Qualification > Sciences
Chemistry- need help mcq questions plz!!!
Deadly_king:
--- Quote from: sunstar on November 14, 2010, 12:07:45 pm ---Can u please explain me again june 2005 Q11 and Q31
nov 2002 Q39 {How is the first one having the tendency to form ethanol } sorry still didn't understand and plz i need help in june 04 Q2, 12, 36,
thank you very much.
--- End quote ---
Nov 02 Q39
It will have a tendency to form ethanol since the equilibrium will be shifting to the left. Hence most H+ and CH3CH2O- will combine together to form ethanol such that they'll be almost absent in the resulting mixture. ;)
Jun 05 p1
11. I'll proceed by elimination. ;)
B cannot be good since rate of formation of methanol is not gong to increase slowly. Rate of reaction given by the gradient should be highest when t = 0.
C also cannot be the answer since rate of reaction is not constant. As reactants decrease in quantity the rate will decrease as well.
D starts well but the concentration of methanol cannot come to a maximum and then decrease a bit until it becomes constant since this is not a reversible reaction.
This leaves us with A.
31. Trigonal planar is the structure of molecules having 3 bond pairs of electrons and zero lone pairs of electrons.
Aluminium has 3 electrons in its outermost shell and in AlCl3, it is sharing all three of them, one with each Cl atom. In other words this molecule has got 3 bond pairs and 0 lone pairs. So it has a trigonal planar shape.
Carbon has 4 electrons in its outermost shell and CH3 it is sharing 3 electrons and the + sign indicate that it has lost the remaining electron. Hence it is also trigonal planar in shape.
Phosphorus has got 5 electrons in its outermost shell. In PH3, it is sharing 3 of them with each H atom such that a lone pair of electrons remains. Therefore, despite having 3 bond pairs it also has one lone pair of electrons which repel all the bond pairs to form a pyramidal shape. Hence it is not trigonal planar.
Answer is B since only 1 and 2 are good.
Deadly_king:
Jun 04 p1
2. You just need to find the equation which balance appropriately. ;)
The obvious answer is B since in the other 3 hydrogen is not balanced.
If you don't know how to balance an equation, you need to review your basics. Try here or here. They might help you. :)
12. Most rapid reaction is one with the lowest activation energy. So A, B and C are eliminated right away.
Hence answer is C.
36. Hydrogen iodide is not purple. It is the iodine vapour (at first iodine gas is emitted, it will then solidify since iodine exists as a solid at room temperature) which results from the decomposition of Hydrogen Iodide that is violet in colour.
So number 1 is rejected. The only answer left is C.
$!$RatJumper$!$:
S05 Q1 Please... im SO lost in it.
Thank you
Deadly_king:
--- Quote from: $!$RatJumper$!$ on November 17, 2010, 06:10:20 am ---S05 Q1 Please... im SO lost in it.
Thank you
--- End quote ---
Combustion of hydrocarbon : That was the first number in p2. ;D So I doubt it'll come again.
Anyway, the equation is as follows.
Hydrocarbon + Oxygen ----> CO2 + H2O + O2(excess)
Volume of hydrocarbon used : 10cm3
Volume of oxygen used : (70 - 20) = 50cm3
Volume of CO2 formed : 30cm3
Volume of H2O liberated : (80 - 50) = 30cm3
NOTE : The volume of H2O liberated is calculated by subtracting the volume of CO2 and excess O2 from the total volumes of reactants. ;)
Ratio of hydrocarbon to CO2 is 10:30 ----> 1:3
So there need to be 3 carbon atoms in the hydrocarbon.
Since volume of CO2 is equal to volume of H2O, the hydrocarbon contains twice number of H atoms and hence is an alkene. :D
Answer is B
$!$RatJumper$!$:
Many thankx :)
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