Chemistry: Post your doubts here!

[leosco1995]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s09_qp_2.pdf Q4 B 2nd part it ask what is formed when a diol(having also a carboxylic group attached)is reacted with carboxylic ......esters are formed but there must be only one ester formed because there is only one carboxylic group that reacts !!why then two esters are formed??? please help!!

Because a carboxylic acid can react with more than on -OH group. BTW, it said in the marking scheme that monoesters were allowed, so even if you didn't have 2 esters but correctly did 1 then I think they marked it right.

 

[DragonCub]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s09_qp_2.pdf Q4 B 2nd part it ask what is formed when a diol(having also a carboxylic group attached)is reacted with carboxylic ......esters are formed but there must be only one ester formed because there is only one carboxylic group that reacts !!why then two esters are formed??? please help!!

Virtually the reaction does NOT limit the amount of the reactants involved. What if two moles of E reacts with one mole of C? The product should be diester then.
On the other hand, the MS has stated "allow either monoester" so drawing only one ester in the product is also correct.

 

[rockerzregmi]

Guys some one plz explain me how do we prepare different concentration of HCL from 2 mol/dm3 . I know its by adding water some one please exam me step by step .. thanx

 

[Mad-Eye]

LiAlH4 in dry ether OR NaBH4 in water.

thnx man

 

[smzimran]

@Everyone:
Complete Chem P5 notes:

http://www.mediafire.com/view/?arx6ngl77dvlga4

 

[raamish]

Hey guys please somebody help me. I dont know how to draw 3d structures of hydrocarbons help

 

[haider159]

plz help me with "question 5 part (b)" of this paper and plz explain me how u did that. thanx

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_w08_qp_2.pdf

 

[Jaf]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s03_qp_2.pdf

how do we do qs 1b(i)???

We know Chlorine has two known isotopes Chlorine - 35 and Chlorine - 37.
The peak at 35 is because of the (35)^Cl+ particle. The peak at 38 is because of the H(37)^Cl+ particle.
(I know, neither the mark scheme nor the examiner's report mention that the particles are supposed to have a positive charge but they are! The mass spectrometer deals with positive ions [but you don't need to know how it works] only. I think they've omitted the positive part because of this very reason [the syllabus doesn't include the working]. Nonetheless, the correct answer, to be on the safe side, is what I stated to be.)

when 50cm^3 of a hydrocarbon Y is burnt it reacts with exactly 300 cm^3 of oxygen to form 200 cm^3 of C02. water is also formed. deduce the equation for this reaction. answer: C4H8 + 6O2 → 4CO2 + 4H2O..please tell the how to work this one out..its frm chemistry coursebook.

Form a general equation first.
CxHy + zO2 ------> xCO2 + (y/2) H2O
Since mole ratio = volume ratio,
50:300:200 = 1:6:4
So z = 6, x = 4
Put in the values in the equation.
C4Hy + 6O2 ------> 4CO2 + (y/2) H2O
We know there are 12 oxygens on the LHS. The carbon dioxide on the right hand side satisfies 8 of these. The moles of H2O need to be such that the remaining oxygens are satisfied. So y/2 = 4 (since one H2O has just one oxygen). y = 8.
C4H8 + 6O2 -------> 4CO2 + 4H2O

 

[Jaf]

Perhaps the some or all of the hydrogen ions are provided by another kind of acid like HCl.

lol chemistry

a) is PbO2 + 4H(+) + SO4(2-) + 4e(-) ------> PbSO4 + 2H2O
b) I guess it is PbSO4 + 2e(-) ------> Pb + SO4(2-) [water here just provides aqueous environment]
Then for (c) PbO2 + 4H(+) + 4e(-) ------> Pb + 2H2O

What's in red is supposed to be 2e(-). I've added what's in green. Honest mistakes I suppose.

 

[Aahliya]

Is there anyone who can help me with Chemistry PAPER 5 ??

 

[hussamh10]

Virtually the reaction does NOT limit the amount of the reactants involved. What if two moles of E reacts with one mole of C? The product should be diester then.
On the other hand, the MS has stated "allow either monoester" so drawing only one ester in the product is also correct.

Thank you very much !!!

 

[hussamh10]

Because a carboxylic acid can react with more than on -OH group. BTW, it said in the marking scheme that monoesters were allowed, so even if you didn't have 2 esters but correctly did 1 then I think they marked it right.

Ok thanks

 

[sumaiyarox:)]

plzzz help
summer 2009 Q3 c , d3 AND 4a
plzzzzz

 

[sararash]

Hi everyone, assalamoalaikum!!

To get things organized in a better way, I am making this thread. As othewise, some queries remain unanswered!

So post your CHEMISTRY doubts in this thread. InshaAllah other people here will help me and you all.

Any Chemistry related notes and links will be added here in this post. Feel free to provide the links to your notes around the forum, or any other websites!
Thanks!
Jazak Allah Khair!

Chemistry Notes:

Data Booklet

http://www.chemguide.co.uk This is the website, which contains almost everything classified according to the syllabus.

Some links & Notes - by 'destined007'

Chemistry worksheets Link shared by hassam

Chemistry Application Booklet: Mistakes and Corrections!

Calculations for A level Chemistry, author E.N. Ramsden third edition ebook download.

 

[Sanis]

color of hydrogen halides plz help i cant find it

 

[DragonCub]

What's in red is supposed to be 2e(-). I've added what's in green. Honest mistakes I suppose.

You're right. That was truly a silly mistake.

 

[sararash]

heyyyyy...............plz sm1 help 4 chem p5.....loads of doubts........may june 2010 p52 Q.2F......PLZZZZZZZ HELP

 

[DragonCub]

color of hydrogen halides plz help i cant find it

If I did not remember the wrong idea they should all be colourless.
Sometimes hydrogen bromide and iodide can show yellow colour due to minor oxidation.

 

[Sanis]

If I did not remember the wrong idea they should all be colourless.
Sometimes hydrogen bromide and iodide can show yellow colour due to minor oxidation.

thx

 

[Saad (سعد)]

As-Salaamu 'Alaykum.

Someone please explain to me the Electron-Pair Repulsion Theory; I don't get it at all. How do we determine what the bond angles and shapes of molecules are?

And whats the shape of/bond angles in an ammonium ion, NH4+?

Jazak Allah.