Chemistry MCQ thread...

[Zishi]

Hi people,

How about posting your problems in MCQS in this thread? This way many people will benefit from your problems(as all the tough MCQs, etc will be there in just on thread).

By the way you may also post your confusions in ANY topic. We'll try to help you clearing your concepts. :good:

 

[hassam]

 

[Zishi]

The answer is A. Combustion is always exothermic, and atomisation is always endothermic. Imagine a energy profile diagram for graphite and diamond, diamond is higher than graphite in it as the reaction from graphite to diamond is endothermic. Enthalpy change of atomisation is also endothermic, so C atoms will be even more "above" in the diagram than graphite and diamond. Combustion is always exothermic, so CO_2 and H_2 0 will be even lower than graphite in the enthalpy profile diagram. This way, it will help you to understand the answer.


--------------------------------------------------------------------------------------------------------------------------------------------

Now my question:

Why doesn't increasing the volume increases the rate of the reaction? Increasing volume increases the concentration, and every book says that increasing concentration of any of the reactants increases the rate of the reaction. Why is the answer B? Why it isn't C?

 

[MHHaider707]

39 Which compounds are able to react with concentrated sulphuric acid to give a product that will
decolourise acidic potassium manganate(VII)?
1 (CH3)3COH
2 CH3CH2CH(OH)CH3
3 CH3CO2H

 

[aquariangurl]

@Zishi, The answer is B, because increasing the volume doesn't increase the concentration, Yes if they had said that a more conc. solution of h2so4 was used then prolly the answer could be C

 

[MHHaider707]

@Zishi, The answer is B, because increasing the volume doesn't increase the concentration, Yes if they had said that a more conc. solution of h2so4 was used then prolly the answer could be C

Yeah rite ! !


Can anyone plzz tell me abt the Q which posted???

 

[Zishi]

@Zishi, The answer is B, because increasing the volume doesn't increase the concentration, Yes if they had said that a more conc. solution of h2so4 was used then prolly the answer could be C

Yeah rite ! !


Can anyone plzz tell me abt the Q which posted???

It's B - carbon atoms with hydroxyl groups have another carbon atoms with hydrogen atoms attached to 'em. This way conc. sulphuric acid will dehydrate them to give alkenes. And of course, alkenes are oxidised by way too string permanagnate ions. This will change their color from purple to colorless.

 

[Zishi]

@Zishi, The answer is B, because increasing the volume doesn't increase the concentration, Yes if they had said that a more conc. solution of h2so4 was used then prolly the answer could be C

Concentraion = moles / volume in dm^3 - so how can you say that changing volume doesn't change something's concentration? :unknown:

 

[sheikfathiu]

can som1 help me solve this with explanation

 

[aquariangurl]

@Zishi, The answer is B, because increasing the volume doesn't increase the concentration, Yes if they had said that a more conc. solution of h2so4 was used then prolly the answer could be C

Concentraion = moles / volume in dm^3 - so how can you say that changing volume doesn't change something's concentration? :unknown:

Ok, My bad, But in this case zinc acts as a limiting reagent, so even when you increase the vol. of h2so4, amt. of h2 collected will remain the same, Do you get it now?

 

[Zishi]

can som1 help me solve this with explanation

If you want replies, then please take a screen shot of it using the "print screen" button on your keyboard. Open paint, and then press "ctrl+v" to get the image. Crop it, and attach it in your post. This way many people will answer it.

 

[Zishi]

@Zishi, The answer is B, because increasing the volume doesn't increase the concentration, Yes if they had said that a more conc. solution of h2so4 was used then prolly the answer could be C

Concentraion = moles / volume in dm^3 - so how can you say that changing volume doesn't change something's concentration? :unknown:

Ok, My bad, But in this case zinc acts as a limiting reagent, so even when you increase the vol. of h2so4, amt. of h2 collected will remain the same, Do you get it now?

Hmm, yeah. This explains it - I was also thinking the same.

 

[hassam]

In which pair do both atoms have one electron only in an s orbital in their ground states?
A Ca, Sc B Cu, Be C H, He D Li, Cr

 

[sheikfathiu]

pls can som1 help explain the soln
20 The compound known as ‘gamma-linolenic acid’ is found in significant amounts in the seeds of
the Evening Primrose plant. There is evidence that the compound may help patients with
diabetes.
CH3(CH2)4CH=CHCH2CH=CHCH2CH=CH(CH2)4CO2H
gamma-linolenic acid
How many cis-trans isomers does gamma-linolenic acid have?
A 3 B 6 C 8 D 12

 

[Zishi]

In which pair do both atoms have one electron only in an s orbital in their ground states?
A Ca, Sc B Cu, Be C H, He D Li, Cr

Write their structures in terms of s,p,d, etc. according to their proton numbers - you'll get the answer.

pls can som1 help explain the soln
20 The compound known as ‘gamma-linolenic acid’ is found in significant amounts in the seeds of
the Evening Primrose plant. There is evidence that the compound may help patients with
diabetes.
CH3(CH2)4CH=CHCH2CH=CHCH2CH=CH(CH2)4CO2H
gamma-linolenic acid
How many cis-trans isomers does gamma-linolenic acid have?
A 3 B 6 C 8 D 12

It's C. Here you go.
cis-cis-cis
cis-cis-trans
cis-trans-cis
cis-trans-trans
trans-cis-cis
trans-cis-trans
trans-trans-cis
trans-trans-trans

 

[hassam]

i dont get abt cromium

 

[hassam]

 

[hassam]

i also dint understand that y they are used for fire extinguishing..it was written in book something abt Br radicals quenching the flame......wats all that

 

[Zishi]

i dont get abt cromium

It has a proton number of 24.

1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 4s^1


Because two has a long hydrocarbon chain with a lot of hydrogen atoms. It has only one Br atom. So only one free radical is formed to quench the fire. Plus due to long hydrocarbon tail, it is also flammable.

EDIT: Its explanation is outta syllabus, but bromine free radicals are highly energetic, so they react with other free radicals in flame to quench it. It's all what we have to know... :roll:

 

[hassam]

hmm..i agree with that flammable THING Abt b....thnxx....but forming one Br radical is nt ryt i guess cos both form one Br free radical.....
this is wat chemguide says
Halogenoalkanes in which all of the hydrogen atoms in the alkane have been replaced by halogen atoms are difficult to burn - hence their use in flame retardants and even in fire extinguishers.