1986 CHEMISTRY ANSWERS
1.
At boiling point, the vapour pressure of a liquid is in
equilibrium with the atmospheric pressure. Shortly afterwards, the liquid
molecules acquire more kinetic energy and begin to rise above the surface of
the liquid as it changes to gas . This process is called evaporation
2.
A divalent metal with an atomic mass of 40 is calcium Equation for
the reaction:
Ca + 2HCl CaCl2 + H2
From the above equation:
1 mole (40g) of Ca produced 1 mole (22.4dm3) of H2
If 22400cm3 of H2 was produced by 40g of Ca
224cm3 of H2 will be produced by:
×
= 0.4g [D]
3.
Unbalanced equation for the reaction
HF + N2F2 2NHF2
From the above equation,
10 cm3 of HF reacted with 5cm3 of N2F2 to form 10cm3 of NHF2
Since volume is directly proportional to mole, the mole ratio to
the simplest form will be 2:1:2.
Multiply each compound with the respective no of mole.
The balanced equation is 2HF + N2F2 2NHF2
[B]
4.
1 mole of a substance contains Avogadros number of particles =
6.02 × 1023
1 mole of NaCl contains 6.02 × 1023 no of atoms
Mass of NaCl = 58.5g
58.5g of NaCl contains 6.02 ×1023 atoms
5.85g of NaCl contains
×
= 6.02 ×1022 atoms
No of atoms of Cl present = 6.02 ×1022 atoms [A]
5.
Equation for the reaction
Mg + 2HCl MgCl2 + H2
From the above equation, 2 moles of HCl reacted ith 1 mole (24g)
of Mg
250cm3 of 0.5mole HCl contains
×
= 1.5g of Mg [B]
6.
From Graham’s law of diffusion, a lighter gas will diffuse as many
times faster than a heavier gas by the ratio of the square root of their
molecular masses.
Molecular mass of O2 = 32g
Molecular masss of CH4 = 16g
Ratio of their square root =
=
This means that CH4 will diffuse
times faster than O2. If
200cm3 of O2 diffuse in 50sec, 80cm3 of CH4 will diffuse in
Since ( t
)
×
= 14.1secs [C]
7.
From Graham’s law of diffusion, the rate (velocity) of diffusion
of gas moleciules is inversely proportional to the square root of its molecular
mass. This can be expressed mathematically as follows
R (U)
U = K
U =
where U = velocity OR U =
[D]
8.
An element with atomic number 12 has 12 electrons.Electronic
configuration of the element is : 2/8/2 This shows a valency of 2.
Element with valency of 2 forms an electrovalent bond [B]
9.
From left to right of the periodic table electronegativity
increases due to increase in the nuclear charge caused by gain of proton. Thus,
there is a greater tendency of an atom to attract an electron to itself. The
energy change that occur when this occurs is called electron affintity
Thus, both electronegativity and electron affinity increases
across the period form left to right. Ionization energy increases across the
period. However this is not constan eg Be has a greater ionization energy than
B, N> O, Mg > Al and P > S.This is due to the greater stability
associated with evenly distributed electron in the subshells [C]
10.
Mass of the dry solid obntained after evaporation = 34.2g .
34.2g of the sugar saturated 50cm3 of the solvent at 400C
Mass of the sugar that will saturate 1000cm3 at the same
temperature =
×
= 684g
Molar mass of the sugar = 342g
Solubility of the sugar at 400C in mol/dm3 =
= 2mol/dm3 [D]
11.
From the graph, the saturated solution from KCland KClO3 contains
the same mass of each salt at 980C. At 980C , the solubility of KCl =
56g/dm3.At 300C, the solubility of KCl = 38g/dm3.On cooling from 980C to 300C
mass of KCl that will crystallize = (56 - 38) = 18g At 980C, the solubility of
KClO3 = 56g/dm3 At 300C the solubility of KClO3 = 6g/dm3
On cooling from 980C to 3000C, the mass of
KClO3 that will crystallize = (56 - 6) = 50g
Total mass of the crystal = (18 + 50) = 68g
% of KCl impurity =
×
×
= 26.5% [D]
12.
