UME
1979 CHEMISTRY ANSWERS
1. Among
the compounds, only ethane & ethyl ethanoate (ester) have sweet adour.
However, ethene decolourizes bromine water while ethyl ethanoate does NOT. Thus,
X is ethylethanoate (D)
2. HCOOCH3
= Methyl methanoate CH
CH = ethyne correct IUPAC names are methyl
ethanoate & ethyne (D)
3. Soap
is made by hydrolysis of fats & oil with caustic soda (saponification).
Margarine is made by hydrogenation of oil. Nylon and artificial rubber are made
by polymerization
4. Cu2+(aq)
+ Zn(s)
Cu(s)
+ Zn2+(aq)
The above equation is a redox reaction whereby copper (II)
ion is reduced to a metallic Cu while zinc metal is oxidized to zinc ion [E]
5. Formula
of potassium permanganate = KMnO4
Sum of the oxidation no in a compound = 0
K + Mn + 4[O] = 0
1 + Mn + 4[-2] = 0
1 + Mn – 8 = 0
Mn = 8 -1 = +7 [A]
6. When
CO2 is bubbled into lime water a white precipitate of CaCO3
is formed CO2 + Ca(OH)2
CaCO3 + H2) on reaction
with further CO2 gives rise to a soluble Ca(HCO3)2
as shown below CaCO3 + H2O +CO2
Ca(HCO3)2 [C]
7. Decrease in the concentration of the product
will favour the forward reaction in all the equations however, since ∆H is only
positive in equation (III) , increase in temperature will favour the forward
reaction in equation (III) while it will favour the backward reaction in
equations (i) and (ii) [C]
8. The
functional group is alkene because of the double bond. The position of the
double bond is at C-1 atom named as (-1- ene) The longest C-chain has 4 C-atoms
named as (but). The branch chain is methyl group at C–3 atom named as
(3-methyl). Combining the names in a reverse order gives the IUPAC name: 3
methylbut-1-ene [D]
9. The
boiling point of methanoic acid is 100.80C which is almost the same
with the boiling point of water (1000C) thus a mixture of water
& methanoic acid cannot be separated directly by distillation or fractional
distillation. Instead, it is first neutralized with NaOH to form HCOONa. The
pure water from the mixture is then
obtained by distillation [C]
10. Cations
present in sodium hydroxide solutions are Na+ and H+. during electrolysis,
these cations migrate to the cathode. H+ being lower than Na+ in the
electrochemical services will be preferentially discharged at the cathode while
Na remains in solution as NaOH, thus its conc. Increases at the cathode [D]
11. Potassium
being a metal ionizes by loss of electron. i.e K
K+
+ e- .
No of electron in a neutral potassium atom = 19. No of electron in K+ = (19 -1)
=18. This is the same as the no of electrons in Argon [C]
12. Amphoteric
oxides are metallic oxides that can react with both acid and base to form
salts. They include the oxides of (ZALT) i.e Zinc, Aluminum, Lead and Tin. Thus,
Aluminum Oxide and Zinc Oxide are the two amphoteric oxides (B)
13. Helium
belongs to the group of element called noble gases. Noble gases are unreactive
due to the completely filled outer electron giving rise to a stable duplet
structure [D].
14. From
Gay Lussac’s Law of combining volume.
2H2 + O
2 H2O
Mole ratio 2 : 1 : 2
Volume ratio = 40 : 20 : 40
Volume of H2 that reacted = 40cm3
Volume of unreacted H2 = (50 - 40)
=10cm3
Volume of the product = 40cm3
Volume
of residual gas = vol. of unreacted gas + volume of product = (10 + 40) = 50cm3 [A]
15. Equation
for complete reaction of propyne with HBr is: C3H4 + 2HBr
C3H6Br2
From the equation: 40g of propyne reacts with 162g of hydrogen
bromide
2g of propyne will react with:
×
=
8.1g [C]
16. When
trioxocarbonates of potassium, calcium & ammonia are heated; potassium
trioxocarbonate does not decompose. Calcium trioxocarbonate decomposes to form
CaO and CO2 while Ammonium trioxocarbonates decomposes to form
ammonia CO2 and water [E].
17.
|
O
|
|
OH
|
|
CH3 – C
|
[E]
18. The
equation for the reaction is as follows: Cl2 + H2O
HOCl+ HCl.
2HOCl
2HCl + O2. Thus, oxygen is given
out with aqueous hydrochloric acid.
19. Saturated
solution of KClO3 is formed at 66oC. Mass of KClO3
solute that dissolved = 12g
Mass of the solvent (water) = 40
Volume of water =
=
=
40cm3 Thus, 12g of KClO3 saturated 40cm3 of
water. Mass of KClO3 that will saturated 1000cm3 of water
=
×
=
300g/dm3 Or 2.45 mol/dm3 [N.C.O]
20. Cations
present in dilute sulphuric acid (H2SO4) is H+. Anions present are SO42- and OH-. During
electrolysis, H+ migrates to the cathode where it is discharged as hydrogen
atom SO42- & OH- migrate to the anode
where OH- is preferentially discharged as OH. 2OH + 2OH
2H2O + O2..Thus, hydrogen
is obtained at the cathode & Oxygen at the anode [C]
21. Oxide
of nitrogen consists of Nitrogen & Oxygen.
|
|
N
|
O
|
|
% composition
|
30.4
|
69.6
|
|
R.A.M
|
14
|
16
|
|
|
= 2.17
|
= 4.35
|
|
Simplest ratio
|
= 1
|
= 2
|
Empirical formula = NO2 [C]
22. When
the charcoal within the enclosed cylinder is ignited, it makes use of oxygen
present in the air and CO2 & H2O are produced as end
product. Thus, the amount of O2 is reduced while the amount of CO2
is increased (D)
23. Moving
from left to right along the periodic table atomic no, atom is mass,
electronegative & no of valence electrons increases while electropositivity
decreases [C]
24. Electrolytic
equation of copper ion:
Cu2+ + 2e -
Cu
64g of Cu requires 2 moles of electron to be deposited as
a metallic Cu.
