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Chemistry: Post your doubts here!

Metanoia

Metanoia

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sweet.sugar said:
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s05_qp_1.pdf
can somebody plz explain some questions from m/j/5
Q1-C
Q2-B
Q13-B
Q14-C (but D is also correct the sum of first two ionization energies for ca is greater)
Q38-A
ANY KIND OF HELP WOULD BE APPRECIATED :)
Click to expand...

Q1.

CxHy + O2 --> CO2 + H2O

from info, 10 cm3 of CxHy reacts with 50 cm3 of O2 to produce 30 cm3 of CO2

1CxHy + 5O2 --> 3CO2 + ?H2O

Comparing both sides of equation,

balancing carbon : x =3
balancing oxygen : 10 = 6 + ? , therefore ? = 4

1CxHy + 5O2 --> 3CO2 + 4H2O

Finally, balancing H: y = 8.

Q2.
From 1st reaction , 1 mole of NaN3 produces 1 mole of Na and 1.5 mole of N2.

The 1 mole of Na produced in 1st reaction will be used in the 2nd reaction, producing 0.1 mol of N2.

So total N2 produced = 1.5 (from 1st reaction) + 0.1 (from 2nd reaction) = 1.6 moles

Q13.

Be and Al has what we call a diagonal relationship.

Be2+ has a smaller charge than Al3+, but Be2+ has a smaller radius than Al3+. This causes their charge density (charge/radius) to be similar which in turns allows them to have similar chemical properties.

This also applies to Li and Mg and other pairings..

Google "diagonal relationship" for more details.
http://chemistry.tutorvista.com/inorganic-chemistry/s-block-elements.html


Q14. Unlikely that Ca's IE is higher than Mg, as the electrons of Ca are in a further shell.
Sum of Mg 1st 2 IE = 736 + 1450
Sum of Ca 1st 2 IE = 590 + 1150

Q38.

C2H4 + H2 --> C2H6
C2H4 --> polyethene (polymerisation)
C2H4 + cold KMno4 --> HOCHCHOH (forming diol)
 
Metanoia

Metanoia

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IGCSE13 said:
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w10_qp_12.pdf Q10 ans B , Q18 ans b , Q20 ans C , Q25 ans D and Q38 ans A Please help if u can
Click to expand...

Q10. Graphite is made up of sheets of hexagonal rings, the angle among the carbon atoms are 120 degrees.

Q18.
A. (Wrong) Br2 is non-polar molecule (ID-ID intermolecular attractions), should be soluble in non-polar solvents.
B. (Correct) Br2 molecule mass is 80 x 2 = 160, which is heavier than air (Mr approximately 30).
C. (Wrong) Br2 vapourise easily (low boiling point) due to ID-ID intermolecular attraction.
D. (Wrong) Gaseous bromine is reddish brown.

Q20. Questions like these are quite time consuming if many atoms are involved. Might be more sensible to spend time on other questions first during the actual exam.

Picture 9.png

Q25. First 3 reactions require heating. The 4th one is reaction of 2,4-DNPH with carbonyl, which can be carried out at room temperature.

38.

I seemed to have 2 and 3 as the answer. The answer provided was A?
 
B

Browny

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http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w02_qp_1.pdf

Can anybody please explain questions 5,13,23,36 and 39?

The problems are as follows:

In 5 why can't A be correct as it is symmetrical.
In 13 why can't A(Mg) be the answer as when it dissolves the pH is around 6.5 which suggests hydrolysis.
In 23 why can't D(180) be the answer as the chain is like -C-C-C-C-C- which shows the C-C-C bond to be 180.
In 36 why is 2(NH3 behaves as a base) is correct.
In 39, actually I don't get a single point!

Thanks in advance and please try these questions out if possible as they are confusing and they might even help you out!:)
 
I

IGCSE13

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Metanoia said:
Q10. Graphite is made up of sheets of hexagonal rings, the angle among the carbon atoms are 120 degrees.

Q18.
A. (Wrong) Br2 is non-polar molecule (ID-ID intermolecular attractions), should be soluble in non-polar solvents.
B. (Correct) Br2 molecule mass is 80 x 2 = 160, which is heavier than air (Mr approximately 30).
C. (Wrong) Br2 vapourise easily (low boiling point) due to ID-ID intermolecular attraction.
D. (Wrong) Gaseous bromine is reddish brown.

Q20. Questions like these are quite time consuming if many atoms are involved. Might be more sensible to spend time on other questions first during the actual exam.

View attachment 44361

Q25. First 3 reactions require heating. The 4th one is reaction of 2,4-DNPH with carbonyl, which can be carried out at room temperature.

38.

