Chemistry MCQ thread...

[kshumaila52]

12
© UCLES 2008 9701/01/M/J/08
30 Compound X, C6H12O, is oxidised by acidified sodium dichromate(VI) to compound Y.
Compound Y reacts with ethanol in the presence of a little concentrated sulphuric acid to give
liquid Z.
What is the formula of Z?
A CH3(CH2)2CH=CHCO2H
B CH3(CH2)4CH2COCH2CH3
C CH3(CH2)4CO2CH2CH3
D CH3CH2CO2(CH2)4CH3

C is correct ans, as C6H12O is an alcohol, so it ll oxidise to carboxylic acid and on reaction with ethanol, ester ll produce

 

[kanz123]

hey shdnt the answer be "B"??...but it's C ???h0w?..cud ne1 xplain dat ?

 

[Hateexams93]

why not D ?

 

[Hateexams93]

hey shdnt the answer be "B"??...but it's C ???h0w?..cud ne1 xplain dat ?

it says nucleophillic addition , so H- has to be negative

 

[Hateexams93]

C is correct ans, as C6H12O is an alcohol, so it ll oxidise to carboxylic acid and on reaction with ethanol, ester ll produce

why not D ?

 

[kshumaila52]

hey shdnt the answer be "B"??...but it's C ???h0w?..cud ne1 xplain dat ?

I just explain it,just few post back...
anyhow
it is very simple, 1st nuclephile means electron rich so it ll be H-, 2ndly in nucleoplillic addition reaction, nucleoplile always attck on carbon atom which become electron partially positive charge due to more electronegative oxygen atom.
hope you ll get it..
is still any confusion???

 

[kshumaila52]

why not D ?

check name of compound, not only carbon atoms, d will form when propanoic acid react with pentanol.

 

[Hateexams93]

why not D ?

check name of compound, not only carbon atoms, d will form when propanoic acid react with pentanol.

oh right ..thank u ) i didn't notice the name

 

[Hateexams93]

Carbon disulphide vapour burns in oxygen according to the following equation.
CS2(g) + 3O2(g) → CO2(g) + 2SO2(g)
A sample of 10 cm3 of carbon disulphide was burned in 50 cm3 of oxygen. After measuring the
volume of gas remaining, the product was treated with an excess of aqueous sodium hydroxide
and the volume of gas measured again. All measurements were made at the same temperature
and pressure, under such conditions that carbon disulphide was gaseous.
What were the measured volumes?
volume of gas after burning / cm3 volume of gas afteradding NaOH(aq) / cm3
A 30 0
B 30 20
C 50 20
D 50 40

 

[Xthegreat]

num. of mole of CS2 = 10/24000 = 4.17 x 10^-4 mole
number of mole of oxygen needed = 3 x 4.17 x 10^-4 = 1.25 x 10^-3 mole
vol of oxygen needed = 1.25 x 10^-3 x 24000 = 30cm3
- so we have 20cm3 of unreacted oxygen

volume of CO2 produced = 10cm3
volume of SO2 produced = 20Cm3
- so volume of gas after burning is 10 + 20 + 20 = 50cm3

when added with NaOH, SO2 will react with NaOH , SO2 + NaOH -> Na2SO3 + H2O, no gas
CO2 + NaOH -> Na2CO3 + H2O

so all we have left at the end is the 20cm3 of oxygen.

ans : C

 

[hassam]

 

[hassam]

 

[hassam]

 

[hassam]

Which of the following statements regarding the chlorides MgCl2, SiCl4 and PCl3 is
correct?
A All three chlorides hydrolyse completely in water to give acidic solution.
B The oxidation states of chlorine in the three chlorides are –2, –4 and –3 respectively.
C All are liquids at room temperature.
D MgCl2 is the only chloride having a giant structure.

 

[Hateexams93]

Red lead oxide, Pb3O4, is used in metal priming paints. It can be made by heating PbO in air.
6PbO(s) + O2(g) → 2Pb3O4(s)
Which two values are needed to calculate the enthalpy change for this reaction?
A enthalpy change of combustion of lead and enthalpy change of formation of Pb3O4
B enthalpy change of combustion of PbO and enthalpy change of formation of Pb3O4
C enthalpy change of formation of PbO and enthalpy change of atomisation of O2
D enthalpy change of formation of PbO and enthalpy change of formation of Pb3O4
HOW TO SOLVE THIS KIND OF PROBLEMS ??????????

 

[Hateexams93]

D ????????? ITS NOT GIANT covalent

 

[Hateexams93]

Which of the following statements regarding the chlorides MgCl2, SiCl4 and PCl3 is
correct?
A All three chlorides hydrolyse completely in water to give acidic solution.
B The oxidation states of chlorine in the three chlorides are –2, –4 and –3 respectively.
C All are liquids at room temperature.
D MgCl2 is the only chloride having a giant structure.

D is the correct answer ?

 

[Zishi]

C

 

[Zishi]

D

 

[Zishi]

C