Chemistry: Post your doubts here!

[knowitall10]

Assalamu 'Alaykum.
Can somebody please post the link to the winter 2011 examners' report?
Jazakum Allahu Khair.

alright never mind, thanks.

 

[strangerss]

Does structural formula mean we have to draw the skeletal formula....for eg can't we write CNCH2CH2CH2CN instead of drawing it in a skeletal form...please please reply
In ms they usually give the skeletal..but isnt the formula i wrote about also structural?

what you wrote is the structural formula and the skeletal formula is completely different

 

[strangerss]

Can someone please help with p34 Oct/Nov 2012 question( g) I didn't know how to find the mean :S

 

[raamish]

in qs 7c(ii) n09 41 how did we determine that 0-ch3 is on 4th position on benzene ring?
answer plzz

 

[messi10]

Can anyone list most poplar electron withdrawing and donating groups?

Regards

 

[biba]

Can anyone list most poplar electron withdrawing and donating groups?

Regards

electron donating groups ; all alkyl groups, -OH, -NH3 ,-OCH3, -N(CH3)2...........
withdrawing groups : -NO2 , -SO3H , -COOH , -CN , -CHO , -CO- , -CO2- , -CONH2- , -CONH-

 

[biba]

in qs 7c(ii) n09 41 how did we determine that 0-ch3 is on 4th position on benzene ring?
answer plzz

em not very good at expalining things..
first go through the notes that i have attached and then try to understand this :
first find the number of carbon atoms by [(100/1.1)x(M+1/M)]
3H gives a singlet,which means that they are directly attached to the more electronegative element or the functional group.. these 3H give u a singlet at chemical shift value of 3.6.. the group is -OCH3...
1H also gives you a singlet at 5.6 that is the chemical shift value of phenol
than the 4H also gives you a singlet so these are the hydrogens of the benzene ring so ur formula becomes
1,4 methoxyphenol.. (the -OH group is on the 1 position of benzene and as it is an electron donating group the -OCH3 is on the 4th position)
i knw my explanation is pathetic ...still...

 

[messi10]

electron donating groups ; all alkyl groups, -OH, -NH3 ,-OCH3, -N(CH3)2...........
withdrawing groups : -NO2 , -SO3H , -COOH , -CN , -CHO , -CO- , -CO2- , -CONH2- , -CONH-

Thank you so much for the help.

Also,
Can you please tell the effect of electron withdrawing and donating groups attached to benzene ring?

I mean, for example if Cl is going to add in benzene in presence of AlCl3
I get confuse on which position to add the Cl (ortho, meta or para), if electron donating is attached and on which to add if electron withdrawing is attached.

I need a little briefing on it.

Your help is appreciated.
Best Regards

 

[Oliveme]

Thank you so much for the help.

Also,
Can you please tell the effect of electron withdrawing and donating groups attached to benzene ring?

I mean, for example if Cl is going to add in benzene in presence of AlCl3
I get confuse on which position to add the Cl (ortho, meta or para), if electron donating is attached and on which to add if electron withdrawing is attached.

I need a little briefing on it.

Your help is appreciated.
Best Regards

Electron-donating groups are -OH (phenol), -NH2 and -CH3 (methyl) - They activate positions 2,4, and 6 on the benzene ring. (2 and 6 are basically the same positions - depends on the way you look at it.)
Electron-withdrawing groups are -NO2 and -COOH groups. They activate positions 3 and 5 on the benzene ring.
Halogens are exceptions. They're meanies. They are electron-withdrawing but activate positions 2, 4 and 6.

 

[gary221]

Electron-donating groups are -OH (phenol), -NH2 and -CH3 (methyl) - They activate positions 2,4, and 6 on the benzene ring. (2 and 6 are basically the same positions - depends on the way you look at it.)
Electron-withdrawing groups are -NO2 and -COOH groups. They activate positions 3 and 5 on the benzene ring.
Halogens are exceptions. They're meanies. They are electron-withdrawing but activate positions 2, 4 and 6.

loved ur explanation..

 

[Oliveme]

loved ur explanation..

Thank you!

 

[Oliveme]

Can someone please help with p34 Oct/Nov 2012 question( g) I didn't know how to find the mean :S

From the list of given numbers, you eliminate 62 because that's way out of given range. Notice that all numbers, except 62, are around about 54.
So mean = (56+54+56+53)/4 = 54.75 and round it off to 54.8 - Always look out for a value that is too out of the range or say is the furthest.

 

[marz_katy]

can any1 xplain me colorimetry??

 

[Jiyad Ahsan]

can any1 xplain me colorimetry??

you mean calorimetry? right?
well its a technique used to measure enthalpy change of reactions.. by experimental calculations.. i'll explain how using a neutralisation reation
lets say you have 50cm^3 of acid and the same amount of an alkali (also assuming that 1 cm^3 weighs 1 gram)
and specific heat capacity is 4.2 joules per gram per degree celsius (assuming same as water)
measure the initial temperature (take it as 22 degrees celsius)
then measure the final temperature when the reaction is complete (take that as 28 degrees celsius)
apply the formula q = m c (change in) T , where m is mass, c is the heat capacity, T is the temperature
so, q = 100 x 4.2 x 6
q=2520 Joules, and thats your enthalpy change of reaction.. hope it helped

 

[xhizors]

you mean calorimetry? right?
well its a technique used to measure enthalpy change of reactions.. by experimental calculations.. i'll explain how using a neutralisation reation
lets say you have 50cm^3 of acid and the same amount of an alkali (also assuming that 1 cm^3 weighs 1 gram)
and specific heat capacity is 4.2 joules per gram per degree celsius (assuming same as water)
measure the initial temperature (take it as 22 degrees celsius)
then measure the final temperature when the reaction is complete (take that as 28 degrees celsius)
apply the formula q = m c (change in) T , where m is mass, c is the heat capacity, T is the temperature
so, q = 100 x 4.2 x 6
q=2520 Joules, and thats your enthalpy change of reaction.. hope it helped

sorry but colorimetry is different
its used to identify the intensity of light being absorbed and emitted when measuring the Rate of Reaction when increasing concentrations!!
I can help further if you like too!

 

[Hirdayesh.B.Shrestha]

How many significant figure do we use in our answers? Is it three or two? Or is it what the values in the question is in?

 

[strangerss]

From the list of given numbers, you eliminate 62 because that's way out of given range. Notice that all numbers, except 62, are around about 54.
So mean = (56+54+56+53)/4 = 54.75 and round it off to 54.8 - Always look out for a value that is too out of the range or say is the furthest.

Oh that explains , thanks a lot for the help jazak alALLAH 5er

 

[marz_katy]

yes plz can u help me to knw more about dis plzzz=)

 

[saadgujjar]

any guess about chemistry paper 34

 

[Jinkglex]

How many significant figure do we use in our answers? Is it three or two? Or is it what the values in the question is in?

usually 3, unless specified otherwise