I seemed to have 2 and 3 as the answer. The answer provided was A?
Click to expand...
Sorry you are right the answer to 38 is C but in Q20 there are 3 carbon atoms not 4
 
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Snowysangel

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Browny said:
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w02_qp_1.pdf

Can anybody please explain questions 5,13,23,36 and 39?

The problems are as follows:

In 5 why can't A be correct as it is symmetrical.
In 13 why can't A(Mg) be the answer as when it dissolves the pH is around 6.5 which suggests hydrolysis.
In 23 why can't D(180) be the answer as the chain is like -C-C-C-C-C- which shows the C-C-C bond to be 180.
In 36 why is 2(NH3 behaves as a base) is correct.
In 39, actually I don't get a single point!

Thanks in advance and please try these questions out if possible as they are confusing and they might even help you out!:)
Click to expand...
Is 39 C?
 
Metanoia

Metanoia

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Browny said:
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w02_qp_1.pdf

Can anybody please explain questions 5,13,23,36 and 39?

The problems are as follows:

In 5 why can't A be correct as it is symmetrical.
In 13 why can't A(Mg) be the answer as when it dissolves the pH is around 6.5 which suggests hydrolysis.
In 23 why can't D(180) be the answer as the chain is like -C-C-C-C-C- which shows the C-C-C bond to be 180.
In 36 why is 2(NH3 behaves as a base) is correct.
In 39, actually I don't get a single point!

Thanks in advance and please try these questions out if possible as they are confusing and they might even help you out!:)
Click to expand...

Q5. The molecule is not symmetrical, you have to view it in its 3D shape, it is tetrahedral.

Q6. MgCl2 dissolves, SiCl4 hydrolyses.

Q23. Each carbon has 4 single bonds, they are tetrahedral shape around each carbon.

Q36. NH3 accepts a H+ to form NH4+, it is a proton acceptor (base)

Q39.
H2SO4 (acid) donates a proton to C2H5OH.
 
080595kat

080595kat

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Can anyone help me with this question?


11 0.144 g of an aluminium compound X react with an excess of water, to produce a gas. This gas
burns completely in O2 to form H2O and 72 cm3 of CO2 only. The volume of CO2 was measured at
room temperature and pressure.
What could be the formula of X?
[C = 12.0, Al = 27.0; 1 mole of any gas occupies 24 dm3 at room temperature and pressure]
A Al 2C3 B Al 3C4 C Al 4C3 D Al 5C3
 
♣♠ Magnanimous ♣♠

♣♠ Magnanimous ♣♠

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080595kat said:
Can anyone help me with this question?


11 0.144 g of an aluminium compound X react with an excess of water, to produce a gas. This gas
burns completely in O2 to form H2O and 72 cm3 of CO2 only. The volume of CO2 was measured at
room temperature and pressure.
What could be the formula of X?
[C = 12.0, Al = 27.0; 1 mole of any gas occupies 24 dm3 at room temperature and pressure]
A Al 2C3 B Al 3C4 C Al 4C3 D Al 5C3
Click to expand...
It can be done by common sense too
see in options you can see only Al and C so what will happen when they react :
Untitled.png
 
Metanoia

Metanoia

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Hi all,

in view of the lack of solutions for past year MCQ papers, I've attempted to create a youtube playlist to explain the answers.

It is a long and laborious process, it is not possible for me to cover a large number of practice papers before your exams on 10 June, I view it as more of a long term project for future batches.

I'm sure that there will be mistakes in my videos as they are done in 1 long sitting, so feel free to point out any mistakes or clarify concepts.

If the videos prove to be useful, I will slowly add on to them in the future.

Here is the link to the June 2013 P11.
http://www.youtube.com/playlist?list=PL4Jmce8VJnNXQDPOYEr7j99fO9nIy55eh

I'll create start a thread elsewhere so the video links can be neatly maintained over time.
 
$$AK$$

$$AK$$

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Hi guys
How are u all?
i need help in http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_w13_qp_12.pdf
Q8)
Q9)
Q13)
Q16)
Q17)
Q20)
THANK YOU GUYS
Even if u can help me only in few of them , I have NO PROBLEM :D
Metanoia said:
Hi all,

in view of the lack of solutions for past year MCQ papers, I've attempted to create a youtube playlist to explain the answers.

It is a long and laborious process, it is not possible for me to cover a large number of practice papers before your exams on 10 June, I view it as more of a long term project for future batches.

I'm sure that there will be mistakes in my videos as they are done in 1 long sitting, so feel free to point out any mistakes or clarify concepts.

If the videos prove to be useful, I will slowly add on to them in the future.

Here is the link to the June 2013 P11.
http://www.youtube.com/playlist?list=PL4Jmce8VJnNXQDPOYEr7j99fO9nIy55eh

I'll create start a thread elsewhere so the video links can be neatly maintained over time.
Click to expand...
 
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