1 mole of electron
(e-) = 1 Faraday (F)
2Faradays of electricity deposits 64g of Cu
0.1F will deposit:
×
= 3.2g
of Cu [E]
25. Ammonium
trioxonitrate when heated decomposes into dinitrogen (I) oxide & water NH4NO3
N2O
+ 2H2O. All the other compounds produce oxygen gas [E]
26. In
the extraction of aluminium, cryolite (Na3AlF6) is used
as an impurity to lower the melting point of the purified bauxite/alumina Al2O3 [B].
27. In
the redox equation, a free chlorine molecule oxidizes Fe2+ to Fe3+
and itself reduced to chloride ion by decreasing the oxidation no from 0 to -1 [B]
28.
An anhydride is a compound formed by removing
water from an acid eg. when H2O are removed from the aqueous
solutions: H2CO3, H2SO3, 2HNO3
the resultant compounds are CO2, SO2 & NO2.
Anhydrides are also called acidic oxides because they form acid with water
giving a pH less than 7 [C]
29.
SO2 is mainly a reducing agent but
only acts as an oxidizing agent in the presence of a stronger reducing agent
like H2S [D]
30.
When calcium hydroxide and ammonium chloride
are heated together, the products are calcium chloride, water & ammonia.
The ammonia is dried by passing it through calcium oxide [D]
31.
Fermentation is the process of breaking down
sugar into ethanol and carbondioxide. Since starch is a type of sugar it can be
converted to ethyl alcohol (ethanol) via fermentation. [B]
32.
In every exothermic reaction, a decrease in
temperature will favour the forward reaction while an increase in temperature
will favour the backward reaction [D]
33.
Hygroscopic substance when exposed to air
absorbs moisture to increase their weight but not enough to form a solution.
Deliquescent substances absorb moisture when exposed to air to form a solution.
Thus Y is hygroscopic while Z is deliquescent [N.C.O]
34.
In the extraction of iron in the bleat
furnace, the iron(III)oxide is reduced by CO while CaO obtained from
decomposition of CaCO3 combines with silicon(IV)oxide impurity to
form calcium trioxosilicate CaSO3 [E]
35.
A decrease in the pressure of a gas is
proportional to the increase in the volume at constant temperature – Boyle’s
Law [D]
36.
36. Vapour density of a gas is the ratio of
the mass of a unit volume of in the gas and the mass of an equal volume of
hydrogen gas VD =
[B]
37.
37. In the equation, the enthalpy change (∆H)
is negative therefore the reaction is exothermic because heat energy is
released into the surroundings [D]
38.
38. In test for chlorides, the sample
solution is first acidified with dilute HNO3 to prevent
precipitation of other salts. Thereafter AgNO3 is added to give
white precipitate of AgCl [C]
39.
39. Hydrogenation is the addition of hydrogen
to an organic compound only unsaturated organic compound such as the homologous
series – alkene & alkyne can take up hydrogen via addition reaction. [C]
40.
40. Hard water is due to the presence of
calcium and magnesium ions in the form of (HCO3-) for temporary
water hardness and (SO42-)for
permanent water hardness.
41. Bronze is an alloy of copper and tin with
copper as the base metal [E]
42. From the equation, 4 atoms of P requires 5
moles of O2
1 mole of a gas occupies 22.4dm3.
thus, 4 atoms of P
requires 5 x 22.4 litres of O2 124g of P requires 5 x 22.4 litres of
O2
12.4g of P requires
x
=
11.2 litres
124g of P will require 11.2 litres [B]
43. Oxidation
of anhydrous iron (II) sulphate yields iron(III)oxide, sulphur (IV)oxide and
sulphur (VI) oxide [C]
44. Separation
of the components of chlorophyll pigment
by chromatography will show the various colours the chemical compound is
made of [E]
45. V1
= 30dm3
P1 = 760mmHg
T1
= 273k
P2 = 380mmHg
T2 = 364K
V2 = ?
Using the general gas equation:
Substitute =
V2 =
= 80dm3
[D]
46. In
the presence of concentrated tetraoxosulphate (VI) acid excess ethanol
undergoes dehydration to form ethoxyethane also known as diethyl ether [A]
47. In
the smelting of iron in the blast furnace, CaCO3 decomposes to CaO
and CO2. The CaO combines with the earthly impurity, SO2
to form CaSiO3 thereby purifying the iron [A]
48. Zn2+,
Al3+ and Ca2+ form white precipitate with small amount of
NaOH. In excess NaOH, Zn2+ and Al3+ dissolve while Ca2+
remains insoluble. However Al does not dissolve in trioxonitrate (V) acid while
Zn does [A]
49. Zinc
reacts with steam at red heat to produce Zinc oxide & hydrogen. It does NOT
react with cold or hot water [C]
50. In
exothermic reaction in temperature causes the equilibrium position to shift to
the right favouring the forward reaction when the forward reaction is favoured
there is increased concentration of the product which is SO3 in the
given equation [C]
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