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Chemistry: Post your doubts here!

GCE As and a level

GCE As and a level

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Metanoia said:
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 Examiners report said this question is withdrawn.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
Click to expand...
Great :)
The soloubility of Group 2 oxides and sulphates and hydroxides in water increases down the group?? :)
 
Hijab

Hijab

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Metanoia said:
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.
Click to expand...

Can u explain this question to me as well?
 

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Metanoia

Metanoia

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

Q23(C),Q29(A)
Click to expand...

w12qp12

Q23
The molecule must be broken in two and
1) contain same carbon atoms
2) both parts must have isomers

So this eliminates D, as 9 carbons molecule cannot form 2 smaller parts with equal carbons.
Isomers from when hydrocarbon have 4 or more carbon, so minimum we must have 8 carbons in the original chain to start with.

The bottom text is the examiners report, if what I typed earlier don't make sense.

"29% of candidates chose the correct answer, C. The most commonly chosen incorrect answer was A, chosen by 41% of candidates. If n=4 (option A) then the molecule C4H10 must be splitting to give C2H6 and C2H4, neither of which have structural isomers. The answer is C because if n=8 the products of cracking are

C4H10 and C4H8, both of which have structural isomers

Q29. Option A takes place under room temperature. The rest requires heating.
 
Metanoia

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Metanoia

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Hijab said:
Can u explain this question to me as well?
Click to expand...
Is the answer C?

The description means we have a mixture of carbonate
1) one which decomposes under heat to give CO2 (CaCO3)
2) one which does not decompose under heat, but later reacts with acid to give CO2 (BaCO3)
 
Hijab

Hijab

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Thank you... Yes its C
Metanoia said:
Is the answer C?

The description means we have a mixture of carbonate
1) one which decomposes under heat to give CO2 (CaCO3)
2) one which does not decompose under heat, but later reacts with acid to give CO2 (BaCO3)
Click to expand...
 
A

_Ahmad

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umair1161 said:
Hello guys!

Can you please help me with:

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

question 17!

Thanks in advance! :)
Click to expand...

Calculate the number of moles of oxygen using Pv=nRT
n=0.0121

they have given us the mass of the elements (1.15g) so we need to check which element will react with 0.0121 mol of oxygen

2Ca + O2 ----> 2CaO
n(Ca)=1.15/40.1=0.0287
using mole ratio 2 moles of Ca react with 1 mole of O2 therefore 0.0287 mole of calcium will react with 0.0143 mol of oxygen (so this is incorrect)

4Na + O2 ----> 2Na2O
n(Na)=1.15/23=0.05
using mol ratio 4 mole of sodium react with 1 mol of oxygen therfore 0.05 mol of sodium will react with 0.0125 mol of oxygen (so this is correct)

answer = D

Hope this makes sense.
 
MaboroshI_I

MaboroshI_I

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loveyuan said:
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w10_qp_11.pdf

question 29
idono why i cant solve isomer question everytime !!!! TT
Click to expand...

An easy, albeit time consuming way to figure these questions out is to simply draw the isomers. You know there's a double bond, so it must show geometric isomerism - cis and trans, that makes 2. But they aren't asking for just geometric isomers, they're asking for all the isomers. Another one would be but-1-ene. So that makes 3. I know this isn't the best way to work it out, but it's better than nothing. :)
 
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