Chemistry: Post your doubts here!

[Suchal Riaz]

Can you help me with Q1 D,
22D,
30A,
33D,
37B of this paper
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w13_qp_13.pdf

if 1,1 dichloroethene polymerise the repeat unit will be -CF2-CH2-
if two repeat units combine the other way around it can be -CF2-CH2-CH2-CF2- of -CH2-CF2-CF2-CH2-
it can't be -CH2-CH2-CH2-CF2(pink color shows wrong group)
only the correct answer follows this pattern:
– CH2 – CH2 – CCl 2 – CCl 2 – CH2 – CH2 –

 

[GCE As and a level]

Can you help me with Q1 D,
22D,
30A,
33D,
37B of this paper
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w13_qp_13.pdf

22D>>This is direct from the book. The reaction with PCl5 is at room temperature,so this will give us the best yield
33D>>Its my doubt too
37B>>
1 Tollens’ reagent and a solution containing acidified dichromate(VI) ions --> It will react with this cuz its aldehyde
2 aqueous bromine and Fehling’s reagent -->Since its an aldehyde,it will react with Fehlings reaagent and as it contains double bond,it will react with Br2
3 2,4-dinitrophenylhydrazine reagent and sodium carbonate-->It will react with 2,4-DNPH , But it cannot react with sodium carmonate as there is no -COOH

Hope it helps

http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_w13_qp_13.pdf

 

[Metanoia]

Sir can you please elaborate Q20

The part on the OH being added or the number of rings?

 

[GCE As and a level]

if 1,1 dichloroethene polymerise the repeat unit will be -CF2-CH2-
if two repeat units combine the other way around it can be -CF2-CH2-CH2-CF2- of -CH2-CF2-CF2-CH2-
it can't be -CH2-CH2-CH2-CF2(pink color shows wrong group)
only the correct answer follows this pattern:
– CH2 – CH2 – CCl 2 – CCl 2 – CH2 – CH2 –

U mean CCl2 ??! Suchal Riaz

 

[Metanoia]

How can we do Q9?
View attachment 45172

Qn 9 :

moles of SO3 2- : moles of electrons : moles of metal
0.0025 : : 0. 005
1 : : 2
1 : 2 : 2

2 mol of metal gained 2 mol of electrons
1 mol of metal gained 1 mol of electron (oxidation state will thus decrease by 1)
Original oxidation state of metal = +3
Final oxidation state = +3 - 1 = +2

 

[Suchal Riaz]

Can you help me with Q1 D,
22D,
30A,
33D,
37B of this paper
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w13_qp_13.pdf

question33:
the questions says that methanoic acid IS stronger acid because it has a LOWER pH. it means that obviously it disassociates more. and if the rest two were true then it would have been weaker acid. tha

 

[ashcull14]

 

[Suchal Riaz]

Can you help me with Q1 D,
22D,
30A,
33D,
37B of this paper
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w13_qp_13.pdf

37:
dichromate will oxidise CHO to CO2H
tollens and fehling reagent will oxidise CHO to CO2H
aqueous bromine will react with C=C
so first two are correct
third is wrong because there is no -CO2H to react with Na2CO3

 

[ashcull14]

 

[Metanoia]

View attachment 45178

11qp12

short answer, cross the Al3+ and C4- charges, though it might not always work.

Long answer
Understand that whatever moles of C we have at the beginning will eventually be the same number of moles of C in CO2 at the end.

Let x and y be the ratio of Al and C respectively.
AlxCy --->???---> CO2

Working backwards from CO2,
moles of CO2 = 72/24 000 = 0.003 moles = moles of C in AlxCy

Going through the options
A Al 2C3, moles of Al2C3 = 0.144/90 = 0.0016, moles of C = 0.0016 x 3 = 0.0048 (incorrect)
B Al 3C4 , moles of Al3C4 = 0.144/129 = 0.0016, moles of C = 0.0016 x 4 = 0.0045 (incorrect)
C Al 4C3 , moles of Al4C3 = 0.144/144 = 0.001, moles of C = 0.001 x 3 = 0.003 (BINGO!)
D Al 5C3

 

[Suchal Riaz]

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i am working on it Metanoia
i had to erase some answer because you had already answered them. please tell me which one you are doing so i do other queries instead.

