Chemistry: Post your doubts here!

[asadalam]

Yeah.

Only AS or A2 as well?

 

[Xylferion]

Only AS or A2 as well?

Just AS.

 

[Metanoia]

Can anyone explain to me these few questions?
4. When a sample of gas is compressed at constant temp from 1500 kPa to 6000 kPa, its volume changes from 76.0cm3 to 20.5 cm3. Which statement are possible explanations for this behavior?
Wrong ans: The gas partially liquefies / Gas is absorbed on to the vessel walls ( Why)?

Thank You.!

I'll add on to the explanation of question 4.

ASSUMING that the gas was an ideal gas.

P1V1 = P2V2
(1500)(76) = (6000)(V2)
V2 = 19 cm^3

This means that if it was an ideal gas, the volume is expected to be 19 cm^3. However, the actual observed volume is 20.5 cm^3.

Since it turned out that the actual volume is MORE than expected, we can't use the possibility that part of the gas became liquid.

Conversely, if the actual volume is less than expected, then it is possible to use the explanation that part of the gas liquifies or is absorbed on walls of vessels.

 

[princess Anu]

It seems correct to me, as for balancing of the stuff in the box it's better to do it but it won't affect your answer.

but how do I balance it? there is no O in the above equation :S

 

[qwertypoiu]

but how do I balance it? there is no O in the above equation :S

Balance everything except the O's, those are essentially coming from combustion. In energy cycles, from my experience so far, one should balance everything like hydrogen, carbon, etc. but oxygen may be ignored.
However, if you really wish, you can add oxygen on both sides above. Oxygen doesn't have enthalpy change of combustion I think, and it's enthalpy change of formation is 0.

 

[Midnight dream]

http://maxpapers.com/syllabus-materials/chemistry-9701-a-level/attachment/9701_w09_qp_21/
Can anyone explain question 3 c?

 

[Lola_sweet]

Can someone please explain this statement:

Student X claimed tht the 17 carbon atoms in the four rings lie all in the same plane

 

[forest822]

1) Use the density and volume to find the mass.

Mass of ice = 1.00 gcm-3 * 1.00 cm3 = 1.00 g
Moles of ice ( ice is frozen water aka H2O ) = 1/18 = 0.0556 moles

It says there in bracket, the volume 1 mol of gas occupies at 298K is 24dm^3.
So 0.0556 moles will give 1.33 dm^3.

This is at 298 K, but we want 596K.

298K = 1.33 dm^3
596K = x
298K * x = 1.33 * 596
298K * x = 792.68
x = 2.66 dm^3
--------------------------------

2) Helium has a complete outers hell and is probably the most inert ( not reactive ) of all the noble gases. A possibility of it bonding with other atoms does not exist. For hydrogen however, it seeks to achieve a noble state, so it does have the possibility of bonding with other atoms. For that reason alone, Helium is more ideal. Ideal gases are based off of the assumption that there are no forces of attraction at all between the gas particles.
--------------------------

3) Like I said above, ideal gases are said to not posses any forces of attraction between the molecules, as a result of this, you find them colliding with each other constantly, so there's hardly anytime for the possibility of attraction between the molecules. The term "interact" means there are forces at work, which isn't the case with ideal gases.
-----------------------

4) You're going to need far more than 6k Pa in order to initiate a partial change in state. The question specifically mentions " constant temperature ", which means that the container and surroundings of the gas molecules isn't going to get any cooler.
This doesn't allow for condensation to take place that easily.
It's also worth noting that the particles are now moving about randomly and colliding with the container much more, "colliding". They can't be absorbed if they're going to collide and then resume moving about.

1) Use the density and volume to find the mass.

Mass of ice = 1.00 gcm-3 * 1.00 cm3 = 1.00 g
Moles of ice ( ice is frozen water aka H2O ) = 1/18 = 0.0556 moles

It says there in bracket, the volume 1 mol of gas occupies at 298K is 24dm^3.
So 0.0556 moles will give 1.33 dm^3.

This is at 298 K, but we want 596K.

