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Author Topic: Chemistry AS Multi Choice Help  (Read 9775 times)

Offline MaNi_DaDuDe

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Re: Chemistry AS Multi Choice Help
« Reply #30 on: November 19, 2009, 04:32:38 am »
I think you take out the moles for each then the %yield ..

I can't type the working on my phone; sorry. :(

Quote from: happy angel on November 19, 2009, 04:11:16 am
Use of the Data Booklet is relevant to this question.

Ethyl ethanoate can be obtained from ethanoic acid and ethanol by the following reaction.

CH3CH2OH + CH3CO2H--> CH3CO2CH2CH3 + H2O

Ethanol (30 g) and ethanoic acid (30 g) are heated under reflux together, and 22 g of ethyl
ethanoate are obtained.

What is the yield of the ester?
A 25 % B 38 % C 50 % D 77 %

the answer is C.. how???
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Offline happy angel

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Re: Chemistry AS Multi Choice Help
« Reply #31 on: November 19, 2009, 04:59:14 am »
its ok mani... i'll work it ouuuuuut.. Thanks anw... am so sleepy!!! my mind not working!
lol... have a nice day...
n Thanks anyway...
c genti
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Offline MaNi_DaDuDe

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Re: Chemistry AS Multi Choice Help
« Reply #32 on: November 19, 2009, 05:18:05 am »
Quote from: happy angel on November 19, 2009, 04:59:14 am
its ok mani... i'll work it ouuuuuut.. Thanks anw... am so sleepy!!! my mind not working!
lol... have a nice day...
n Thanks anyway...
c genti


Yes, it's 9 AM at your end.

Get some rest and then attempt papers with a fresh mind.

Good night, err morning.

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Offline iq101

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Re: Chemistry AS Multi Choice Help
« Reply #33 on: November 19, 2009, 11:24:24 am »
Can someone please do November 07 question 16?!
Why cant it be A?
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Offline happy angel

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Re: Chemistry AS Multi Choice Help
« Reply #34 on: November 19, 2009, 01:33:07 pm »
Quote from: iq101 on November 19, 2009, 11:24:24 am
Can someone please do November 07 question 16?!
Why cant it be A?

hey dude the answer IS A.. LOLSS check the marking scheme well... ::)
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Offline iq101

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Re: Chemistry AS Multi Choice Help
« Reply #35 on: November 19, 2009, 02:31:58 pm »
Quote from: happy angel on November 19, 2009, 01:33:07 pm
hey dude the answer IS A.. LOLSS check the marking scheme well... ::)

I mean why cant it be C :P sorry.
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Offline 123

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Re: Chemistry AS Multi Choice Help
« Reply #36 on: November 19, 2009, 03:05:52 pm »
Quote from: iq101 on November 19, 2009, 02:31:58 pm
I mean why cant it be C :P sorry.
Man put up the question...there will b less confusion!! heres the question:

16 Compound X on refluxing with aqueous sodium hydroxide gave mixture Y which on distillation
with acidified potassium dichromate(VI) produced propanone. Mixing Y with dilute nitric acid and
aqueous silver nitrate gave a cream precipitate.
What could compound X be?
A CH3CHBrCH3
B CH3CHICH3
C CH3CH2CH2Br
D CH3CH2CH2I

Ok so ur right that is could C...since it has Br...which is the only 1 that gives cream ppt
But if u react choice C  with NaOH u get  a primary alcohol, right???
Then if u react that primary alcohol with dichromate u get a aldehyde & finally carboxylic acid.
But the question says that u get ketone(propanone)...which is the oxidation a secondary alcohol
So if u react choice A with NaOH u get Secondary alcohol as Br replaces OH on that 2nd C atom
Then that secondary alcohol gives ketone on oxidation....

I hope this proves its A.... :P
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Offline 123

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Re: Chemistry AS Multi Choice Help
« Reply #37 on: November 19, 2009, 03:18:38 pm »
Quote from: happy angel on November 19, 2009, 04:11:16 am
Use of the Data Booklet is relevant to this question.