NaHSO4 contains a replaceable H+ and thus is acidic.
Na2CO3 hydrolysis in water into weak acid (H2CO3) and strong base NaOH
and thus is basic.
NaCl is a neutral salt of strong acid and strong
base and thus is neutral in water
NaHCO3 just like Na2CO3 hydrolysis in water a
weak acid and strong base and thus forms a
basic soluition with PH greater than 7. Other salt
that undergoes hydrolysis in water are Na2S,
AlCl3. [D]
13.
An acidic salt contains replaceable H+ Among
the salts provided in the options only NaHSO4
contains replaceable H+ and thus is
acidic
14.
Sugar is an organic compound whose molecules are held together by
covalent bond. Dissolution of sugar in water does not lead to formation of
mobile ions. Hence, it does NOT conduct electricity [D]
15.
Aqueous
solution of K2SO4 contains
the following ions
(cations)include 2K+ and
H+, negative
ions (anions) include SO42- and
OH-.
During
electrolysis, negative ions migrate towards
the
anode, thus SO42- AND OH- will migrate to
the
anode [A]
16.
Current (I) passed = 6.5A
Time
of flow (T) = 1hour
1hour
= 3600secs.
Q=
IT = 6.5 × 3600 = 23,400 = 2.34× 104C
[D]
17.
A redox reaction occurs between
oxidizing agent
and
a reducing agent. Oxidation is the addition of
electronegative
element while reduction iis the
removal
of electronegative element or addition of
electropositive
element. In option D, the
electropositive
element – Zn was added to 2HCl,
reducing
it to H2 (oxidation state changed from +1
to
0) while the electronegative element Cl2 was
added
to Zinc, oxidizing it to ZnCl2, (oxidation
state
of Zinc changed from 0 to + 2)
18.
From the equation, the oxidation state
of Mn
changed
from +4 in MnO2 to +2 in MnCl2. This
means that the oxidation state of Mn was reduced
by
2 and reduction means addition of electron. Thus, 2 electrons was transferred
to Mn atom. [A]
19.
Equation for the reaction:
NH4OH
+ HCl NH4Cl + H2O
From
the above equation, 1 mole of NH4OH neutralized 1 mole of HCl to form 1 mole of
NH4Cl and H2O. 20cme of 0.1M NH4OH =
×
= 0.002mole
0.002mole
of NH4OH on neutralization liberated 102joules of energy.
1
mole of NH4OH will liberate
×
=
51,000 joules =51kilojoule
Since
heat was liberated (evolved)
H IS negative = -51KJ/mol
20.
For a pressure to have effect, one of the
reactants or products must be gaseous with unequal no of moles. From the
equation the no of moles of
gaseous
rectant = 1, the no of moles of gaseous product = 0.
Since
M
V
, The reaction proceeds with decrease in volume (from 1 to 0)
Any
reaction that proceed with decrease in volume will also proceed with increase
in pressure since V
Hence, increase in pressure will drive the equilibrium to the right
favouring the forward reaction [B]
21.
% composition of O2 in air = 21% this
means that 100cm3 of air contains 21cm3 of O2 . Volume of air that will contain
10cm3 of O2 =
= 47.6
50cm3 [C]
22.
Metals such as Mg, Al, Zn, Fe, Pb and Cu
do not react with cold water but with steam after heating each of them to red
hot [D]
23.
When steam is passed through a red hot
carbon at a temperature of 12000C, THE result is the formation of water gas
which is a mixture of hydrogen and carbon (ii) oxide [C]
24.
Option C and D contains 3 substances.
However , option C contains the substances in the order of – efflorescent
(Na2CO3.10H2O), deliquescent (FeCl3) and hygroscopic (conc. H2SO4) [C]
25.