298K = 1.33 dm^3
596K = x
298K * x = 1.33 * 596
298K * x = 792.68
x = 2.66 dm^3
--------------------------------

2) Helium has a complete outers hell and is probably the most inert ( not reactive ) of all the noble gases. A possibility of it bonding with other atoms does not exist. For hydrogen however, it seeks to achieve a noble state, so it does have the possibility of bonding with other atoms. For that reason alone, Helium is more ideal. Ideal gases are based off of the assumption that there are no forces of attraction at all between the gas particles.
--------------------------

3) Like I said above, ideal gases are said to not posses any forces of attraction between the molecules, as a result of this, you find them colliding with each other constantly, so there's hardly anytime for the possibility of attraction between the molecules. The term "interact" means there are forces at work, which isn't the case with ideal gases.
-----------------------

4) You're going to need far more than 6k Pa in order to initiate a partial change in state. The question specifically mentions " constant temperature ", which means that the container and surroundings of the gas molecules isn't going to get any cooler.
This doesn't allow for condensation to take place that easily.
It's also worth noting that the particles are now moving about randomly and colliding with the container much more, "colliding". They can't be absorbed if they're going to collide and then resume moving about.

Hope any of that made sense!

Thank you for the solution. May I know what is the condition that gas will be absorbed on to the vessel wall?

 

[qwertypoiu]

Can someone please explain this statement:

Student X claimed tht the 17 carbon atoms in the four rings lie all in the same plane

A plane refers to a 2D surface at a certain angle, like one face of a cuboid.I don't know if this makes sense, I don't know how to explain a plane in writing, so here's a picture:

The points A, B, and C are on the same plane, meaning they are on the same SURFACE.

For example, the carbon atoms in one layer of graphite lies on one plane.

I have seen the question you are talking about.
The student X claims that all these 17 carbons are lying in one particular plane, it cannot be true, since the carbons are sp3 hybridised and so bonded to 4 groups each, which means it will form a 3D shape with 109.5 degrees as the bond angle, which is NOT in one plane.

 

[Lola_sweet]

A plane refers to a 2D surface at a certain angle, like one face of a cuboid.I don't know if this makes sense, I don't know how to explain a plane in writing, so here's a picture:

The points A, B, and C are on the same plane, meaning they are on the same SURFACE.

For example, the carbon atoms in one layer of graphite lies on one plane.

I have seen the question you are talking about.
The student X claims that all these 17 carbons are lying in one particular plane, it cannot be true, since the carbons are sp3 hybridised and so bonded to 4 groups each, which means it will form a 3D shape with 109.5 degrees as the bond angle, which is NOT in one plane.

Thank u so much

 

[Midnight dream]

A plane refers to a 2D surface at a certain angle, like one face of a cuboid.I don't know if this makes sense, I don't know how to explain a plane in writing, so here's a picture:

The points A, B, and C are on the same plane, meaning they are on the same SURFACE.

For example, the carbon atoms in one layer of graphite lies on one plane.

I have seen the question you are talking about.
The student X claims that all these 17 carbons are lying in one particular plane, it cannot be true, since the carbons are sp3 hybridised and so bonded to 4 groups each, which means it will form a 3D shape with 109.5 degrees as the bond angle, which is NOT in one plane.

And also in the same question, it was asked whether the compound exhibits cis-trans isomerism. And the answer was No. So does it mean that cyclic alkenes never exhibit isomerism?

 

[forest822]

Hello. Can anyone please explain to me these questions? Thank you

1. The density of this nitrogen to be 1.2572gdm-3 at stp. Chemically, pure nitrogen has a density of 1.2505gdm-3 at stp. Which gas was present in atmospheric nitrogen to cause this discrepancy?
A. Argon B. Helium C. Methane D. Neon. ANS: Argon (Why?)

2. A sample of mg of an organic compound is vaporised in a gas syringe and occupies V cm3 at TK and p atm. What is the relative molecular mass of the compound, Mr?
A. Mr= (m.22400.T)/p.V.273 B. Mr= (m.22400.T+273)/p.V.273 C. Mr= (m.22400.273.p)/V.T D. (m.22400.273.p)/V.(T+273) ANS: A

 

[qwertypoiu]

And also in the same question, it was asked whether the compound exhibits cis-trans isomerism. And the answer was No. So does it mean that cyclic alkenes never exhibit isomerism?

