Qualification > Sciences
Chemistry AS Multi Choice Help
iq101:
--- Quote from: happy angel on November 19, 2009, 01:33:07 pm ---hey dude the answer IS A.. LOLSS check the marking scheme well... ::)
--- End quote ---
I mean why cant it be C :P sorry.
123:
--- Quote from: iq101 on November 19, 2009, 02:31:58 pm ---I mean why cant it be C :P sorry.
--- End quote ---
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
123:
--- 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???
--- End quote ---
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
iq101:
--- 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
--- End quote ---
THANKYOU so much !!!
123:
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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