In the titration of 2 samples of hard
water with soap, the water before boiling contains both temporary and permanent
hardness while the water after boiling contains only permanent hardness. From
the titration result, volume of soap for permanent hardness (water after
boiling) = 20.0 – 15.0 = 5cm3
Volume
of soap after the permanent and temporary hardness (water before boiling) =
25.0 – 0.0 = 25cm3
Volume
of soap for temporary hardness = (volume of soap for both temporary and
permanent hardness – volume of soap for permanent hardness) = 25.0 – 5.0 =
20.0cm3
Ratio
of the volume of soap for permanent to temporary hardness =
=
1: 4 [B]
26.
Petrol of high sulphur content consist of
three elements namely carbon , hydrocarbon and sulphur. Combustion of carbon in
a limited supply of air causes the release of CO in the exhaust fumes of motor
vehicles which is the largest single cause of CO emission
Combustion
of sulphur also yields SO2
[B]
27.
Oganic
wastes like sewage when dumped into the sea are broken down by bacteria into
simpler substances. In the process the bacteria make use of the dissolved
oxygen in the water thereby depleting the amount of oxygen needed by the
aquatic organisms for survival. [A]
28.
NO
combines readily with oxygen at room temperature to form brown fumes of NO2
which is a higher oxide. SO2 combines with oxygen at a temperature
of about 4500C at pressure of 1 atm in the presence of Vanadium (V) oxide to
yield SO3 which is a higher oxide [C]
29.
H2S
react with Pb(NO3)2
or CH3COOPb to form a black residue of PbS which causes the
blackening of the filter paper. Cl2 when in contact with moist blue
litmus paper turns red and thereafter bleaches. Elements present in H2S
and Cl2 are H, S and Cl [A]
30.
Sulphides
soluble in acidic solution include: K2S, Na2S, (NH4)2S,
CaS, MgS, ZnS, NiS, CoS, MnS and FeS while sulphides insoluble in acidic
solution include: SnS, PbS, CuS, CdS, HgS, Sb2S3, As2S3
and Bi2S3. Among these, only K2S, Na2S
and (NH4)2S are soluble [C]
31.
When
chlorine is passed into water, the result is the formation of oxochlorate (I)
acid and hydrochloric acid
|
2HOCl
|
|
sunlight
|
|
2HCl + O2 [C]
|
32.
The
only stable trioxocarbonates are the trioxocarbonates of Na, K and NH3
while other trioxocarbonates are unstable. Aluminiumtrioxocarbonates does NOT
exist because aluminium salts reacts with Na2CO3 to form
Al(OH)3. Thus, none of the metals in the option forms a stable CO3 [N.C.O]
33.
Z
reacts with NaOHto form salt shows that
Z must be acidic. In the options only NaHS and NaHSO3 are
acidic. However, metallic sulphides (NaHS) reacts with dilute acid (HCl) to
liberate SO2 which does not givea yellow suspension of sulphur with
conc. H2SO4 [A]
34.
Ammonia
gas can only be dried withsilica gel orquicklime (CaO) because it does not
reactwiththem. Other drying agent agents such as conc. H2SO4
and CaCl2 are not used for drying ammonia because it reacts with
them. [B]
35.
Copper
reacts with concentrated trioxonitrate (V) acid to form copper (II)
trioxonitrate (V), water and nitrogen (IV) oxide:Cu + 4HNO3 Cu(NO3)2 + 2H2O
+ 2NO2
36.
The
same copper reacts with dilute HNO3 to form copper (II) trioxonitrate (V),
water and nitrogen (II) oxide
3Cu + 8HNO3 3Cu(NO3)2
+ 2H2O + 2NO2
In the second equation, x,
y & z correspond with 3, 8 & 2
[B]
37.
Iron
(III) oxide does NOT dissolve in caustic soda. Thus, the bauxite (Al2O3)
containing the iron (III) oxide impurity
is dissolved in NaOH to form a solution containing the soluble aluminate
(III) NaAl(OH)4 and the insoluble Fe2O3. The iron (III) oxide is then filtered
off as a sludge [B]
38.
