Chemistry: Post your doubts here!

[Ahmed Aqdam]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w09_qp_12.pdf
Q3 answer is C buts isn't it suppose to be A?
Q20 answer is A
Q23 isn't moles of H2 suppose to be 3?? answer is A
Q28 answer is B
Q33 isnt 3 suppose to be correct too if the activation energy is higher than 500 the reaction wouldnt have even started
Q40 i have no idea answer is C

3: x is 104 y is 109 z is 107 so C is correct.
y has no lone pair so greatest angle x has two so smallest.
20:

Q2o. IF all carbons are single bonds, CnH2n+2O, C20H42O (20 carbons , 42 hydrogen, 1 oxygen)

Since 1 aldehyde group is present, subtract 2 H, C20H40O (20 carbons , 40 hydrogen, 1 oxygen)

Since it contains a ring, subtract another 2H, C20H38O (20 carbons , 38 hydrogen, 1 oxygen)

Since retinal contains only 28 H, the missing 1o H are due to 5 double bonds.

23: H2 does not break CO bond.

28: Reactant should have 1 less carbon as one is added by reaction with sodium cyanide. So C is wrong.
CN will be attached in place of Br and then it will form carboxylic acid. Only B will form the correct carboxylic acid.

33:The activation energy remains same at different temperatures.

40: A would mean polymer has same number of molecules as monomer and would be identical to it so not possible.
If all 6.02x10^23 molecules react, it will form one molecule but 1/6.02x10^23 moles.

 

[Hijab]

Can some one plz plz plz explain the question to me... I would really be grateful

 

[Metanoia]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w06_qp_1.pdf

Q3(C), 4(D), 21(C) and 25(B)
Need help!

w06qp1

Q3.
P is option A
Ca is option B
Kr is option D
So that leaves element X as option C.

Q4.
Use any of the alkane as an example and construct a balance equation for complete combustion, for here, I'll use CH4

I prefer to use table, but not sure if the alignment shows when typed out.
CH4 (g) + 2O2 (g) --> CO2 (g) + 2H2O (l)
10 .............70 ................0 .................. - .......... initial
-10 ............-20 ............ +10 ............. - .......... change
0 ............ 50 .............. 10 ............ - ............ final

Total gases at the end = 50 + 10 = 60 cm3, this fits option D.

Q21
This is a bit tedious, you need to draw and figure out the number of structural isomers for the following halogenalkanes.
C2H5Cl (1 isomer)
C2H4Cl2 (2 isomers)
C2H3Cl3 (2 isomers)
C2H2Cl4 (2 isomers)

Q25. Divide by n throughout for easy balancing.
C3H6 + O2 --> CO2 + H2O
C3H6 + 4.5 O2 --> 3CO2 +3H2O

 

[Metanoia]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w09_qp_12.pdf
Q3 answer is C buts isn't it suppose to be A?
Q20 answer is A
Q23 isn't moles of H2 suppose to be 3?? answer is A
Q28 answer is B
Q33 isnt 3 suppose to be correct too if the activation energy is higher than 500 the reaction wouldnt have even started
Q40 i have no idea answer is C

I'll just add on to Q40 since Ahmed Aqdam has addressed the other questions.

Q40. Requires a bit of reasoning, if 1 mole of monomer join together, we will get less than 1 mole of polymer.
If we end up with 1 mole of polymer in the end, it means none of the monomer has joined to any other!

Its like 1000 bricks (monomer) will build one house (polymer) , the number of monomer is expected to be more than polymers.
Hope it makes sense.

 

[Metanoia]

Can some one plz plz plz explain the question to me... I would really be grateful

There is a lone pair of electrons on the N, so there are 2 bond pairs and 1 lone pair. The closest angle would be 120 degrees.

 

[Metanoia]

For May June 2012 paper variant 11

Please explain
--> q 7 the answer is c
Q 6 the answer is B
Question 13 answer is 13

Q 14 answer is B
Question 38 answer is A

Dunno but found this paper real tough! Please help!!!!

Q6. need to make sure units are correct
pV= nRT
R = 8.31
T= 303K
n= 0.56/28= 0.02 mol
p= 120 000 Pa

You can solve for v (in terms of m^3)

Q7.
Formation of propanone

3C (s) + 3H2 (g) + 1/2 O2 (g)--> C3H6O (g)

heat of reaction = heat of combustion of reactants- heat of combustion of products
= 3(-394)+3(-286)-(-1786)
=-254

Q13. ? What answer?

Q14.
I'm guess the hurdle is getting the balanced equation?

10Al + 3Ba(NO3)2 --> 5Al2O3 + 3BaO + 3N2

moles of barium nitrate = 0.783 / 261 = 0.003
moles of N2 = 0.003
vol of N2 at RTP = 0.003 x 24000 = 72 cm^3

Q38.
Mr of alcohol = 74
Moles of alcohol used = 70/74 = 0.946
Moles of alcohol converted into products = 0.946 x 62% = 0.586

1. If 0.586 mol of butanone was obtained, it would weight 0.586 x Mr of butanone = 0.586 x 72 = 42.2 g (statement 1 true)
2. If 0.586 mol of butanoic acid was obtained, it would weight 0.586 x Mr of butanoic acid = 0.586 x 88 = 51.6 g (statement 2 true)
3. If 0.586 mol of 2-methylpropanoic acid was obtained, it would weight 0.586 x 2-methylpropanoic acid = 0.586 x 88 = 51.6 g (statement 3 true)

 

[Hijab]

I

There is a lone pair of electrons on the N, so there are 2 bond pairs and 1 lone pair. The closest angle would be 120 degrees.

Isn't it that atoma with 2 bond pairs and 1 lone pair have bond angle between 104-107??