Ethyl ethanoate can be obtained from ethanoic acid and ethanol by the following reaction.

CH3CH2OH + CH3CO2H--> CH3CO2CH2CH3 + H2O

Ethanol (30 g) and ethanoic acid (30 g) are heated under reflux together, and 22 g of ethyl
ethanoate are obtained.

What is the yield of the ester?
A 25 % B 38 % C 50 % D 77 %
the answer is C.. how???
Hmmm... i thought it was B. 38%.  It should b C. 50%.... heres why i think:
 using mass & Mr of ethanol & ethanoic acid
u find n moles of each reactants.
So
n(ethanol) = m/Mr = 30/46 = 0.65mol
n(ethanoic acid) = m/Mr = 30/60 = 0.5mol
As 1mol of reactants give 1 mol of ester
Ethanol is in excess, so all of it will not b used to make ester
But ethanoic acid will b used up fully (limiting reagent)to make that ester
So use the ethanoic acid
n moles acid = n moles ester.  Find the theoretical mass of ester
theo. mass of eser = Mr x n    ;  actual m = 30g
Then,      % yield= (actual m / theo. m) x 100%

I hope this is right!  :o



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Offline iq101

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Re: Chemistry AS Multi Choice Help
« Reply #38 on: November 19, 2009, 03:22:20 pm »
Quote from: 123 on November 19, 2009, 03:05:52 pm
Man put up the question...there will b less confusion!! heres the question:

16 Compound X on refluxing with aqueous sodium hydroxide gave mixture Y which on distillation
with acidified potassium dichromate(VI) produced propanone. Mixing Y with dilute nitric acid and
aqueous silver nitrate gave a cream precipitate.
What could compound X be?
A CH3CHBrCH3
B CH3CHICH3
C CH3CH2CH2Br
D CH3CH2CH2I

Ok so ur right that is could C...since it has Br...which is the only 1 that gives cream ppt
But if u react choice C  with NaOH u get  a primary alcohol, right???
Then if u react that primary alcohol with dichromate u get a aldehyde & finally carboxylic acid.
But the question says that u get ketone(propanone)...which is the oxidation a secondary alcohol
So if u react choice A with NaOH u get Secondary alcohol as Br replaces OH on that 2nd C atom
Then that secondary alcohol gives ketone on oxidation....

I hope this proves its A.... :P

THANKYOU so much !!!
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Offline 123

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Re: Chemistry AS Multi Choice Help
« Reply #39 on: November 19, 2009, 05:00:37 pm »
Your welcome!  1up on my reputation power will b much appreciated!!  ;D

Anyone figured out what questions will  b repeated...in Chemistry paper 1 O/N 2009???
Which paper r rest of u guys taking - paper 11 or paper 12???
I'm gonna do paper 12 (9701/12)
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Offline vince_24

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Re: Chemistry AS Multi Choice Help
« Reply #40 on: November 19, 2009, 06:56:07 pm »
Taking /12 in 1 hr 30 min ;d
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Offline MaNi_DaDuDe

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Re: Chemistry AS Multi Choice Help
« Reply #41 on: November 19, 2009, 08:27:23 pm »
Quote from: vince_24 on November 19, 2009, 06:56:07 pm
Taking /12 in 1 hr 30 min ;d

Ditto

Good luck!
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Offline MaNi_DaDuDe

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Re: Chemistry AS Multi Choice Help
« Reply #42 on: November 19, 2009, 11:48:57 pm »
WHAT A RETARDED PAPER.  >:(
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Offline 123

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Re: Chemistry AS Multi Choice Help
« Reply #43 on: November 19, 2009, 11:59:43 pm »
HUH...u already gave the paper1/12/11??
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Offline AndrewCedric

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Re: Chemistry AS Multi Choice Help
« Reply #44 on: November 20, 2009, 03:14:20 am »
Quote from: MaNi_DaDuDe on November 19, 2009, 11:48:57 pm
WHAT A RETARDED PAPER.  >:(

Wht happened?? Just did paper 12.. its okAy.. ;)
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