Chemistry: Post your doubts here!

[flashguy412]

hey thanks hamid.

also, u know when making optical isomers. does it matter which group is on the top of the chiral carbon atom. for example, if we had butan-2-ol, then we clearly have a chiral carbon atom. now does it matter whether (when drawing the displayed formula of the optical isomer) which group of atoms we place right above the chiral carbon atom. like can it be a hydrogen OR a hydroxy group OR a methyl. does it really matter.?

 

[OakMoon!]

The 3D shape does matter. But not the placement of different molecules, they can be interchanged because it doesn't matter.

 

[flashguy412]

ok one more thing. lets say we have but-2-ene. we're told to draw the cis trans isomers. now in the marking scheme, it says H3C (linked to one of the carbon atoms) whereas i would normally write CH3. does that matter?

 

[OakMoon!]

No. It just shows that C is the one attached to other C and not the H3.

 

[Dayyanah]

@ flashguy412
from what we were told, cambridge penalises if the bond line is touching the wrong atom ie if its touching the H instead of O as in -OH and H3 instead of C in -CH3.
hope i havent confused you more :no:

 

[OakMoon!]

But if you write CH3 and link C with C, it's fine. But when making a 2D shape and writing the CH3 on the left side it's better to write it as H3C as shown in the diagram below.

 

[panda222]

Ok I dont question 1 (what topic is it, I dont remember learning that)

Can someone plzzz help. I have my exam tomorow.

http://www.xtremepapers.com/CIE/Interna ... 6_qp_2.pdf

 

[yuliana95]

Ok I dont question 1 (what topic is it, I dont remember learning that)

Can someone plzzz help. I have my exam tomorow.

http://www.xtremepapers.com/CIE/Interna ... 6_qp_2.pdf

It's on qualitative analysis and mole concept..

Answers:
(a i) NH3
(a ii)NH4+
(a iii)Fe(OH)2
(b )BaSO4
(c i)FeSO4
(NH4)2SO4
(c ii)Mr of Salt1: FeSO4 = 151.9
Mr of Salt2: (NH4)2SO4 = 132.1
(c iii) xH2O = 392 – (132.1 + 151.9) = 108
x = 108/18 = 6

 

[panda222]

Can someone explain question 2 c please. Thanks

http://www.xtremepapers.com/CIE/Interna ... 7_qp_2.pdf

 

[AAestar]

Can someone explain question 2 c please. Thanks

http://www.xtremepapers.com/CIE/Interna ... 7_qp_2.pdf

 

[Dayyanah]

everyone sitting for Chem P2 tomorrow, good luck!!

odd qn, anyone from dar? :beer:
what times the paper?

 

[Dayyanah]

do we need to know the flame tests?? coz we were told we have to be able to identify the metals if we're give the flame colour but i haven't seen them in the past papers
has anyone else come across it?? do we have to know 'em?

 

[no.mercy]

can someone explain me the answer of this MCQ...

 

[XPFMember]

Assalamoalaikum!

See in 1 mol of CaCl2 there are 2 Cl- ions! so in 0.02 mol, there'll be 0.02 x 2 = 0.04 mol of Cl- ions

conc. = n / vol.
=0.04 / 0.2 = 0.2 mol dm-3

remember ...u must convert vol. to dm3

 

[no.mercy]

Assalamoalaikum!

See in 1 mol of CaCl2 there are 2 Cl- ions! so in 0.02 mol, there'll be 0.02 x 2 = 0.04 mol of Cl- ions

conc. = n / vol.
=0.04 / 0.2 = 0.2 mol dm-3

remember ...u must convert vol. to dm3

Thankuuuuuuuuuuuuuuuuuuuuuuuuu

 

[leosco1995]

This is probably a very dumb O-level type of question but how do you figure out the number of bonds in covalent molecules? Like for example, how many bonds are there in PCl3, N2H4 and CS2? I have always sucked at bonding since O-levels so any help here would be appreciated.

 

[yuliana95]

This is probably a very dumb O-level type of question but how do you figure out the number of bonds in covalent molecules? Like for example, how many bonds are there in PCl3, N2H4 and CS2? I have always sucked at bonding since O-levels so any help here would be appreciated.

Try drawing the structure first.. then see the number of bonds.. Try www.chemguide.co.uk and see some notes from there..

 

[no.mercy]

Can someone explain me the answer of this question

 

[OakMoon!]

Can someone explain me the answer of this question

B! Just the trend of successive ionisation energies in a period. Check out the concept in your book. It's simple.

 

[student]

EXPLAIN the strength, high melting point and electrical insulating
properties of ceramics in terms of their giant molecular structure.