BCSE 2004
SECTION A
Question 1
(a) Answer the following questions by selecting suitable words from the given below list.
Lead nitrate, carbon tetrachloride, chlorine, zinc, acetic acid, dilute hydrochloric acid,
fluorine, platinum
(i) Which element has the highest electro negativity?
Answer: Fluorine
(ii) Which compound contains free ions only?
Answer: Hydrochloric acid.
(iii) Which metal is brittle?
Answer: Zinc
(iv) Which acid is used for cooking?
Answer: Acetic acid
(v) Which salt produces a crackling sound on heating?
Answer: Lead nitrate.
(vi) Which catalyst is used during the manufacture of nitric acid by Ostwald’s Process?
Answer: Platinum.
(b) State whether the following statements are true or false.
(i) One litre of nitrogen and two litres of carbon dioxide contain the same number of
molecules at S.T.P (FALSE)
(ii) Potassium nitrate gives nitrogen dioxide on heating. (FALSE)
(iii) Brass is an alloy of copper and zinc. (TRUE)
(iv) Water is a polar solvent. (TRUE)
(v) Concentrated sulphuric acid is used for drying Sulphur dioxide. (TRUE)
(vi) Ionization energy increases across the period. (TRUE)
(c) Name the terms used to describe the following:
(i) The amount of energy released when an extra electron is added to a neutral gaseous
atom. (Electron Affinity)
(ii) A base soluble in water. (Alkali-Sodium hydroxide)
(iii) The process of coating iron with molten zinc. (Galvanisation)
(iv) The process of heating crude rubber with Sulphur to make it stronger and more
elastic. (Vulcanization)
(v) The type of reaction between methane and chlorine. (Substitution reaction/
Halogenations/ Chlorination)
(vi) An electrically charged atom or group of atom. (Ions)
(d)
(i) What is a polar molecule?
Answer: A covalent bond in which one of the bonded atoms acquires slight positive
charge and the other slight negative charge is called polar molecule. (All ionic
compounds are polar molecule)
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(ii) There are three gas cylinders. Cylinder A contains 14g of nitrogen, cylinder B
contains 32g of oxygen and C contains 2g of hydrogen at S.T.P. What are the
volumes occupied by these three gases? Justify your answer.
Answer:
Molecular formula of nitrogen is N2 and hence its molecular mass is 28g.
28g of nitrogen measures 22.4 litres. Therefore, 14g of nitrogen will measure
11.2litres.
Molecular formula of oxygen is O2 and hence its molecular mass is 32g. Therefore,
32g of oxygen occupies 22.4 litres.
Molecular formula of hydrogen is H2 and hence its molecular mass is 2g. Therefore,
2g of hydrogen will occupy 22.4 litres.
Therefore, 2g of hydrogen will occupy same volume as that of 32g of the oxygen.
(iii) Complete the following ionic equations.
 Fe2+ −e
(Fe3+)
 Cl + (e)
Cl−
 N + ( 3 e)
N3−
 Al −3e
(Al3+)
(iv) Define the term molar volume.
Answer: The volume occupied by one mole of any gas at S.T.P.
(e)
(i) Hydrogen and nitrogen react to form ammonia according to the following equation:
3H2 + N2
2NH3
Calculate the volume of ammonia produced at S.T.P when 1.5 litres of hydrogen reacts
excess of Nitrogen. (H=1; N=14)
Ratio of volume between hydrogen and ammonia is 3: 2.
Therefore, 3 volumes of hydrogen give 2 volumes of nitrogen
3 Vol. of hydrogen
2Vol. of Ammonia
1.5 litres of hydrogen
‘X’
1 .5 X 2
Therefore,
= One litre.
3
(ii) The element X has electronic configuration 2, 8, 1
1. Give the valence electron of X.
Answer: 1
2. To which group does the element X belong?
Answer: I
3. Predict the period to which the element X belongs.
Answer: 3
4. Is the element X, a metal or a non-metal?
Answer: Since the valence electron is less than three, it is a metal.
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(f)
(i) Define the term electrolysis.
Answer: The process of decomposition of an electrolyte in the molten or in aqueous
solution, by passage of an electric current through it is called Electrolysis.
