Chemistry: Post your doubts here!

[Shahroz95]

W.S!
Kp is simply the equilibrium constant in terms of partial pressures. Since gases can occupy any container, there volume is indefinite. The Kp provides a better measure of the equilibrium constant in reactions involving gases as conc. = moles/volume, which is difficult to determine.
To find partial pressure, use: p = n/N x P, where
n: no. of moles of the particular gas (whole partial pressure is being found)
N: total no. of moles (of all gases)
P: total pressure (of all gases).

Once you have the partial pressures, Kp is found in exactly the same way as Kc. Just use the partial pressures where you'd use concentrations.

Thanks!! Though this piece of info came too late to save me for tomorrow, but it sure will save me on Friday! Thanks again!

 

[khanrocks20]

can u please give some organic chem notes????

 

[MEGUSTA_xD]

can u please give some organic chem notes????

http://www.xtremepapers.com/community/threads/all-notes-o-level-0r-igcse-a-level.4866/

 

[smzimran]

Can anyone explain how to do question number 2, 15 and 31 from this paper i've attached ?

Q2:
100g fertiliser contains 15g Nitrogen
1g will contain (15/100)g Nitrogen
14g fertiliser will contain (15/100) * 14 = 2.1g = (2.1/14) mol = 0.15 mol Nitrogen
Now,
concentration of nitrogen atoms = no. of moles/volume
= 0.15/5
= 0.03 mol/dm3
So A is correct.

 

[smzimran]

Can anyone explain how to do question number 2, 15 and 31 from this paper i've attached ?

Q15:
Total mass of limestone burnt = 1000 + 200 = 1200 million tonnes
Molecular mass Mr of limestone = 100
So, number of moles of limestone burnt = 1200/100 = 12 mol (Forget million tonnes)
The equation of decomposition of limestone is:
CaCO3 ---> CaO + CO2
The ratio between limestone and CO2 is 1:1
So 12 mol of limestone produce 12 mol CO2
Now,
mass of 12 mol CO2 = 12 * 44 = 528
So B is correct.

 

[smzimran]

Can anyone explain how to do question number 2, 15 and 31 from this paper i've attached ?

Q31:
Write the balanced equations of all three compounds to check which one requires exactly 2.5 mol O2.
1 C2H5OH + 3O2 --> 2CO2 + 3H2O
2 C2H2 + 2.5O2 --> 2CO2 + H2O
3 CH3CHO + 2.5O2 --> 2CO2 + 2H2O

As 2 and 3 are correct, the correct answer must be C.

 

[omg]

can any1 upload the examiner report for nov 11 p4???

 

[omg]

Q2 (a) (iv) pls helpppp!!!!

 

[smzimran]

Q2 (a) (iv) pls helpppp!!!!

As we know all three are 1st order reactions,
Look at experiment 1
[CH3CHO] is already 0.20
We need to increase [CH3OH] 2 times and increase [H+] 4 times.
Therefore (both being 1st order), rate will increase 2 * 4 = 8 times.
Final relative rate = 8 * (relative rate in experiment 1) = 8 * 1 = 8

 

[omg]

may 2oo7 paper 5 q2 part (e). i am not getting how they got the value of x!!!!!!!!!!!!!!!!!!!!!!!! PLEASE HELLP

 

[smzimran]

may 2oo7 paper 5 q2 part (e). i am not getting how they got the value of x!!!!!!!!!!!!!!!!!!!!!!!! PLEASE HELLP

Please post the link to this paper, it would be helpful and time saving..

 

[omg]

Please post the link to this paper, it would be helpful and time saving..

http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_s07_qp_5.pdf i spnt like an hour on it bt didnt get it :?

 

[abcde]

21: http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_s06_qp_1.pdf
How come Warfarin has only 1 chiral centre? Aren't there 3 (why are 1 and 2 not viable chiral centres)?

 

[tom ed]

20 The compound known as ‘gamma-linolenic acid’ is found in significant amounts in the seeds of
the Evening Primrose plant. There is evidence that the compound may help patients with
diabetes.
CH3(CH2)4CH=CHCH2CH=CHCH2CH=CH(CH2)4CO2H
gamma-linolenic acid
How many cis-trans isomers does gamma-linolenic acid have?
A 3 B 6 C 8 D 12

 

[smzimran]

21: http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_s06_qp_1.pdf
How come Warfarin has only 1 chiral centre? Aren't there 3 (why are 1 and 2 not viable chiral centres)? View attachment 4921

Iam not sure about this but i think that is because the 1 and 2 carbon atoms are not bonded to 4 atoms, the C-H bond is broken and H+ lost before an external bond is formed there is a pie electron cloud in the benzene ring....
Hope iam corect

 

[smzimran]

20 The compound known as ‘gamma-linolenic acid’ is found in significant amounts in the seeds of
the Evening Primrose plant. There is evidence that the compound may help patients with
diabetes.
CH3(CH2)4CH=CHCH2CH=CHCH2CH=CH(CH2)4CO2H
gamma-linolenic acid
How many cis-trans isomers does gamma-linolenic acid have?
A 3 B 6 C 8 D 12

C 8

 

[CaptainDanger]

C 8

Seeing the structure there are 3 of the possibilities ... So you used 2^3? Right?

 

[smzimran]

Seeing the structure there are 3 of the possibilities ... So you used 2^3? Right?

Absolutely, thats the method

 

[tom ed]

is nt ths

Absolutely, thats the method

method supposed to be used for finding enantiomers....where n is the number of chiral centres......

 

[smzimran]

is nt ths
method supposed to be used for finding enantiomers....where n is the number of chiral centres......

In that method n is the number of double bonds + the number of chiral centers however as there are no chiral centres in consideration here, the double bonds must be counted.
I use this formula:
Total Number of isomers = 2^(a+b)
where
a is the number of double bonds
b is the number of chiral centres