 

[ashcull14]

11qp12

short answer, cross the Al3+ and C4- charges, though it might not always work.

Long answer
Understand that whatever moles of C we have at the beginning will eventually be the same number of moles of C in CO2 at the end.

Let x and y be the ratio of Al and C respectively.
AlxCy --->???---> CO2

Working backwards from CO2,
moles of CO2 = 72/24 000 = 0.003 moles = moles of C in AlxCy

Going through the options
A Al 2C3, moles of Al2C3 = 0.144/90 = 0.0016, moles of C = 0.0016 x 3 = 0.0048 (incorrect)
B Al 3C4 , moles of Al3C4 = 0.144/129 = 0.0016, moles of C = 0.0016 x 4 = 0.0045 (incorrect)
C Al 4C3 , moles of Al4C3 = 0.144/144 = 0.001, moles of C = 0.001 x 3 = 0.003 (BINGO!)
D Al 5C3

THNKS N nice explanation

 

[ZaqZainab]

question33:
the questions says that methanoic acid IS stronger acid because it has a LOWER pH. it means that obviously it disassociates more. and if the rest two were true then it would have been weaker acid. tha

dumb me thanks again

w13qp13

Q1.
N in NH4+ is -3, increases by 4 units to from +1 in N2O
N in NO3- is +5, decreases by 4 units to from +1 in N2O

Q22.
A. Is not good as we might get multi substitutions
B. Is not good as we get dichloropropane
C. Cl- will not replace OH
D. Cl will replace at the OH position

Q30.
A. CH2-CH2-CCl2-CCl2-CH2-
Coloured are the repeating units that come from the monomers

Q33. Lower pH means higher conc of H+, due to more H+ released. Which statements are you confused about?


Q37. Which statement are you unsure of?
Statement 3: No COOH group to react with sodium carbonate

Thanks and for
Q37 the first one how can it be oxidized when no alcohol is present

 

[ZaqZainab]

i am working on it Metanoia
i had to erase some answer because you had already answered them. please tell me which one you are doing so i do other queries instead.

lol happens with me so many times but i don't erase it might help the person understand more

 

[GCE As and a level]

dumb me thanks again

Thanks and for
Q37 the first one how can it be oxidized when no alcohol is present

Aldehyde is present which get oxidised to -COOH

 

[Snowysangel]

ZaqZainab this is how to solve

Won't it be x2= 4 (1-x)^2 -> x= 2(1-x) -> x= 2-2x -> 3x=2 -> x=2/3?

 

[Suchal Riaz]

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calcium has 2+ charge so there must be two CO2H
in case (1) there will be two ethanoic acids: 2CH3CO2- = C4H6O4(2-)
in case two it is butandioc means there are di=two CO2H groups in one molecule: O2CCH2CH2CO2=C4H6O4(2-)
in case three: there is again same butanedioc acid(it's isomer) so same formula: C4H6O4(2-)
so (2) and (3) are correct

 

[Suchal Riaz]

Won't it be x2= 4 (1-x)^2 -> x= 2(1-x) -> x= 2-2x -> 3x=2 -> x=2/3?

yes i made a mistake. it had to be x²

 

[Suchal Riaz]

dumb me thanks again

Thanks and for
Q37 the first one how can it be oxidized when no alcohol is present

aldehyde will be oxidised to CO2H

 

[Ahmed Aqdam]

but the answer to question 23 is c tht is 4 esters

Yes, two with methanoic acid, one with ethanoic and one with propanoic. So total are 4.