(ii) Classify the following as – strong electrolyte, weak electrolyte and non-electrolyte.
(Dilute sulphuric acid, ethanol, ammonium hydroxide, aqueous solution of sodium
chloride, distilled water, acetic acid)
Strong Electrolyte
Weak Electrolyte
Non-Electrolyte
Dilute sulphuric acid
Ammonium hydroxide
Ethanol
Aqueous NaCl
Acetic acid
Distilled water
(iii) Dorji took a white crystalline salt X and carried out the following tests in the
laboratory. The salt was tested as follows:
(A) It is soluble in water.
(B) Addition of barium chloride solution to the aqueous solution of salt X gave a
white precipitate.
(C) A flame test of the salt gave a persistent yellow coloration.
Predict the possible identity of salt X and write the equation for the reaction of
the aqueous solution of the salt with barium chloride solution.
Answer: Sodium sulphate (Na2SO4)
Na2SO4 + BaCl2
BaSO4 + 2NaCl
(g)
(i) Wangmo passes chlorine gas into a test-tube containing water. When this test-tube was
left exposed in sunlight, bubbles of gas appeared.
1. Name the gas formed.
Answer: Oxygen
2. What will be the action of this solution on litmus?
Answer: Blue litmus will change into red.
(ii) The functional group of alcohol is ─OH.
1. What is a functional group?
Answer: An atom or group of atoms which determines the characteristic properties of
all the members of the same homologous series is known as the functional group of
that homologous series.
(ii) Name the functional group in alkenes.
Answer: C=C (carbon-carbon double bond/ ethylenic bond/ olefinic bond)
(iii) What is the difference between saturated and unsaturated hydrocarbons?
Answer: In saturated hydrocarbons, each of the carbon will have single bond with the
constituent atoms of carbon but, in the unsaturated hydrocarbons one carbon
atom will be linked by double bond or triple bond with other carbon atoms.
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SECTION B
Question 2
(a)
(i) Define the term empirical formula.
Answer: It is the formula of a compound which gives the simple whole number ratio of
the atoms of various elements present in one molecule of the compound.
(ii) A chemical compound has empirical formula ZY3. Its vapour density is equal to the
empirical formula weight. Find the molecular formula of the compound.
Answer: Since the empirical formula weight is equal to the vapour density:
2 X V.D = Molecular weight
Therefore, 2 X Empirical weight= Molecular weight.
Hence, 2 X ZY3 = Z2Y6
(b)
(i) Calculate the weight of urea (NH2CONH2) which can produce 560kg of nitrogen.
(N=14; H=1; C=12; O=16)
Answer: Nitrogen atoms= 2X14=28g
Hydrogen atoms=4X1=4g
Carbon atoms=1X12=12g
Oxygen atoms = 1X 16=16g
Molecular weight= 60g
60g of urea
28g of nitrogen
‘X’ weight of urea
560kg of nitrogen.
560kgX 60 g
Hence,
= 1200kg
28 g
Therefore, 560kg of nitrogen can be obtained by using 1200kg.
(ii) If on e acre of field requires 7kg of nitrogen, how much urea is required for 5 acres?
(N=14; H=1; C=12; O=16)
Answer: One acre of field requires 7kg of nitrogen; therefore 5acres of field will require
35kg of nitrogen.
60g of urea
28g of nitrogen
“X” kg of urea
35kg of nitrogen.
60 gX 35kg

 75kg
28 g
Hence, 5 acres of land requires 75kg of urea.
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(c)
(i) The formula of the washing soda is Na2CO3.10H2O. Calculate the percentage of
water of crystallization in washing soda. (Na=23; C=12; O=16; H=1)
Sodium=2x23=46
Carbon=1x12=12
Oxygen=3x16=48
Water=10x18=180
Total molecular weight=286
180
 100 =62.9%
% of water of crystallization=
286
(ii) What is an acid salt? Give one example.
Answer: Salt containing replaceable hydrogen ions in their molecules.
Example-NaHSO4.
Question 3
(a) The following questions are related to electrolysis of aqueous solution of sulphuric
acid using platinium electrodes.
