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Chemistry: Post your doubts here!

Jaf

Jaf

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sumaiyarox:) said:
what volume of oxygen ,measured at room temperature and pressure,is required for the complete combustion of 8.65g of H2S ?
plzz help..howz this done..??
Click to expand...
First write the balanced equation.
2H2S + 3O2 ------> 2H2o + 2SO2
(Note:For AS you're not required to know this equation by heart but whenever such a question appears, there are either hints in it about the products or the products are stated explicitly. Nonetheless, you're expected to know the oxidation/combustion products of H2 and S2. )
Moles of H2S = Mass/Molar Mass = 8.65/((32.1 + 2) = 173/682
Moles of O2 = (3 x (173/682))/2 (since mole ratio H2S:O2 = 2:3 from the equation) = 519/1364
Volume = moles x 24 = (519/1364) x 24 = 9.13 dm^3
 
Amy Bloom

Amy Bloom

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Hello there. Can anybody check my answer this?:

Oxides of nitrogen in the atmosphere contribute to the formation of acid rain by catalysing the oxidation of SO2 t0 SO3. Write equations to show how this occurs and describe the type of catalysis observed here.
 
Amy Bloom

Amy Bloom

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smartangel said:
also, Nov 2010 Q 4 and 9!!!
http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w10_qp_11.pdf
Click to expand...

For question 4
Maybe somebody else can verify this, i'm not so sure:

First you try to write an equation for the reaction, which is as follows:

SO2 + I- ----> I2 + SO42-

Then, you find the oxidation numbers of S in SO2 and SO42-and you'll see a change from +4 to +6.
So the answer is D

For question 9
First you find the ▲H = 200 x 4.2 x (66-18) =40320 Jg-1oC-1
Let the energy released when 1.60 g of fuel is burned be x J
0.45x has been absorbed by 200g of water
So, 0.45x = 40320
Hence x = 89600 J
Now, When 1.60g is burned, 89600 J of energy is released
So, when 1g of fuel is burned, 89600/1.60 = 56000 J of energy is released.
Answer is B.

Hope my answer has been of help to you. :)
 
Jaf

Jaf

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smartangel said:
need help with June 2010 paper1 Q 4,6,21,29,40
http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s10_qp_11.pdf
Click to expand...
4)
P - One Br-Br bond is breaking. So enthalpy change is -193J
Q - One Cl-Cl bond is forming. So enthalpy change is +244J
R - One C-Cl bond is forming. So enthalpy change is +340J
S - One C-H bond is breaking. So enthalpy change is -410J.
Answer is therefore D.

6) NH4NO3 is really NH4+ and NO3-.
Oxidation state of N in NH4+:
x + 4 = +1
x = -3

Oxidation state of N in NO3- :
x + (-2 x 3) = -1
x = +5

Oxidation state of N in N2O:
2x + (-2) = 0
x = +1

So changes in oxidation numbers are:
-3 ---> +1 = +4
+5 ---> +1 = -4
Answer is D

21) Here, they're testing your knowledge of what happens in a free radical substitution reaction. A C-H bond is changed to a C-Cl.

In the compound given, work clockwise with the carbon atoms. In the top most carbon atom, any of the hydrogens being replaced will give the same X radical. So this gives one possible X radical. The next two carbons will also have only one possible distinct replacement. However, these replacements will give radicals which are identical to the replacement given by the first carbon. Try doing this on a paper if you don't get how.
Another possible replacement is on any one of the 2 hydrogens on the second-last carbon on the left. The last one is possible on any one of the 3 hydrogens on the last carbon.
Hence a total of 3 possible X radicals are possible. Ans: C

29) butan-2-ol can form only 2 straight chain alkenes plus one alkene with an alkyl side chain. Ans: B

40) in 3 both reactants are gases. They can't be 'heated under reflux'. Ans: B
 
S

smartangel

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Amy Bloom said:
For question 4
Maybe somebody else can verify this, i'm not so sure:

First you try to write an equation for the reaction, which is as follows:

SO2 + I- ----> I2 + SO42-

Then, you find the oxidation numbers of S in SO2 and SO42-and you'll see a change from +4 to +6.
So the answer is D

For question 9
First you find the ▲H = 200 x 4.2 x (66-18) =40320 Jg-1oC-1
Let the energy released when 1.60 g of fuel is burned be x J
0.45x has been absorbed by 200g of water
So, 0.45x = 40320
Hence x = 89600 J
Now, When 1.60g is burned, 89600 J of energy is released
So, when 1g of fuel is burned, 89600/1.60 = 56000 J of energy is released.
Answer is B.

