Chemistry MCQ thread...

[sakibfaiyaz]

Oh yes, thanks

 

[xHazeMx]

you welcome .. if u have more questions post them as it would be beneficial for all

 

[sakibfaiyaz]

The answer to this is B.
But why should it react differently with Conc H2SO4?...they are both primary alcohols (so rate of dehydration would be the same would it not?)....And even if one of them were secondary, then C would also have been the right answer( As secondary alcohols undergo Sn2 and Sn1 but primary alcohols always undergo Sn2 reaction)

 

[Zishi]

The answer to this is B.
But why should it react differently with Conc H2SO4?...they are both primary alcohols (so rate of dehydration would be the same would it not?)....And even if one of them were secondary, then C would also have been the right answer( As secondary alcohols undergo Sn2 and Sn1 but primary alcohols always undergo Sn2 reaction)

No, that's not the thing. For dehydration, the carbon atom with hydroxyl group must be attached with another carbon atom with at least one hydrogen atom. In the first alcohol, this isn't the case - so that won't be dehydrated.

 

[Xthegreat]

as you said, concentrated sulphuric acid is for dehyrdration.

(CH3)3CCH2OH cannot undergo dehydration.
- dehydration requires the alcohol to remove the -OH group from one carbon and another -H atom from the carbon next to it
- (CH3)3CCH2OH cannot do this

 

[sakibfaiyaz]

@Xthegreat: oh right! I get it now. Thanks
@Zishi: Dont you mean the first alcohol cannot be dehydrated?

 

[Zishi]

@Xthegreat: oh right! I get it now. Thanks
@Zishi: Dont you mean the first alcohol cannot be dehydrated?

Oops, sorry. That was a typo. Yes I mean that first one can't be dehydrated.

 

[Hateexams93]

Chlorofluorocarbons (CFCs) have been widely used in aerosol sprays, refrigerators and in making
foamed plastics, but are now known to destroy ozone in the upper atmosphere.
Which of the following will not destroy ozone, and therefore can be used safely as a replacement
for CFCs?
A CHBr3 B CCl3CBr3 C CHClFCClF2 D CH3CH2CH2CH3
WHY ITS D ???and NOT A ???????????

 

[xHazeMx]

Chlorofluorocarbons (CFCs) have been widely used in aerosol sprays, refrigerators and in making
foamed plastics, but are now known to destroy ozone in the upper atmosphere.
Which of the following will not destroy ozone, and therefore can be used safely as a replacement
for CFCs?
A CHBr3 B CCl3CBr3 C CHClFCClF2 D CH3CH2CH2CH3
WHY ITS D ???and NOT A ???????????

Br acts as free radical too so its usage is strictly forbidden as it would deplete the ozone layer, the first 3 choices contains either Br or Cl. Hence, the only possible answer is D and sometimes they can replace D with hydro-fluorocarbons which is also correct

 

[sakibfaiyaz]

A small increase in temperature, gives us a Boltzman distribution curve whose peak is to the right of the original curve right? So how is the answer B here? Should it not be D? where the peak is now much more to the right (at 500 degrees)

 

[xHazeMx]

i think we got a special case here, at 500 C nitrogen START reacting so i think it is shifted to the right side slightly as it is obvious and it has the same area under the curve which indicates the number of the total moles having the activation energy or not or more so i think its B

 

[xHazeMx]

is polyethene
flammable?
releases toxic gases when burnt?
odourless ( doesnt have a smell ) ?
reactive ?
biodegradable?
and what are its uses ?

 

[aquariangurl]

Usually, when the temperature increases, the peak of the curve shortens & moves towards the right, but the area under the curve remains the SAME, but in D, the area decreases..so the most appropriate answer is B

 

[hassam]

the plastic bags u see.......are made of polyethene....so its non-biodegradable....odourless burnt----unreactive

 

[Zishi]

is polyethene
flammable?

yes

releases toxic gases when burnt?

NO

odourless ( doesnt have a smell ) ?

Yes

reactive ?

NO

biodegradable?

NO

and what are its uses ?

Shopping bags, plastic objects, etc

 

[xHazeMx]

how about PVC ?

 

[Zishi]

how about PVC ?

NO. That's made from the monomer chloroethene. and PVC is itself abbreviation of poly vinyl chloride. Are there any chlorine atoms in polyethene?

 

[xHazeMx]

A compound X has all of the following properties:

it is a liquid at room temperature and atmospheric pressure;
it does not mix completely with water;
it does not decolorise acidified potassium manganate(VII).

What could X be?

A ethane

B ethanoic acid

C ethanol

D ethyl ethanoate
why B is not possible?

 

[Zishi]

A compound X has all of the following properties:

it is a liquid at room temperature and atmospheric pressure;
it does not mix completely with water;
it does not decolorise acidified potassium manganate(VII).

What could X be?

A ethane

B ethanoic acid

C ethanol

D ethyl ethanoate
why B is not possible?

Because ethanoic acid mixes completely with water.

 

[Hateexams93]

A sample of chlorine containing isotopes of mass numbers 35 and 37 was analysed in a
mass-spectrometer.
How many peaks corresponding to +2Cl were recorded?
A 2 B 3 C 4 D 5
WHY ITS B ?