(i) Mention all the ions present in the solution.
Answer: The ions present are H+, SO42−, and OH−.
(ii) Write the reactions taking place at anode and cathode.
Answer:
Reaction at Anode
Reaction at Cathode
−
−
OH −e
OH
2H+ +2e−
2H
4OH
2H2O + O2
2H
H2
(iii) What is the ratio by volume of the products formed at the cathode and anode?
Answer: The products formed at cathode and an anode is hydrogen and oxygen
respectively. Their ratio of volume is 2:1.
(iv) If sulphuric acid is replaced by copper sulphate, copper will be discharged at the
cathode. Why?
Answer: In the electrochemical series, copper lies below the position of hydrogen and
therefore, gets the preferences of discharging at cathode.
(b) Name one hydroxide each which is soluble in:1. Excess NaOH
Answer: Zinc hydroxide, lead hydroxide and aluminium hydroxide.
2. Excess NH4OH
Answer: Copper hydroxide
(c) You are provided with ammonium hydroxide solution. Using the reagent, how would
you distinguish the following ions?
Fe2+, Fe3+ and Cu2+
Answer: When the salts of the following ions are treated with the ammonium
hydroxide solution, ferrous ion will give the dirty green precipitate due to formation
of ferrous hydroxide, ferric ion will give reddish brown precipitate due to formation
of ferric hydroxide and copper will give blue precipitate which dissolves in excess of
ammonium hydroxide solution.
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Question 4
(a) A laboratory assistant has kept dilute nitric acid and dilute hydrochloric acid in two
separate bottles but forgotten to label them. As a chemistry student, how would you
help to identify these two acids?
Answer: The action of dilute nitric acid on proteins forms a yellow coloured compound
known as Xanthoproteic acid. Therefore, the acid that gives yellow colour with the
skin will be labeled as nitric acid and the other as dilute hydrochloric acid.
(b) Two solutions A and B have pH 2 and pH 11 respectively.
(i) Which of these is an acid?
Answer: Solution A whose pH value is 2.
(ii) Which solution would give pink colour with phenolphthalein?
Answer: Solution B, whose pH value is 11.
(c)
(i) How would you identify sodium carbonate and sodium sulphate using hydrochloric
acid only?
Answer: The reaction of hydrochloric acid with sodium carbonate releases carbon
dioxide gas with brisk effervescence. However, the reaction of hydrochloric acid with
sodium sulphate releases Sulphur dioxide gas having pungent suffocating ordour. The
released gas if passed through lime water, the lime water will turn milky due to the
presence of CO2 gas. From this test we can identify the two salts
(ii) Give one test for hydrogen chloride gas.
Answer: When a glass rod dipped in ammonia solution is brought near to the mouth of
the gas jar, there is formation of thick dense white fumes.
(d) Zinc oxide is reduced by coke according to the following equation:
ZnO + C
Zn + CO
What mass of zinc would be formed if 70kg of zinc oxide is reduced by coke?
(Zn=65; O=16; C=12)
Answer: Molecular mass of zinc oxide is 81g
81g of zinc oxide
65g of zinc
70kg of zinc oxide
‘X’
70kgX 65 g
Therefore, X=
= 56kg
81g
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Question 5
(a)
(i) State the modern periodic law.
Answer: The physical and chemical properties of elements are periodic function of their
atomic numbers.
(ii) What is electro negativity?
Answer: The tendency of an element to attract the shared pair of electrons towards itself
when combined in a compound is known as electronegativity.
(iii) Atomic radius decreases as we move from right to left in the periodic table. Why?
Answer: Due to the increase in the nuclear charge of each successive element, the
electrons are pulled more strongly towards the nucleus and therefore atomic radius
decreases across the period.
(iv) Na, Li and K belong to the same group in the periodic table based on their similar
properties.
1. Name one property common to all these three metals.
Answer: Alkali metals whose hydroxides are soluble in water. Very reactive metals
and forms salt with halogens.
2. Arrange them in order of their increasing metallic character.
Answer: Li, Na and K
3. What is the usual name of this group?
Answer: Alkali metals.
