,

http://www.scribd.com/doc/118237530/Edexcel-A2-Chemistry-Hodder

How useful is it? and what Q's from the book should we do for the Unit 5 exam?

Thanks!

1 month... :afraid:

Original post by LeaX

1 month... :afraid:

Ah thanks for reminding :frown:

aha

You will be okay :biggrin:

Especially since you revise in your sleep too :wink:

By the way... how's unit 4 ? :smile:

I'm starting chemistry next weekend, as I have 3 exams before then. I haven't done any unit 4 since January :frown:

have unit 2 to prepare for as well ! :tongue:

damn it... I hope this all benefits my unit 5 as well though :smile:

Reply 1032

blossomx

Original post by posthumus

Ah thanks for reminding :frown:

aha

You will be okay :biggrin:

Especially since you revise in your sleep too :wink:

By the way... how's unit 4 ? :smile:

I'm starting chemistry next weekend, as I have 3 exams before then. I haven't done any unit 4 since January :frown:

have unit 2 to prepare for as well ! :tongue:

damn it... I hope this all benefits my unit 5 as well though :smile:

Hahaha.

I decided against resitting unit 4, I think I might resit it next year instead. You're sitting unit 2, 4 and 5? Good luck!

Reply 1033

Gnome :)

Urgh. Just done the Jan13 paper and got an E.

:frown:

It's the memorising things that's letting me down; I can't remember any of the reactions, reagents, conditions etc, both organic and inorganic.

Time to plaster posters all over the house, methinks.

Reply 1034

posthumus

Original post by Gnome :)

Urgh. Just done the Jan13 paper and got an E.

:frown:

It's the memorising things that's letting me down; I can't remember any of the reactions, reagents, conditions etc, both organic and inorganic.

Time to plaster posters all over the house, methinks.

Don't worry too much about it at this stage, I was getting Us for unit 4 in December but then towards the exam in January I started getting atleast As in all my past papers (although in the end I got a D lol :redface:

)

... TSR helped me a lot ! :smile:

aha and that sounds like a good idea, have fun ! :tongue:

Reply 1035

GeorgeL3

Original post by Gnome :)

Urgh. Just done the Jan13 paper and got an E.

:frown:

It's the memorising things that's letting me down; I can't remember any of the reactions, reagents, conditions etc, both organic and inorganic.

Time to plaster posters all over the house, methinks.

Yeah that test was a bit **** wasn't it. I blame it on that copper question.
****ing copper.

Reply 1036

Gnome :)

Original post by posthumus

)

... TSR helped me a lot ! :smile:

aha and that sounds like a good idea, have fun ! :tongue:

Thanks

How are things coming along for you and your ridiculous number of exams? :tongue:

Reply 1037

Gnome :)

Original post by GeorgeL3

Yeah that test was a bit **** wasn't it. I blame it on that copper question.
****ing copper.

Stupid, stupid copper.

And it's not effing pink! I had bright pink in my mind, not coppery brownish "pink" so got really confused :frown:

Reply 1038

posthumus

Original post by Gnome :)

Thanks

How are things coming along for you and your ridiculous number of exams? :tongue:

aha okay so far I guess, thanks for asking :redface:

I have one tomorrow as well in fact ! :tongue:

The only retakes I'm dreading is for Chemistry really :frown:

How's unit 2? & pardon me if I've forgotten... are you doing unit 4 as well ? :smile:

Reply 1039

GeorgeL3

Original post by Gnome :)

Stupid, stupid copper.

And it's not effing pink! I had bright pink in my mind, not coppery brownish "pink" so got really confused :frown:

Yeah I know, even Wikipedia says a freshly exposed surface has a reddish-orange color.
Ah well, it's just copper and chromium then can ask us stuff like that for so as long as we learn them both it'll be fine.