Chemistry MCQ thread...

[Zishi]

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.....
Halogenoalkanes in which all of the hydrogen atoms in the alkane have been replaced by halogen atoms are difficult to burn - hence their use in flame retardants and even in fire extinguishers.

Hmm, yes.

 

[Hateexams93]

12 Which chlorine compound has bonding that can be described as ionic with some covalent
character?
A NaCl B MgCl2 C AlCl3 D SiCl4
WHY ITS B ?????

 

[aquariangurl]

^Thats because if you look at the rest of the compounds, NaCl has Ionic bonding, AlCl3 also has Ionic bonding, SiCl4 has a giant covalent structure, So obviously the only answer that makes sense is MgCl2

 

[Zishi]

^Thats because if you look at the rest of the compounds, NaCl has Ionic bonding, AlCl3 also has Ionic bonding, SiCl4 has a giant covalent structure, So obviously the only answer that makes sense is MgCl2

NO! SiCl4 is simple covalent structure. (Mg is a small ion with a relatively high charge, therefore polarizing)

 

[aquariangurl]

Thanx for pointing that out.
Verdict, I gotta study -.-

 

[hassam]

or to put it simply AlCl3 is purely covalent as SiCl4 while NaCl is ionic...

 

[Zishi]

or to put it simply AlCl3 is purely covalent as SiCl4 while NaCl is ionic...

You can't say that so easily. AlCl3 has ionic bonding at room temperature(with a lot of covalent character), but as it vaporises, its structure changes to covalent from ionic.

 

[Saturation]

12 Which chlorine compound has bonding that can be described as ionic with some covalent
character?
A NaCl B MgCl2 C AlCl3 D SiCl4
WHY ITS B ?????

what I think is that it's because when you dissolve MgCl2 in water the pH is 6.5.

Although my teacher said that is was C and that the ms must be wrong! :unknown:

 

[Zishi]

12 Which chlorine compound has bonding that can be described as ionic with some covalent
character?
A NaCl B MgCl2 C AlCl3 D SiCl4
WHY ITS B ?????

what I think is that it's because when you dissolve MgCl2 in water the pH is 6.5.

Although my teacher said that is was C and that the ms must be wrong! :unknown:

Mark scheme is always right. It just went wrong for once only - i.e Winter 2010 QP12.

 

[Xthegreat]

AlCl3 and SiCl4 are a COVALENT COMPOUND! not ionic properties whatsoever.

the question says ionic with some covalent character. this only happens when
- the charge on the cation is large ( Mg > Na )
- the cation is small ( Mg < Na )
- the size of the anion is large

since both anion are the same, and Mg is smaller in size and has a greater charge, it has a higher covalent character then Na.

* just in case you forget, the main bondings in MgCl2 and NaCl are IONIC BONDS.

 

[Hateexams93]

AlCl3 is also covalent , but with covalent character , thats the only thing that i was confused about

 

[Hateexams93]

also ionic * but with covalent character, sorry

 

[smartangel]

Can some one please explain May/jun 09 Q4?

 

[alphabravocharlie]

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

Aoa. Shouldn't the answer be B, 6. For cis-trans isomers we need a C=C. There are only 3 double bonds so answer should be 6.

 

[Xthegreat]

how is AlCl3 IONIC?!!!
- AlCl3 is pure covalent
- exists in simple molecular form
- held together by weak VDW

AlCl3 IS COVALENTLY BONDED!!!

 

[Zishi]

also ionic * but with covalent character, sorry

Right.

Can some one please explain May/jun 09 Q4?

Draw the electronic configurations of atoms with protons number 13 and above. That will tell you 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

Aoa. Shouldn't the answer be B, 6. For cis-trans isomers we need a C=C. There are only 3 double bonds so answer should be 6.

Wasalam. No, it is C, i.e 8. Number of cis-trans isomers of a "straight-chain" compound with n double bonds is 2^n, NOT 2n. See my explanation about it in previous posts.

how is AlCl3 IONIC?!!!
- AlCl3 is pure covalent
- exists in simple molecular form
- held together by weak VDW

AlCl3 IS COVALENTLY BONDED!!!

Believe it OR not. AlCl3 is ionic at room temperature(but with a lot of covalent character).

 

[smartangel]

please explain this:

0.200 mol of a hydrocarbon undergo complete combustion to give 35.2 g of carbon dioxide and
14.4 g of water as the only products.
What is the molecular formula of the hydrocarbon?
A C2H4 B C2H6 C C4H4 D C4H8

the answer is D.

 

[Zishi]

Chemistry MCQ thread...

I hope this helps.

 

[MHHaider707]

please explain this:

0.200 mol of a hydrocarbon undergo complete combustion to give 35.2 g of carbon dioxide and
14.4 g of water as the only products.
What is the molecular formula of the hydrocarbon?
A C2H4 B C2H6 C C4H4 D C4H8

the answer is D.

First of all calculate the moles of CO2 and H2O....

moles of CO2 = 35.2/44 = 0.8 mol
moles of H2O = 14.4/18 = 0.8 mol

now using equations of combustion of hydrocarbons check that which hydrocarbon gives 0.8 mol of CO2 and 0.8 mol of H2O!!


Answer will be D!!!!

 

[smartangel]

N09 paper 11 Q 28?