 

[Metanoia]

I

Isn't it that atoma with 2 bond pairs and 1 lone pair have bond angle between 104-107??

Nope, 2 bonds and 2 lone pairs will be around 104.

Something like this
4 bond 0 lone = 109.5
3 bond 1 lone = 107
2 bond 2 lone = 104.5

 

[Hijab]

Nope, 2 bonds and 2 lone pairs will be around 104.

Something like this
4 bond 0 lone = 109.5
3 bond 1 lone = 107
2 bond 2 lone = 104.5

So for 2 bond pairs and 1 Lone pair its around 120?

 

[Metanoia]

So for 2 bond pairs and 1 Lone pair its around 120?

If 3 bond, 0 lone would be 120.

If 2 bond, 1 lone, would be slightly below 120, still approximately 120 given the 4 options in the MCQ.

 

[GCE As and a level]

If 3 bond, 0 lone would be 120.

If 2 bond, 1 lone, would be slightly below 120, still approximately 120 given the 4 options in the MCQ.

Did u reached my second doubt or still :/
Plz i need help

 

[_Ahmad]

http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_w12_qp_12.pdf

Q23(C),Q29(A)

 

[doremon]

http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_s12_qp_11.pdf

Question 2 . Any help will be highly appreciated.

 

[doremon]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w12_qp_12.pdf

Q23(C),Q29(A)

for question number 29 read the COURSEBOOK pg-240

 

[ZaqZainab]

3: x is 104 y is 109 z is 107 so C is correct.
y has no lone pair so greatest angle x has two so smallest.
20:

23: H2 does not break CO bond.

28: Reactant should have 1 less carbon as one is added by reaction with sodium cyanide. So C is wrong.
CN will be attached in place of Br and then it will form carboxylic acid. Only B will form the correct carboxylic acid.

33:The activation energy remains same at different temperatures.

40: A would mean polymer has same number of molecules as monomer and would be identical to it so not possible.
If all 6.02x10^23 molecules react, it will form one molecule but 1/6.02x10^23 moles.

I thought more the lone pairs more the replution and hence more the angle
Q20 i am confused about A and B??

 

[ZaqZainab]

I'll just add on to Q40 since Ahmed Aqdam has addressed the other questions.

Q40. Requires a bit of reasoning, if 1 mole of monomer join together, we will get less than 1 mole of polymer.
If we end up with 1 mole of polymer in the end, it means none of the monomer has joined to any other!

Its like 1000 bricks (monomer) will build one house (polymer) , the number of monomer is expected to be more than polymers.
Hope it makes sense.

so it can be any number more than 1???

 

[Hijab]

If 3 bond, 0 lone would be 120.

If 2 bond, 1 lone, would be slightly below 120, still approximately 120 given the 4 options in the MCQ.

Thankyou

 

[aaaamfa]

I am terribly sorry. Here is the correct explanation. Hope this helps

equilibrium: x moles of R, therefore there must be 2x moles of Q (as the ratio of Q to R is still 2:1). P will be 2-2x moles (we subtract Q from P this time because P is the same as Q in equilibrium in this equation)
Add them up: x + 2x + 2 - 2x = x + 2 so B is the correct answer

Oh ok just one last question how did u get P as 2-2x shouldnt it be 2x same as Q

 

[Metanoia]

Metanoia , ZaqZainab ,@ EVERY ONE
Plz guys i need help in these
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w05_qp_1.pdf
Q3...Answer B>>>I cant understand the question :s
Q12..Answer D>>>Shouldnt the answer be B ,cuz for Mg--> Mg+0.5 O2=MgO
for Al --> 2Al+1.5 O2=Al2O3
for S --> S+O2 = So2
Q17..Answer B>>>I cant understand a single word
Q18..Answer C
Q20..Answer B>>>Should i include the Structural isomer also ?
Q24..Answer C
Q30..Answer B
Q31..Answer A>>>Wat the exactly want??
Q33..Answer A>>> How could i know that 2 and 3 are also correct
Q34..Answer B

Q_IS there any way that could help me to know if the reaction is Electrophillic or Nucleophillic??
THNX GUYS

w05qp1

Q5. Least tedious method is to use data booklet and look at the the 2nd IE of
A. Mg
B. Al
C. Si
D. P

You will see that Al has lower 2nd IE (1820) compared to the elements next to it on the Periodic Table, Mg (1450) and Si (1580)

Q12. Examiners report said this question is withdrawn. My suspicions is that students were expected to view sulfur element as S8, which many did not.

Q17. Ammonia donates a lone pair of electron to Ag+ and forms a ligand. This might not be in your syllabus since its under transistion metals.

Q18. Apply concept of ammonium salt + base --> salt + water + NH3.
Nitrogen nutrient would be lost in when NH3 is released.

Q20. Draw the isomers you manage to get, and we'll see what you are missing.

Q24. Are you able to draw the alkenes that each option gives? Then I point out whats missing.

Q30. Look at both ends, the polymer is held by ester bonds. So hydrolysis will break it up.

Q31. The chlorine molecule is made up of Cl-37 and Cl-35. Radius is the same for both. Nucleon and mass is different.

Q33. They gave the clue that its similar structure to graphite.

Q34. Statement 1 means eqm 1 shift to the right as OH neutralised, Ca5(PO4)3OH (s) thus dissolves

Statement 2 means eqm 2 shift to the left, PO4 3- decreases.
When PO4 3- decreases, eqm 1 shift to right, Ca5(PO4)3OH (s) thus dissolves

 

[Metanoia]

so it can be any number more than 1???

from 1 mole of monomer, the moles of polymer formed can be any number less than 1.