Hope my answer has been of help to you. :)
Click to expand...
thanks a lot! :)
 
S

smartangel

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Jaf said:
4)
P - One Br-Br bond is breaking. So enthalpy change is -193J
Q - One Cl-Cl bond is forming. So enthalpy change is +244J
R - One C-Cl bond is forming. So enthalpy change is +340J
S - One C-H bond is breaking. So enthalpy change is -410J.
Answer is therefore D.

6) NH4NO3 is really NH4+ and NO3-.
Oxidation state of N in NH4+:
x + 4 = +1
x = -3

Oxidation state of N in NO3- :
x + (-2 x 3) = -1
x = +5

Oxidation state of N in N2O:
2x + (-2) = 0
x = +1

So changes in oxidation numbers are:
-3 ---> +1 = +4
+5 ---> +1 = -4
Answer is D

21) Here, they're testing your knowledge of what happens in a free radical substitution reaction. A C-H bond is changed to a C-Cl.

In the compound given, work clockwise with the carbon atoms. In the top most carbon atom, any of the hydrogens being replaced will give the same X radical. So this gives one possible X radical. The next two carbons will also have only one possible distinct replacement. However, these replacements will give radicals which are identical to the replacement given by the first carbon. Try doing this on a paper if you don't get how.
Another possible replacement is on any one of the 2 hydrogens on the second-last carbon on the left. The last one is possible on any one of the 3 hydrogens on the last carbon.
Hence a total of 3 possible X radicals are possible. Ans: C

29) butan-2-ol can form only 2 straight chain alkenes plus one alkene with an alkyl side chain. Ans: B

40) in 3 both reactants are gases. They can't be 'heated under reflux'. Ans: B
Click to expand...
thank you so much!! its amazing how you guys help others! God bless you :)
 
Jaf

Jaf

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Amy Bloom said:
For question 4
Maybe somebody else can verify this, i'm not so sure:

First you try to write an equation for the reaction, which is as follows:

SO2 + I- ----> I2 + SO42-

Then, you find the oxidation numbers of S in SO2 and SO42-and you'll see a change from +4 to +6.
So the answer is D
Click to expand...

Fine. Take credit for my answer. :mad:
 
Student12

Student12

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How do you do the questions mentioned below of the paper attached?
Question 1 : The ans. is A. what i did is 25dm x 1.0x10^-2 you get the moles of KOH but we need to find the concentration of calcium sulphate. It doesnt make sense what i did !! EXPLAIN mee?
Q4 : why is it D?
Q8: why is it D ?
Q11: how do you do this question ? o_O
Q14 ??
Q17: why D and not A
Q20: how do you know alcohols have the ratio of hydrogen to carbon the highest.
Q23: why B and not A ?
Q32 ??

I know they are aloot of questions but Help would be appreciated ! :)
 

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smzimran

smzimran

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panda222 said:
Need help with Question 3 on this paper: (Summer 2011 Qp 21)

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s11_qp_21.pdf

What topic is it? Is it Group 2? I couldn't get any answers correct from Question 3 C till the end of the question. Help me. Exam is in less then a month.
Click to expand...
Aoa,
Yes, it is from group II
What is it that you do not get ?
Its a straight forward questions testing knowledge of the reactions of calcium and its compounds
:)
 
W

whocares

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Why is it that sound waves are not normally considered as having particle-like properties, nor raindrops as having wave like properties?
 
H

hm12

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The decreasing order of ease of hydrolysis of acyl chloride , alkyl chloride and aryl chloride is as follows : CH3COCl>CH3CH2Cl>C6H5Cl

can anyone explain why is that so?
 
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