Chemistry MCQ thread...

[Mobeen]

saturation u were right .. q5 is D .. it is w08 qp1 q 38 . answer is D ..

phew! so what I said earlier makes sense then? It's chloro flouro carbons which are used in extinguishers? (and btw some of the HALOGENOALKANES are extremely flammable!)

^ yeah u were right ..

 

[abdullah181994]

is there any formulae for balancing hydrocarbons contianing oxygen like alchols for their combustion reaction.

 

[CommonSense]

the general formula for an alcohol is CnH2n+1OH. So when combusted, nCO2 will be formed and (2n+2)/2 water will be formed. for that u will need
[2n+{(2n+2)/2}-1]/2 oxygen molecules. Simplifying this will give us:
CnH2n+1OH + (1.5n)O2 --------> nCO2 + (n+1)H2O
Example:
C5H11OH + 7.5O2 ----------> 5CO2 + 6H2O
You will notice that the number of moles of carbondioxide formed = the number of carbons in the alcohol; the number of moles of water formed = the number of hydrogens divided by two. The number of oxygens required it is the total number of oxygens on the right hand side minus all the oxygens in the compound to be combusted then divide by two. This rule is the same for all organic compounds and through it u can derive all the formulas u want!!
NB I found that out myself as I have no text book to study from, so I recommend u to check my information just to make sure.

 

[CommonSense]

is there any formulae for balancing hydrocarbons contianing oxygen like alchols for their combustion reaction.

I am sorry if u think I am picky, but hydrocarbons donot contail oxygen. You could have said organic compounds just to be more accurate. :sorry:

 

[ninjas4life]

16 Chlorine compounds show oxidation states ranging from –1 to +7.
What are the reagent(s) and conditions necessary for the oxidation of elemental chlorine into a
compound containing chlorine in the +5 oxidation state?
A AgNO3(aq) followed by NH3(aq) at room temperature
B concentrated H2SO4 at room temperature
C cold dilute NaOH(aq)
D hot concentrated NaOH(aq)

and
plz explain

from m/j 03

 

[diwash]

How many different substution products are possible,in prnciple, when a mixture of ethene and bromine are allowed to react? (0/N 04)
A. 3
B. 5
C. 7
D. 9

Can anyone explain how is the answer D.

 

[diwash]

is there any formulae for balancing hydrocarbons contianing oxygen like alchols for their combustion reaction.

I devloped this formula myself and it works for all cases..

CxHyOz + (x+ y/4- z/2) o2-------------------x Co2 + (y/2) H2o

 

[aliya_zad]

Chlorine is both oxidised and reduced when it reacts with water or with sodium hydroxide solution – disproportionation reactions.

With water: Cl2(aq) + H2O(l) " HOCl(aq) + HCl (aq)

With cold dilute NaOH solution: Cl2(aq) + 2NaOH(aq) ---> " NaCl (aq) + NaOCl (aq) + H2O(l)

With hot concentrated NaOH solution: 3Cl2(aq) + 6NaOH(aq) "-----> 5NaCl(aq) + NaClO3(aq) + 3H2O(l)
and with hot conc. naoh the oxidation no changes from 0 to -1 and +5.

 

[aliya_zad]

16 Chlorine compounds show oxidation states ranging from –1 to +7.
What are the reagent(s) and conditions necessary for the oxidation of elemental chlorine into a
compound containing chlorine in the +5 oxidation state?
A AgNO3(aq) followed by NH3(aq) at room temperature
B concentrated H2SO4 at room temperature
C cold dilute NaOH(aq)
D hot concentrated NaOH(aq)

and
plz explain

from m/j 03

Chlorine is both oxidised and reduced when it reacts with water or with sodium hydroxide solution – disproportionation reactions.

With water: Cl2(aq) + H2O(l) " HOCl(aq) + HCl (aq)

With cold dilute NaOH solution: Cl2(aq) + 2NaOH(aq) ---> " NaCl (aq) + NaOCl (aq) + H2O(l)

With hot concentrated NaOH solution: 3Cl2(aq) + 6NaOH(aq) "-----> 5NaCl(aq) + NaClO3(aq) + 3H2O(l)
and with hot conc. naoh the oxidation no changes from 0 to -1 and +5.
So the answer has to be D.

 

[diwash]

Oct/Nov 04 ..Qno 38...HElp

 

[aliya_zad]

pls explain. Im a little confused over here. Should we consider the no of moles produced or consider the bonds...

 

[aliya_zad]

Oct/Nov 04 ..Qno 38...HElp

It can be all 3.
1)2 chlorines present, and therefore rate of reaction will be faster and the amount produced for a given tim e will be greater.
It can be 2 and 3 as well coz the c-br and c-cl bond energies are lower than that of c-cl. therefore reaction can proceed at a faster rate.

 

[ninjas4life]

can someone plzzz ans this? and explain..

 

[diwash]

@ninjas...In the residue given how many total double bond are left...if u count then u will get 2 left from the topmost residue..and 1 left with second and last residue...so there are total of
4 carbon double bond (C=C) still left then from the original molecule...this means that 5 carbon double bond has been removed by hydrogenation from the original molecule. As a H2 molecule can remove a single double bond, 5 H2 molecules has reacted .so the anwer is B.

 

[diwash]

How many different substution products are possible,in prnciple, when a mixture of ethene and bromine are allowed to react? (0/N 04)
A. 3
B. 5
C. 7
D. 9

Can anyone explain how is the answer D.

Some one please Help me with this.

 

[kshumaila52]

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.

no
if u are finding cis traans then apply formula 2 power n where n is number of double bonds..so 2 power3 is equal to 8....this is what our teacher said

 

[Xthegreat]

How many different substution products are possible,in prnciple, when a mixture of ethene and bromine are allowed to react? (0/N 04)
A. 3
B. 5
C. 7
D. 9

Can anyone explain how is the answer D.

Some one please Help me with this.

you actually typed the question wrongly, it's suppose to be ethane.
well talk about ethane before we go to ethene.
so ethane + bromine = free- radical substituition.
its actually very very long to type and the answer is 9.
* u should check your book under free-radical substituition, I M 100% SURE they have the exact mechanism there.
i'll show you a few,
C2H6 + Br' -> C2H5' + HBr ( that is 1 )
C2H5' + Br2 -> C2H5Br + Br'
C2H5Br + Br' -> C2H4Br' + HBr
* the substitution goes on and on, see the pattern?

and if you are using ethene, then it depends whether you are using AQUEOUS bromine or Br in CCl4.
if in aqueous bromine then there are 2 types.
if u use Br in CCl4 then there is only 1 product.

 

[ninjas4life]

3. Use of the Data Booklet is relevant to this question.
Most modern cars are fitted with airbags. These work by decomposing sodium azide to liberate
nitrogen gas, which inflates the bag.
2NaN3 ---> 3N2 + 2Na
A typical driver’s airbag contains 50 g of sodium azide.
Calculate the volume of nitrogen this will produce at room temperature.
A 9.2 dm3 B 13.9 dm3 C 27.7 dm3 D 72.0 dm3

9 Which substance, in 1 mol dm–3 aqueous solution, would have the same hydrogen ion
concentration as 1 mol dm
3 of hydrochloric acid?
A ethanoic acid
B nitric acid
C sodium hydroxide
D sulphuric acid

 

[hassam]

well that should be nitric acid....cos ethanoic is a weak onne H2SO4 gives 2H+ per molecule so u r left up with HNO3

 

[hassam]

WEN WILL EXAM COME....come'on cambridge...these holidayxx are hell boring