Chemistry: Post your doubts here!

[Student12]

When ethanal, CH3CHO, reacts with HCN and the organic product is hydrolysed by aqueous
acid, organic compound Y is formed.
When propanal, C2H5CHO, is heated under reflux with acidified potassium dichromate(VI),
organic compound Z is formed.
What is the difference in relative molecular mass of compounds Y and Z?
A 12 B 14 C 16 D 17

how is it C ?

 

[user]

Moreover if you google images of bromine vapour -you can see it's reddish brown not purple

nopes..i knew that...!

i don't know what's wrong i downloaded these papers before the site update..and the ms i have says its D

check the attachment..

 

[mushoo]

URGEEEEEENNNTTTTTTTT...OCTO NOV...2011 VARIENT 11 PAPER 1...Q 6,914,15,20,23,25,28,29...plzz....


http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_w11_qp_11.pdf

see, we just have to divide the energy that is given by 6.
cos while the have broken N-F bond 6 tyms, so simply divide it by 6 to get the bond energy fr 1! -_-

 

[gary221]

Hey any idea how 2 solve this??

Ans is C

 

[Zsiddiqui]

question 11 and 23 :/
http://www.xtremepapers.com/papers/CIE/Cambridge%20International%20A%20and%20AS%20Level/Chemistry%20(9701)/9701_s11_qp_12.pdf
http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_s11_ms_12.pdf

 

[KurayamiKimmi]

Kimmi pls try ths ques.:/

A monomer undergoes addition polymerization.A 1 mol sample of the monomer is completely polymerized.
How many moles of polymer might , theoretically, be formed?
1. 1
2. 10^-6
3. 1/ 6.02*10^23 (1/ Avogadro's cons.)

the ans is C, cox 1 is nt the ans.
but hw cum the ans are 2 &3 ??

this is because many monomers join up to make one mole of a polymer
for example 10,000 monomers will join up to give one mole of polymer
so one mole of a monomer will be equal to only 1/10,000th mole of the polymer
therefore the answer has to be lesser than 1

 

[user]

Assalamoalaikum wr wb!

Since when is gaseous bromine purple...

This question is from N'10_12 Q 18

View attachment 8952

its not
and the answer is B i think

the ans is B ofcrse
as the air is almst of nitrogen and oxygen,
and O2 = 32
N2 = 28
total mass = 60.

while Br2 itself is 160.
threfre its simply denser than air! :/

nopes..i knew that...!

i don't know what's wrong i downloaded these papers before the site update..and the ms i have says its D

check the attachment..

i really dunno what has happened...i had the ms and er downloaded before the site update....and in both of them answer is stated as B

and in the paper i checked now on the site..in both ms and er it says D

 

[hm12]

When ethanal, CH3CHO, reacts with HCN and the organic product is hydrolysed by aqueous
acid, organic compound Y is formed.
When propanal, C2H5CHO, is heated under reflux with acidified potassium dichromate(VI),
organic compound Z is formed.
What is the difference in relative molecular mass of compounds Y and Z?
A 12 B 14 C 16 D 17

how is it C ?

CH3CHO+HCN---->CH3CH(CN)OH which when hydrolysed by acid gives CH3CH(COOH)OH Mr of Y therefore=90
C2H5CHO when oxidised gives C2H5COOH Mr=74
therefore difference=90-74=16

 

[KurayamiKimmi]

nopes..i knew that...!

i don't know what's wrong i downloaded these papers before the site update..and the ms i have says its D

check the attachment..

yea but sometimes the marking scheme can also be wrong -(its made by humans afterall)
and besides you just can't agree to option D -its absurd!

 

[lavanyamane]

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_s08_qp_1.pdf
Q.2 please?

 

[user]

yea but sometimes the marking scheme can also be wrong -(its made by humans afterall)
and besides you just can't agree to option D -its absurd!

i know...but i'm kinda shocked..when it's all same how does the answer for this question changed
anyway thanks...

i was thinking how at all can it be purple, when we've always been learning its brown..

 

[Zsiddiqui]

question 11 and 23 :/
http://www.xtremepapers.com/papers/CIE/Cambridge%20International%20A%20and%20AS%20Level/Chemistry%20(9701)/9701_s11_qp_12.pdf
http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_s11_ms_12.pdf

 

[roadtrip9o9]

http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_s11_qp_11.pdf question 2 , 5,8,9,10,11,13,16,21,23,31,34

 

[Asimadnan]

May/June 11 P 12 ques 11.
can any one plz explain the answer!

 

[KurayamiKimmi]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s08_qp_1.pdf
Q.2 please?

N : P :K ratio
15:30:15
percentage nitrogen =(15/15+30+15) =0.25*100
25% of fertiliser;s mass is Nitrogen
reccommended usage is 14g
% of nitrogen in this =(25/100)*14
mass of nitrogen=3.5 g
no of moles =mass/Mr =3.5/14*2=0.125
concentration of N=0.125/5=0.025=0.03

 

[Casablanca]

May/June 11 P 12 ques 11.
can any one plz explain the answer!

Yeah, I don't know this one either. I'll type the question so it's easier for people to answer:

0.144 of an aluminum compound X react with an excess of water to produce a gas. This gas burns completely in O2 to form H2O and 72cm^3 of CO2 only. The volume of CO2 was measured at room temp. and pressure. What could be formula of X?

A. Al2C3
B. Al3C4
C. Al4C3
D. Al5C3

I thought it was C, but I've been wrong before, so I don't know...

 

[geek101]

Hey any idea how 2 solve this??
View attachment 8956
Ans is C

1 cant be right cuz when you heat under reflux with alkali the -Cl is going to be replaced by -OH, on acidifying dehydration will occur so no way a carboxylic acid is produced. if one is wrong the answer is C

 

[geek101]

Yeah, I don't know this one either. I'll type the question so it's easier for people to answer:

0.144 of an aluminum compound X react with an excess of water to produce a gas. This gas burns completely in O2 to form H2O and 72cm^3 of CO2 only. The volume of CO2 was measured at room temp. and pressure. What could be formula of X?

A. Al2C3
B. Al3C4
C. Al4C3
D. Al5C3

I thought it was C, but I've been wrong before, so I don't know...

a member has answered this on one of the recent pages, you can see there

 

[KurayamiKimmi]

http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_w10_qp_11.pdf

Can anyone Please solve questions 5-10-12-21? pretty please ^^

 

[Casablanca]

When trying to increase the value of Kc for an exothermic reaction, should the temperature be increased or decreased? :/