No. Cycloalkenes are restricted from having cis-trans isomers, but those which have more than 8 carbons DO have cis-trans isomerism.
Metanoia Please confirm this is true right??

 

[Midnight dream]

http://maxpapers.com/syllabus-materials/chemistry-9701-a-level/attachment/9701_w09_qp_21/
Xylferion Please can you answer 3 part c?

 

[The Sarcastic Retard]

http://maxpapers.com/syllabus-materials/chemistry-9701-a-level/attachment/9701_w09_qp_21/
Xylferion Please can you answer 3 part c?

3c) You'll have to draw a double hump diagram for this. It's essentially the same as the simpler energy diagram except we break the single hump into 2 to represent the intermediate steps. There should be two peaks and the first one should be higher than the second. If you recall free radical substitution mechanism you know the intermediate steps. The first hump represents the formation of methyl free radical and HCl and the second represents the chloromethane formation. The reason the second hump is lower is because the reaction can only proceed as long as the energy barrier for the following step is less then the preceding one. The activation energy is labeled for the first hump.
-lilcloud!

 

[Studydayandnight]

GUYS PLEASE HELP!!!!!!

Question 2 (b) May june 2014/23

I am struggling with this and my exams start in less than 10 days! :'(

http://maxpapers.com/wp-content/uploads/2012/11/9701_s14_qp_23.pdf

 

[Midnight dream]

3c) You'll have to draw a double hump diagram for this. It's essentially the same as the simpler energy diagram except we break the single hump into 2 to represent the intermediate steps. There should be two peaks and the first one should be higher than the second. If you recall free radical substitution mechanism you know the intermediate steps. The first hump represents the formation of methyl free radical and HCl and the second represents the chloromethane formation. The reason the second hump is lower is because the reaction can only proceed as long as the energy barrier for the following step is less then the preceding one. The activation energy is labeled for the first hump.
-lilcloud!

Thanks.....yaar.

 

[The Sarcastic Retard]

GUYS PLEASE HELP!!!!!!

Question 2 (b) May june 2014/23

I am struggling with this and my exams start in less than 10 days! :'(

http://maxpapers.com/wp-content/uploads/2012/11/9701_s14_qp_23.pdf

This simply means u haven't grasped your concept of moles properly. Physical chemistry needs calculation practice.
2bi) n = cv = 40*0.4*0.001 = 0.016mol.
ii) n = cv = 25*0.12*0.001 = 0.003mol.
iii) intitial - excess will be left to react with ammonia = 0.016-0.003 = 0.013mol.
iv) ratio is 1:1 hence no change. 0.013mol.
v) 0.413/63.5 = 0.0065mol of copper. 1:2 ratio hence x=2.
vi) 63.5 + (14*2) + (1*8)+(32.1*2)+(16*8)+(6*18) = 399.7

 

[Midnight dream]

3c) You'll have to draw a double hump diagram for this. It's essentially the same as the simpler energy diagram except we break the single hump into 2 to represent the intermediate steps. There should be two peaks and the first one should be higher than the second. If you recall free radical substitution mechanism you know the intermediate steps. The first hump represents the formation of methyl free radical and HCl and the second represents the chloromethane formation. The reason the second hump is lower is because the reaction can only proceed as long as the energy barrier for the following step is less then the preceding one. The activation energy is labeled for the first hump.
-lilcloud!

your getting good wid chem probs..

 

[Studydayandnight]

This simply means u haven't grasped your concept of moles properly. Physical chemistry needs calculation practice.
2bi) n = cv = 40*0.4*0.001 = 0.016mol.
ii) n = cv = 25*0.12*0.001 = 0.003mol.
iii) intitial - excess will be left to react with ammonia = 0.016-0.003 = 0.013mol.
iv) ratio is 1:1 hence no change. 0.013mol.
v) 0.413/63.5 = 0.0065mol of copper. 1:2 ratio hence x=2.
vi) 63.5 + (14*2) + (1*8)+(32.1*2)+(16*8)+(6*18) = 399.7

Thanks so much for this. I don't get one thing though, how can you tell the mole ratios in the (iv) part? By comparing the NH4+ ions to what in which equation?

(I know I may come across as stupid but moles and organic are my weakness :'( )