4. Why does the reducing power of these metals increases down the group?
Answer: Since the metallic character increases down the group, the more metallic
elements are likely to give electrons readily away from their last shell. Therefore,
addition of electrons to other elements is termed as reduction. So the reduction
properties increase.
(b) What do you observe when:
(i) Hydrogen sulphide gas is passed through ferric chloride solution?
Answer: The reddish brown solution of ferric chloride solution will change into a
dirty green solution of ferrous chloride.
(ii) Sulphur dioxide is passed through acidified potassium dichromate solution?
Answer: The orange colour of acidified potassium dichromate changes to green
coloured chromium sulphate.
(c) Why is bleaching action of Sulphur dioxide not permanent?
Answer: Bleaching action of Sulphur dioxide is not permanent because the colour is
restored on exposure to the air. The reduced colourless product is oxidized to the
original coloured state.
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Question 6
(a)
(i) Write an equation for the laboratory preparation of nitric acid from sodium nitrate.
Answer: NaNO3 + con. H2SO4  200 °C NaHSO4 + HNO3
(ii) During the laboratory preparation of nitric acid, all glass apparatus is used. Why?
Answer: Nitric acid is a very corrosive solvent which attacks almost all the substance
including rubber and therefore all the apparatus are made from rubber.
(b) Give the balanced equation for the following
(i) Copper with concentrated nitric acid.
Answer: Cu + 4HNO3
Cu (NO3)2 +2 NO2 + 2H2O
(ii) Copper with dilute nitric acid.
Answer: 3Cu + 8HNO3
Cu (NO3)2 +2 NO + 4H2O
(iii) Chlorine with cold, dilute sodium hydroxide.
Answer: 2NaOH + Cl2
NaCl + NaClO + H2O
(iv) Chlorine with hot, concentrated sodium hydroxide.
Answer: 6NaOH + 3Cl2
5NaCl + NaClO3 + 3H2O
(c) What would you see when crystals of CuSO4.5H2O are placed in concentrated
sulphuric acid? What is the reason for the observation?
Answer: The blue crystals of hydrated copper sulphate lose its water of crystallization
and become colourless. This is because, the hygroscopic nature of concentrated
sulphuric acid results in absorption of the water of crystallization.
(d) Name two important ores of iron?
Answer: Haematite and Magnetite.
Question 7
(a) The following questions are related to the extraction of aluminium by Hall’s process.
(i) Name the chief ore of aluminium.
Answer: Bauxite
(ii) Give the equation for the reaction when aluminium hydroxide is heated.
Answer: Al (OH) 3 1500°C Al2O3 + 3H2O
(iii) Write the function of Cryolite during the extraction of aluminium.
Answer: Cryolite reduces the fusion temperature of alumina and also increases the
conductivity.
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(iv) Write the equations for the reactions that take place at the cathode and anode.
Answer:
Reactions at cathode
Al3+ + 3e−
Al
Reactions at anode
4AlO3 3−
2Al2O3 + 3 O2 + 12e−
(v) The anode is replaced from time to time during the electrolytic reduction of
alumina. Why?
Answer: During the process of electrolysis, the oxygen produced at anode burns the
anode and to prevent that, we replace the anode time to time.
(b) Ethane and ethene react with chlorine to form monochloroethane and
dichloroethane respectively.
(i) Name the type of reaction formed between chlorine and ethene?
Answer: Addition reaction and chlorination or halogenation.
(ii) What feature makes ethene to undergo such a reaction?
Answer: Because of its unsaturated nature, it combines with other elements to make
itself more stable.
(c)
(i) Give an equation for the laboratory preparation of ethanol.
Answer: C2H5Br + KOH
C2H5OH + KBr
(ii) Give the structural formula of ethanol.
Answer:
H H
‫׀ ׀‬
H─ C─ C─ O H
‫׀ ׀‬
H H
(iii) Name the chemical used to make alcohol spurious.
Answer: Methanol
N.B: If you do not make use of this note, I will curse, to your extreme failure because I
have sacrificed precious hours of my life. BUT! If you bring success to your life,
perhaps it would be my inspiration even to work harder for the rest of my life. Hope
you all will not disgrace my expectation.
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