Posted by Kyo on November 03, 2007 at 08:44:37:
In Reply to: Ans to (03 Nov) Qns posted by Kyo on November 03, 2007 at 08:26:38:
Q33 (04 Nov 07)
At sea level, standard atmospheric pressure (ie. 1.01325 x 10^5 Pa)
causes mercury in a dish to rise 760 mm up a glass column. A mixture of
two alkanes (with molar masses 16.0 g and 30.0 g respectively) is stored
in a container at 294 mmHg. The gases undergo complete combustion to produce
CO2 that has a pressure of 356 mmHg when measured at the same temperature
and volume as the original mixture. Calculate the percentage composition
of the mixture.
Q34 (04 Nov 07)
Compound A, C10H12O, caused precipitation of silver metal on addition to a solution containing [Ag(NH3)2]+ ions. It reacted with LiAlH4 in dry ether to give compound B, C10H14O, which was converted by hydrobromic acid to compound C, C10H13Br. Treatment of compound C with potassium cyanide in dry ethanol gave compound D, C11H13N, which in turn, after boiling under reflux with aqueous sodium hydroxide followed by acidification, gave compound E, C11H14O2. When compound E was heated with thionyl chloride, it gave compound F, C11H13ClO, which reacted on warming with the Lewis acid aluminium chloride to give compound G (shown in the diagram above).
(i) Deduce and explain, giving equations whenever relevant, the identities and structural formulae of compounds A to F.
(ii) Using Lewis / Kekule structures, draw the electron-flow (step-by-step)
mechanism for converting compound F to G.
Q35 (04 Nov 07)
a) 10cm3 of NaOH (aq) has a pH of 10. What mass of H2SO4 (s) must be
added to decrease pH by 1?
b) Given Ka ethanoic acid = 1.8 x 10-5, calculate the pH of a buffer
system (containing 1.0 mol/dm3 ethanoic acid and 1.0 mol/dm3 sodium ethanoate)
after the addition of 0.1 mol/dm3 of gaseous HCl.
c) Given Ka ethanoic acid = 1.8 x 10-5 and Kb ethanoate ion = 5.6 x
10-10, calculate the pH in the titration of 25 cm3 0.1 mol/dm3 ethanoic
acid by 0.1 mol/dm3 sodium hydroxide using
i) 10 cm3 of NaOH
ii) 25 cm3 of NaOH
iii) 35 cm3 of NaOH
Q36 (04 Nov 07)
a) i) Calculate the solubility (g/dm3) of AgCl in a 6.5 x 10-3 mol/dm3
AgNO3 (aq). (Ksp AgCl = 1.6 x 10-10)
ii) Calculate the molarity of NH3 (aq) needed to initiate the precipitation
of Fe(OH)2 (s) from a 0.003 mol/dm3 solution of FeCl2 (aq). (Given Ksp
Fe(OH)2 = 1.6 X 10-14 and Kb NH3 = 1.8 x 10-5)
b) Sea water which contains 0.054 mol/dm3 of Mg2+ (aq) ions, can be
used to obtain magnesium by adding solid calcium hydroxide in a displacement
redox reaction, which yields Mg(OH)2 (s).
i) Given Ksp of Mg(OH)2 = 2.0 X 10-11, calculate [Mg2+] ions in a saturated
solution of Mg(OH)2.
ii) What % of the original magnesium in the seawater can this method
extract?
Q37 (04 Nov 07)
Gaseous hydrogen and gaseous iodine react to produce gaseous hydrogen
iodide.
a) When the two reactant gases are mixed together in a 3 hydrogen :
2 iodine ratio at a certain temperature, there is 42% conversion of hydrogen
gas to hydrogen iodide at equilibrium. Calculate Kp for this reaction if
the total pressure at equilibrium is 5 x 107 Pa.
b) A 1dm3 vessel contains 0.30 mol of I2, 0.40 mol of H2, and 0.10
mol of HI at initial conditions. Given Kc = 0.191 at this temperature,
(i) compare the rates of the forward and backward reactions until equilibrium
is reached; and (ii) calculate the number of moles of H2 that must be added
into the vessel (at initial conditions) to obtain 0.15 mol of HI at equilibrium.
Q38 (04 Nov 07)
Using Lewis / Kekule structures, draw the electron-flow diagram to
describe the mechanisms for the following reactions :
(Note : The following are the mechanisms for some common reactions
encountered at 'A' level / H2 / H1 Organic Chemistry. You will not be
required to draw out all of these mechanisms during the exams; only a few
are required and examinable. What is important for you to understand
however, is that these mechanisms are *not* meant to be memorized, but
(are meant) to be worked out for yourself. In other words, if you have
well and truly mastered your organic chemistry principles, you *will* be
able to work out *all* of these mechanisms all by yourself, no memorization
at all needed. Practice is the key to drawing mechanisms, which are indeed,
regarded as perhaps the most enjoyable aspect of Organic Chemistry. The
beauty of the logical, natural, (indeed) philosophical flow of electrons
during a reaction pathway are captured, recreated and experienced through
one's mind, heart and hands; in the iteration, elucidation and crafting
of a reaction mechanism pathway. Enjoy! ^_^
1) Halogenation of Benzene / MethylBenzene
2) Alkylation/Acylation of Benzene / MethylBenzene
3) Nitration of Benzene / MethylBenzene
4) Acyl Chlorides + Alcohol / Phenol --> Ester
5) Acyl Chlorides + Ammonia / Amine --> Amide
6) Acyl Chlorides + Water --> Carboxylic Acid
7) Nitration of Phenol (dil vs conc HNO3)
8) Bromination of Phenol (aqueous vs CCl4)
9) Bromination of Phenylamine (aqueous)
10) Reduction of Ester to Alcohol by LiAlH4
11) Reduction of Acyl Chloride / Carboxylic Acid to Alcohol by LiAlH4
12) Reduction of Ketones / Aldehydes to Alcohol by LiAlH4
13) Nucleophilic Addition of Cyanide ion to Ketone / Aldehyde to form
Hydroxynitrile / Cyanohydrin
14) Nucleophilic Substitution of HalogenoAlkane by Ammonia to form
Amine
15) Mechanism of Tri-IodoMethane Test
16) Reaction of Thionyl Chloride with Carboxylic Acid to form Acyl
Chloride
17) Reaction of Thionyl Chloride with Alcohol to form HalogenoAlkane
18) Acid Hydrolysis of Esters
19) Acid Hydrolysis of Amides
20) Acid Hydrolysis of Nitriles
.