Aluminium
is used for making overhead electric cables because of its lightness and good
electrical conductivity. Duralumin
is an alloy of Al, Cu, Mg and Mn Containers for nitric acid are made of Al or
Fe because conc. HNO3 does NOT attack them due to the formation of a surface
coating of oxide. Containers for caustic soda are NOT made of Al because it
dissolves Al to form aluminate (III) NaAl(OH)4
[C]
39.
Decomposition of zinc trioxocarbonate produces zinc oxide and CO2
as shown below:
ZnCO3
ZnO + CO2.
The zinc oxide is an insoluble solid which turns yellow when hot and
white when cold [C]
40.
Flame test for the ions: Na+, Ca2+ and K+ are as folllows: Na+ =
golden yellow, Ca2+ = brick-red, K+ = lilac (pale purple) Thus, KCl and K2SO4
will give a flame colouration of lilac. However, only K2SO4 will form a white
precipitate with acidified barium chloride due to the presence of SO42- [C]
41.
Glucose is a strong reducing agent due to the presence of CHO group. It reduces oxidizing agents like
Fehling's solution, Tollen's reagent and Na2CrO4 to Cu2O, Ag and Cr3+ respectively.
However, this only takes place after the glucose has been boiled [B]
42.
Leaded petrol contains lead (IV) tetraethyl which is added to
petrol as an anti-knock to improve the efficiency of the fuel combustion [B]
43.
Fats and oil hydrolysis in alkalis such as caustic soda (NaOH) and
potash (KOH) to form soap and glycerol. The process is called saponification
[B]
44.
The
molecular formula C3H8 is only possible in organic compound containing only one
atom of oxygen eg alkanol (ROH), alkanal (RCHO), alkanone (RCOR') and ether
(ROR'). However propanal (C2H5CHO) and propanone CH3COC2H5 contains 6H - atoms
while propanol (C3H7OH) and methoxyethane (CH3OC2H5) contain 8-atoms. But
propanol has 2 - isomers based on the position of the OH group namely
propan-1-ol and propan-2-ol. Hence, isomers with the molecular formula C3H8 are
propan-1-ol, propan-2-ol and methoxyethane
45.
Homologous
series whose name ends "oic acid" is a tricarboxylic acid with the
functional group - COOH. The name "pent" indicates 5-C atoms. The
name "-2-en" indicates the presence of double bond at C-2 atom.
Putting the structures together gives
|
H - C
- C -
C
C - C
|
|
O
|
|
O - H
|
|
H H H
H
|
|
H H
|
[C]
46.
When
an ethanol is heated with conc. H2SO4 at 1700C, an ester called ethylhydrogen
tetraoxosulphate is formed which decomposes in the presence of excess H2SO4 to
form ethene. The overall process is called dehydration of ethanol since a
molecule of water is extracted from it.
C2H5OH + H2SO4 C2H5HSO4 + H2O
C2H5HSO4 C2H4 + H2SO4
[C]
47.
Methane
in the presence of ultraviolet ray undergoes substitution reaction with
chlorine to form the following compounds:
CH4 + Cl2 ch3cL + HCl
CH3Cl + Cl2 CH2Cl2 + HCl
CH2Cl2 + Cl2 CHCl3 + HCl
CHCl3 + Cl2 CCl4 + HCl
Thus, C2H5Cl is not a
product of substitution reaction of methane with chlorine [B]
48.
Alkyl
group has the general formula CnH2n+1. When this combines witha halide X, the
result is CnH2n+1X [B]
49.
Nylon
is made polymerization. Soap is made by saponification. Artificial rubber is
made by polymerization. Butane is made by hydrogenation of butene or butyne.
Margarine is made by hydrogenation of oil. Thus, nylons and artificial rubber
are made by polymerization [B]
50.
Carbohydrate
(starch) in the presence of enzyme like zymase found in the yeast is readily
converted to ethanol (alcohol) and carbon (IV) oxide. The process is called
fermentation. [B]
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