Chemistry: Post your doubts here!

[Mstudent]

Which pair of reactions could have the same common intermediate?
W CH3CH2CH3 → intermediate → (CH3)2CHCN
X CH3CH(OH)CH3 → intermediate → (CH3)2C(OH)CN
Y CH3CH=CH2 → intermediate → CH3CH(OH)CH3
Z CH3CO2CH2CH2CH3 → intermediate → CH3CH2CH2Br
A W and X B W and Y C X and Z D Y and Z

Ans is B

The intermediate for
W: CH3CHCH3+ (Hydrogen is lost from central carbon )
X: CH3C(OH)CH3+ (Hydrogen is lost from central carbon)
Y:CH3CHCH3+ (Hydrogen is ADDED to Ch2 due to markonikov's rule)
Z:CH3CH2CH2OH (Alcohol from ester hydrolysis)

So W and Y have the same intermediates!

hope this helped

 

[dumbledore.]

Ans: C

 

[Zaki ali asghar]

Yo, listen, you confused it with hydrolysis of ester with dilute Sodium Hydroxide which does result in a sodium salt of carboxyllic acid and the alcohol!

thanks man,got it.

 

[dumbledore.]

Ans: C

1. Neutral.. Carboxyl group acidic iss liye D k ilawa sab neutral
2. Orange ppt with 2 4 dnph...SIRF aldehyde aur ketones dete hain ppt. B mein ester hai so NO
3. HCl with PCl5...sirf alcohol aur carboxylic acid dete hain. Iss liye C

 

[Mstudent]

1. Neutral.. Carboxyl group acidic iss liye D k ilawa sab neutral
2. Orange ppt with 2 4 dnph...SIRF aldehyde aur ketones dete hain ppt. B mein ester hai so NO
3. HCl with PCl5...sirf alcohol aur carboxylic acid dete hain. Iss liye C

Could you plz translate it in english so that I can understand (I don't understand urdu very well even tho Im Bangladeshi!)

 

[dumbledore.]

Could you plz translate it in english so that I can understand (I don't understand urdu very well even tho Im Bangladeshi!)

1. It says it is neutral. So D option is incorrect as carboxyl group is present.
2. Orange ppt can only be given by aldehydes and ketones. So B option is incorrect as it contain ester group.
3. Only alcohol and carboxylic groups gives HCl with PCl5.
Hence C is the right answer.

 

[Mstudent]

1. It says it is neutral. So D option is incorrect as carboxyl group is present.
2. Orange ppt can only be given by aldehydes and ketones. So B option is incorrect as it contain ester group.
3. Only alcohol and carboxylic groups gives HCl with PCl5.
Hence C is the right answer.

OH ok, thank you very much!

 

[darks]

Both of these please. Thanks : )

 

[DeadbeatCIE]

I have no idea about the kind of approach towards these type of questions any help would be kindly appreciated.

S16qp11

 

[Thisansa]

View attachment 62398 View attachment 62399
I have no idea about the kind of approach towards these type of questions any help would be kindly appreciated.

S16qp11

For question number 3:
First you have to find the Mr of the whole molecule= 207.2+ 4[(12*2)+(1*5)] = 323.2
Then you gotta find the total mass of carbon= 4(12*2)= 96
Then to find the percentage of Carbon in the molecule you have to divide the total mass of C by the total mass of the molecule and multiply by 100.
% of C = (96/323.2)*100 = 29.7%
so the answer is C

For question number 17:
You have to build up an equation for the neutralization reaction taking place which will be CaCO3 +2HCl --> CaCl2 + CO2 + H2O
Then find the number of mol of HCl reacted by equation n=cv = 0.5*(36/1000)=0.018mol
The amount of mol of CaCO3 reacted will be half of that because the molar ratio is 1:2 so, mol of CaCO3= 0.018/2 = 0.009mol
Next we have to find the Mr of CaCO3 which is = 40.1 + 12 + (16*4) = 100.1
then we can find the mass of CaCO3 by rearranging the equation n=m/Mr to m= n*Mr
So mass of CaCO3 will be = 0.009*100.1= 0.9 g
Then the percentage of CaCO3 in the rock can be found by dividing mass of CaCO3 by mass of rock and multiplying by 100
So, %of CaCO3= (0.9/2)*100 = 45%
Thus, B is the answer.

Hope it helps!

 

[Thisansa]

View attachment 62394 View attachment 62395
Both of these please. Thanks : )

For question 4:
The equation we must use is pV=nRT, rearrange the equation to get p=nRT/V
since n and R are constant you can consider that pressure is directly proportional to T and is inversely proportional to V
so you can build up 2 equations p1/p2 = T1/T2 and p1/p2 = V2/V1
we can combine these two equations and get p1/p2 = T1/T2 * V2/V1
so then its just substitutions where p1= 1*10^5
T1= (273+20)= 293 K
T2= (273+100)= 373K
V1= 1V
V2= 1V+ 3V= 4V
If you substitute correctly you'll get 3.18*10^4 so the answer is A

 

[amina1300]

Q 11 20 29
http://papers.gceguide.com/A Levels/Chemistry (9701)/9701_w16_qp_13.pdf

 

[amina1300]

http://papers.gceguide.com/A Levels/Chemistry (9701)/9701_w16_qp_12.pdf
Q30 Q26

 

[Mstudent]

Q 11 20 29
http://papers.gceguide.com/A Levels/Chemistry (9701)/9701_w16_qp_13.pdf

29 should be A. count the number of peaks before the =O, the peaks represent the number of carbon atoms

 

[Metanoia]

Cyanohydrins can be made from carbonyl compounds by generating CN–
ions from HCN in the
presence of a weak base.
In a similar reaction, –
CH2CO2CH3 ions are generated from CH3CO2CH3 by strong bases.

Which compound can be made from an aldehyde and CH3CO2CH3 in the presence of a strong
base?
A CH3CH(OH)CO2CH3
B CH3CO2CH2CH(OH)CH3
C CH3CH2CH(OH)CH2CO2CH3
D (CH3)2C(OH)CH2CO2CH3

this question is from w09qp1. pls help. the ans is C but I dont know why

Hopefully, the explanation in this video would help

 

[Metanoia]

Any trick for this kind of question? :/

Acids can be viewed as proton donors, for options A, B and D, the underlined substances are acids as they donated (lost their protons)

Option B, on the other hand, lost OH-.

 

[Rohith99]

Hey I need help in the Mcq paper 11 for chemistry which I'm gonna write on Wednesday 7th June if anyone writes it before 5am gmt pls share the question paper or the questions which come in it its a deep request someone pls help..it would be a huge favor for me.. Thanks.. Reply asap of anyone sees this msg

 

[Metanoia]

CAN SOMEONE THIS QUESTION??
View attachment 62348View attachment 62349

ANSWER = B

 

[Metanoia]

Hey I need help in the Mcq paper 11 for chemistry which I'm gonna write on Wednesday 7th June if anyone writes it before 5am gmt pls share the question paper or the questions which come in it its a deep request someone pls help..it would be a huge favor for me.. Thanks.. Reply asap of anyone sees this msg

Use this forum to clarify your doubts on past year papers.

An exam is but an academic indicator, please do not compromise the bigger test which is your integrity. It is unfair to all other students giving their honest efforts.

 

[Metanoia]

Why it's not A?

Bond dissociation energy is a measurement of the strength of the covalent bond holding the atoms together within the molecules, and not the measurement of the strength of the bond among molecules.