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AS Chemistry Doubts

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nikhil000

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may june 2002 paper 1 question no .21 ..I don't have any clue how to approach these kind of questions
 
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panda222

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Ash1994 said:
0.200 mol of a hydrocarbon undergo complete combustion to give 35.2 g of carbon dioxide and
14.4 g of water as the only products.
What is the molecular formula of the hydrocarbon?
A C2H4 B C2H6 C C4H4 D C4H8
HOW DO WE FIND THE Mr OF the HYDROCARBON?
Click to expand...


Here's the answer: (if you have any further questions let me know)

1) The Molar mass of Co2 and H20 are given. Hence we can calculate the number of moles.

2) Then we use mole ratios. Divide everything by the lowest number in the equation to get a much more simpler equation.

3) The equation then suggests we must have 4 carbons and 8 Hydrogens. Hence answer is D.

Working done below. Hope that helped.

t7fqu1.jpg
 
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panda222

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raamish said:
http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w10_qp_12.pdf

PLease explain qs 7 and 8 pllllz
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Regarding question 8. From what i understand, the values given next to the question are incorrect. The question paper clearly tells you to use the data booklet and hence you must. If you use the Data booklet you get the right answer. Without the data booklet, i cant help you get the right answer.

As for Question 7. I've done the working below: (it looks pretty complicated, but its really not that hard).

24ywh0k.jpg
 
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workaholic

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floral123 said:
kindly explain that why not -405???
u said that bonds are formed so sign should be negative...
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The question asked for the bond energy not the enthalpy change for the reacton and as bond energies are always +ve thus it cant be -ve.............Had the qst asked for the enthalpy change the ans would have been negative.....:)
 
floral123

floral123

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workaholic said:
The question asked for the bond energy not the enthalpy change for the reacton and as bond energies are always +ve thus it cant be -ve.............Had the qst asked for the enthalpy change the ans would have been negative.....:)
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Jazakallah...u solved my many questions(y) ... i had the same problem about the sign...kindly can u give few lines about how to solve these sort of questions....i mean the tips for these questions...i ll be grateful...:)
 
perkypearl

perkypearl

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fasih58 said:
I'm starting this thread just to separate AS students... from that one thread were all are posting and things are pretty messed up!

So, ONLY AS STUDENTS post your doubts and challenging questions here...

I'm sure it will help me and YOU!
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oct/nov 2009 ppr 11 Question#2,5,7,11,13,19,20,21,24,29,28,30 help me in these questions explanation please
 
perkypearl

perkypearl

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workaholic said:
The answer is 8...........
In order to identify the no of cistrans isomers u need to first identify the stereocentres.If a compound has a C-C double bond and each of the C in the double bond is bonded to seperate groups then the compound has a stereocentre.In the above case there are 3 such centres.The total number of cistrans isomers is given by the formula 2^n so 2^3=8. There you go......;)
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how many separate groups should be there with carbon double bond ? and is stereocentre and chiral centre same ?
 
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Paparazzi478

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Scafalon40 said:
Calculate the no of moles of N2 from first equation.
Calculate the no of moles of Na from first equation.
Calculate the no of moles of N2 from the second equation too using the no of moles of Na calculated from the first equation.
Add all the moles of N2 together.
Click to expand...
okay boy genius they didn't give us the masses ..... so how am i suppose to find the moles?
 
perkypearl

perkypearl

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oct/nov 2009 ppr 11 Question#2,5,7,11,13,19,20,21,24,29,28,30 help me in these questions explanation please
ppl plz help me in these questions ...............
 
iFuz

iFuz

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perkypearl said:
how many separate groups should be there with carbon double bond ? and is stereocentre and chiral centre same ?
Click to expand...
No, Chiral centre is different!
If I'm right stereocentre is in CisTrans Isomerism and that means AB-C=C-AB
In this C=C is stereocentre as It has 2 different groups on each side...

Correct me if i'm wrong! I think i am wrong ! :(
 
iFuz

iFuz

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perkypearl said:
oct/nov 2009 ppr 11 Question#2,5,7,11,13,19,20,21,24,29,28,30 help me in these questions explanation please
Click to expand...

Why are you doing paper 11 ? Paper 12 is our..
btw leme try the question you've asked!
2. Find moles of CO2 and H2O Both would be 0.8

so CxHy + O2 ------> xCO2 + y/2 H2O

By mole - mole relation...
we have moles corresponding to 0.2 moles of Hydrocarbon... find for 1 Mole ... i.e 4 moles in each case..
so X = 4
and Y/2 = 4
y = 8

C4H8 ANSWER!

5. A all are C to Cl... will cancel
B ... It has dipoles.... C to O , H to C , H to C
C ... C to O , C to Cl , C to Cl *last 2 will cacnel each other
D Both are C to O and will cancel..
B ANSWER

7 Oxidition = GOLEH ( Gain of Oxygen, Loss of Electrons & Hydrogen)
Reduction = LOGEH ( Loss of Oxygen, Gain of Electrons & Hydrogen)
Easy way to remember = GOLI LAGI :p

11 A is correct, Cu+ and Cu2+ are obtained at end of reactions... so they are catalyst
B is straight away wrong, as Cu is oxidized so its Reducing agent
C in Reaction I Ag ppt is formed which darkens it ,,, wo light is absorbed in it...
D Ag is reduced*

13 C. its a fact... SiCl4 + H2O ---> SiCl2 + 4H+ (ion) + 4 Cl- (ion) 9.1 (i) of syllabus.. look at that

19. A its a FACT!

20 In such structures.. all junction points are chiral centres... except ones with double bonds! :)

21. If you can draw it you'll find that out... other wise cram it! ;) Draw it and you'll be left with some additional Hydrogen atoms needs to be justified with double bonds.... and its a Cis retinal... be careful with that.........
A cyclohexane ring alters the C:H ...

24 C-C double bonds = 2 ..
Hydrogen atoms required 4....
and moles of H2 required would be 2... 2 x 2 = 4

Same with bromine.... 2 Br2 = 4 atoms of bromine to dissolve double bond

28 29 30 PM me the answers when you get em!
 
XPFMember

XPFMember

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Paparazzi478 said:
On collision, airbags in cars inflate rapidly due to the production of nitrogen.
The nitrogen is formed according to the following equations.
2NaN3 → 2Na + 3N2
10Na + 2KNO3 → K2O + 5Na2O + N2
How many moles of nitrogen gas are produced from 1 mol of sodium azide, NaN3?
A 1.5 B 1.6 C 3.2 D4.0

Can someone tell me how the answer is B?
Click to expand...
Assalamoalaikum wr wb!
Ok so they said, there's 1 mol of sodium azide..right?

from eqn 1, we know 2 moles produce 3 moles of nitrogen, so one mole will produce 3/2 moles of nitrogen and one mole of sodium...

now from eqn 2, we know 10 moles of sodium produce one mole of nitrogen, we got only one mole of sodium therefore how many nitrogen will be there..=> 1/10

total moles of nitrogen = 3/2 + 1/10 = 1.6 ;)
 
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