Chemistry: Post your doubts here!

[Metanoia]

help plzView attachment 57400
this is a paper 5 question, that means no application booklet, so how am i supposed to write the equation without it

In acidic solutions, balance the equations in order of AOHE (atoms, oxygen, hydrogen and electrons).

1. Balance the atoms apart from oxygen and hydrogen.
2. Balance the oxygens by adding water molecules.
3. Balance the hydrogens by adding hydrogen ions.
4. Balance the charges by adding electrons.

For example, MnO4- to Mn2+

MnO4- --> Mn2+ .....................( Step 1 not necessary as Mn is already balanced)

MnO4- --> Mn2+ + 4H2O ................. (balancing the O atoms)

MnO4- + 8H+ --> Mn2+ + 4H2O .........(balancing the H atoms)

MnO4- + 8H+ + 5e- --> Mn2+ + 4H2O........... (balancing the charges)

You can try for the HCOO- to CO2.

 

[Metanoia]

Solve these :~
Q : 6, 8, 17, 18, 19, 30, 39 and 4o

https://www.xtremepapers.com/commun...qs-9701-and-9702-discussion-here.42450/page-4 (my doubts are at last posts of the page 4 and starting of page 5) Thanks.

Qn 6.
Original Mr (CaCl2) = 111
Final Mr (CaCl2.2H2O) = 147
% Increased in Mr =(36/111) x 100% = 32%

Qn8.
Moles of Cr2O7 2- = 0.00131 mol
Moles of Fe2+ = 0.00131 x 6 = 0.00786 mol
Mass of Fe = 0.00786 x 56 = 0.44 g
% of Fe = (0.44/1) x 100% = 44%

Qn17.
Describing an element that forms an amphoteric oxide, so answer is Al.

Qn 18.
P4O10 + 6H2O --> 4H3PO4

Qn 19.
CO2 is released when HCl is added to CaCO3.

Qn30.
You can post your isomers, and I'll help to see what's missing.

Qn39.
Statement 1: true, hydrazone is orange
Statement 2: false, ethanoic acid is colorless
Statement 3: false, ethan-1,2- diol is colorless.

Qn40.

 

[manya]

In acidic solutions, balance the equations in order of AOHE (atoms, oxygen, hydrogen and electrons).

1. Balance the atoms apart from oxygen and hydrogen.
2. Balance the oxygens by adding water molecules.
3. Balance the hydrogens by adding hydrogen ions.
4. Balance the charges by adding electrons.

For example, MnO4- to Mn2+

MnO4- --> Mn2+ .....................( Step 1 not necessary as Mn is already balanced)

MnO4- --> Mn2+ + 4H2O ................. (balancing the O atoms)

MnO4- + 8H+ --> Mn2+ + 4H2O .........(balancing the H atoms)

MnO4- + 8H+ + 5e- --> Mn2+ + 4H2O........... (balancing the charges)

You can try for the HCOO- to CO2.

Ooooo thankyu so much
jazakalla khair

 

[SadiaMaryam]

Qn39.
Statement 1: true, hydrazone is orange
Statement 2: false, ethanoic acid is colorless
Statement 3: false, ethan-1,2- diol is colorless.

But for Statement #2, I thought that ethanol + acidified potassium dichromate will turn from orange to green, forming an aldehyde, isn't that right??

 

[awesomaholic101]

23rd question please. If possible please draw out the isomers?
Is there any tip for these type of questions? Any particular way to work it out? I feel like it's only a waste of time coz in the end, i don't get it right anyway.

 

[krishnapatelzz]

why the answer is c and not d :/

 

[awesomaholic101]

View attachment 57442why the answer is c and not d :/

The question asks for the MAJOR product. The carbocation intermediate that is formed for C to be the product is more stable as it is tertiary. That is, the carbon atom with the +ve charge has 3 other carbons attached to it. The more stable the carbocation, the more of that product formed.
For D however, the intermediate carbocation would only be secondary.

 

[Metanoia]

But for Statement #2, I thought that ethanol + acidified potassium dichromate will turn from orange to green, forming an aldehyde, isn't that right??

It could be ethanal or ethanoic acid, depending on the conditions. The question is asking for an organic product that is colored, and neither ethanal or ethanoic acid is colored. The green color is due to the Cr3+ ions.

 

[Metanoia]

View attachment 57441
23rd question please. If possible please draw out the isomers?
Is there any tip for these type of questions? Any particular way to work it out? I feel like it's only a waste of time coz in the end, i don't get it right anyway.

No short cut. At the most, you can focus on the tertiary isomers first to narrow down the options to 2 choices. Then move on to the secondary isomers.

 

[awesomaholic101]

No short cut. At the most, you can focus on the tertiary isomers first to narrow down the options to 2 choices. Then move on to the secondary isomers.

ok ... so more practice atleast should help?

 

[nehaoscar]

When you say equilibrium shifts to the left, which equilibrium equation are you referring to? And how do you conclude it would shift to the left?

Since it's added to the Fe cell the equation would be FeSO4 == Fe^2+ + SO4^2-
So as Na2SO4 is added the concetration of SO4^2- ions increases so the equilibrium shifts to the left??

For weak acid,

[H+] = square root (Ka*[acid])

[H+] = square root (8.9 x 10^-4 *(0.1)) = 9.43 x 10^-3

pH = - log [H+] = - log (9.43 x 10^-3) = 2.02

For the second step you have done (8.9 x 10^-4 *(0.1)) instead the mark scheme has done (8.9 x 10^-4 *(0.2)) so the pH works out to be 1.87 instead of 2.02

So I'm confused how they got 0.2 as concentration :/

 

[SadiaMaryam]

It could be ethanal or ethanoic acid, depending on the conditions. The question is asking for an organic product that is colored, and neither ethanal or ethanoic acid is colored. The green color is due to the Cr3+ ions.

 

[The Sarcastic Retard]

View attachment 57441
23rd question please. If possible please draw out the isomers?
Is there any tip for these type of questions? Any particular way to work it out? I feel like it's only a waste of time coz in the end, i don't get it right anyway.

Practice. Once you will, it will be easy for you.

 

[awesomaholic101]

Practice. Once you will, it will be easy for you.

hopefully, and thanx

 

[The Sarcastic Retard]

Qn 6.
Original Mr (CaCl2) = 111
Final Mr (CaCl2.2H2O) = 147
% Increased in Mr =(36/111) x 100% = 32%

Qn8.
Moles of Cr2O7 2- = 0.00131 mol
Moles of Fe2+ = 0.00131 x 6 = 0.00786 mol
Mass of Fe = 0.00786 x 56 = 0.44 g
% of Fe = (0.44/1) x 100% = 44%

Qn17.
Describing an element that forms an amphoteric oxide, so answer is Al.

Qn 18.
P4O10 + 6H2O --> 4H3PO4

Qn 19.
CO2 is released when HCl is added to CaCO3.

Qn30.
You can post your isomers, and I'll help to see what's missing.

Qn39.
Statement 1: true, hydrazone is orange
Statement 2: false, ethanoic acid is colorless
Statement 3: false, ethan-1,2- diol is colorless.

Qn40.
View attachment 57440

Q30 : Are they correct?
Q39 : statement 2 isnt color changes from orange to green. No?
Q40 : How statement 1 is correct?
THanks.

 

[Metanoia]

Since it's added to the Fe cell the equation would be FeSO4 == Fe^2+ + SO4^2-
So as Na2SO4 is added the concetration of SO4^2- ions increases so the equilibrium shifts to the left??

That is not the correct half equation to use. The equation should be a redox equation.

Fe2+ --> Fe3+ + e-

Adding the Na+ and SO42- ions to the solution does not affect the equilibrium.

For the second step you have done (8.9 x 10^-4 *(0.1)) instead the mark scheme has done (8.9 x 10^-4 *(0.2)) so the pH works out to be 1.87 instead of 2.02
View attachment 57445
So I'm confused how they got 0.2 as concentration :/

I have relooked at the question and spotted my mistake.

The concentration of Fe2(SO4)3 is o.1 mol/dm3. But since we are using concentration of Fe3+ , we do need to multiply it by 2 as there are two Fe3+ ions for each Fe2(SO4)3.

I will be editing my original answer incase it misleads others.

 

[donewithtime80]

http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_w14_qp_12.pdf

hi, please help me solve these questions:
4 - no idea how to go about this
15- i got my answer as 0.2826... why multiply it *100?
28- idk why it's not B
31- V 3+ has 20 electrons and shells are filled... then how are the electrons unpaired ?
37- y c?

answers:

4- A
15- B
28- C
31-A
37- c

Thanks in Advance

 

[Metanoia]

Q30 : Are they correct? View attachment 57448
Q39 : statement 2 isnt color changes from orange to green. No?
Q40 : How statement 1 is correct?
THanks.

Qn 30. A bit hard for me to see your picture. I've attached my solution for you to compare.


Qn 39. The green color is due to the Cr3+ ions, not the organic products.

Qn 40. CN- is a negative ion and is thus a stronger nucleophile than the neutral HCN.

 

[The Sarcastic Retard]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w14_qp_12.pdf

hi, please help me solve these questions:
4 - no idea how to go about this
15- i got my answer as 0.2826... why multiply it *100?
28- idk why it's not B
31- V 3+ has 20 electrons and shells are filled... then how are the electrons unpaired ?
37- y c?

answers:

4- A
15- B
28- C
31-A
37- c

Thanks in Advance

4) 1 * 10^5 * v = 293 (n and R is const.)
v = 2.93 * 10^-3
Now, for whole system, volume is 4v
p * 4(2.93 * 10^-3) = 373
p = A.

15) n = 15 * 10^-3 * 2 = 0.03mol
25 cm^3 ---> 0.03 mol
250 cm^3 = 0.3mol
1 : 1 ratio
0.3 * (39+39+16) = B

28) Organic acid produced is butanoic acid (CH3CH2CH2COOH) C4H8O2 --> C2H4O = C

31) Electron removal is from 4s orbital. 2 electrons removed from that orbital and 1 wil be removed from 3d orbital so one unpaired electron present in 3d.

37) The answer is B not C.

 

[Metanoia]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w14_qp_12.pdf

hi, please help me solve these questions:
4 - no idea how to go about this
15- i got my answer as 0.2826... why multiply it *100?
28- idk why it's not B
31- V 3+ has 20 electrons and shells are filled... then how are the electrons unpaired ?
37- y c?

answers:

4- A
15- B
28- C
31-A
37- c

Thanks in Advance

Qn 4.


Qn 15. Most likely you've overlooked the info that only 25 cm3 out of the 250 cm3 was used for titration.

Qn 28.


Qn 31. You have to write out the configuration of V atom first and then remove electrons from the outer shells.

Qn 37. I have actually explained a similar question (qn 40) in this post
https://www.xtremepapers.com/commun...st-your-doubts-here.9859/page-732#post-942168