ICSE Class 10 Chemistry Board Exam Question Paper 2019 with Solutions

ICSE Class 10 Chemistry Board Exam Question Paper with Solutions

SECTION I (40 Marks)

Attempt all questions from this Section

Question 1

(a) Choose the correct answer from the options given below: [5]
(i) An electrolyte which completely dissociates into ions is
A. Alcohol
B. Carbonic acid
C. Sucrose
D. Sodium hydroxide

Answer: (D) Sodium hydroxide

Sodium hydroxide is a strong electrolyte that dissociates completely into sodium and hydroxide ions in aqueous solution.

Teacher's Note:
a) Strong electrolytes are ionic compounds or strong acids/bases that undergo almost 100 percent ionization in solution.
b) Alcohol, carbonic acid, and sucrose are either non-electrolytes or very weak electrolytes.

(ii) The most electronegative element from the following elements is [1 Mark]
A. Magnesium
B. Chlorine
C. Aluminium
D. Sulphur

Answer: (B) Chlorine

Electronegativity increases across a period from left to right. Among the given options, chlorine is in Period 3, Group 17, making it the most electronegative.

Teacher's Note:
a) Electronegativity is the measure of the tendency of an atom to attract a bonding pair of electrons.
b) Remember that fluorine is the most electronegative element in the periodic table, followed by oxygen, nitrogen, and chlorine.

(iii) The reason for using aluminium in the alloy duralumin is [1 Mark]
A. Aluminium is brittle.
B. Aluminium gives strength.
C. Aluminium brings lightness.
D. Aluminium lowers melting point.

Answer: (C) Aluminium brings lightness.

Aluminium is a very light metal, which makes duralumin ideal for aircraft bodies where low weight and high strength are required.

Teacher's Note:
a) Duralumin consists mainly of aluminium, along with copper, magnesium, and manganese.
b) Students often confuse the purpose of alloying components; aluminium provides lightness, while copper and magnesium impart strength.

(iv) The drying agent used to dry HCl gas is [1 Mark]
A. Conc. H₂SO₄
B. ZnO
C. Al₂O₃
D. CaO

Answer: (A) Conc. H₂SO₄

Concentrated sulphuric acid is a strong dehydrating and drying agent that does not react with hydrogen chloride gas.

Teacher's Note:
a) Basic drying agents like calcium oxide (CaO) cannot be used because they would react chemically with acidic HCl gas.
b) Always select a drying agent that is chemically inert to the gas being dried.

(v) A hydrocarbon which is a greenhouse gas is [1 Mark]
A. Acetylene
B. Ethylene
C. Ethane
D. Methane

Answer: (D) Methane

Methane is a potent greenhouse gas released from natural sources and human activities such as agriculture and fossil fuel extraction.

Teacher's Note:
a) Greenhouse gases trap heat in the atmosphere, contributing to global warming.
b) Methane is the simplest alkane with the chemical formula CH₄.

(b) Fill in the blanks with the choices given in brackets: [5 Marks]
(i) Conversion of ethanol to ethene by the action of concentrated sulphuric acid is an example of ____________. (Dehydration/dehydrogenation/dehydrohalogenation)

Answer: dehydration.

Teacher's Note:
a) Dehydration involves the removal of a water molecule from a compound.
b) Concentrated sulphuric acid acts as a dehydrating agent in this reaction.

(ii) When sodium chloride is heated with concentrated sulphuric acid below 200°C, one of the products formed is __________. (Sodium hydrogen sulphate/sodium sulphate/chlorine) [1 Mark]

Answer: sodium hydrogen sulphate.

Teacher's Note:
a) The reaction produces sodium hydrogen sulphate and hydrogen chloride gas.
b) Sodium sulphate is formed only at higher temperatures above 200°C.

(iii) Ammonia reacts with excess chlorine to form _________. (Nitrogen/nitrogen trichloride/ammonium chloride) [1 Mark]

Answer: nitrogen trichloride.

Teacher's Note:
a) When ammonia is in excess, it reacts with chlorine to form nitrogen and ammonium chloride.
b) When chlorine is in excess, it forms explosive nitrogen trichloride (NCl₃).

(iv) Substitution reactions are characteristic reactions of __________. (Alkynes/alkenes/alkanes) [1 Mark]

Answer: alkanes.

Teacher's Note:
a) Saturated hydrocarbons (alkanes) undergo substitution reactions because all valencies of carbon are satisfied by single covalent bonds.
b) Unsaturated hydrocarbons (alkenes and alkynes) typically undergo addition reactions.

(v) In Period 3, the most metallic element is __________. (Sodium/magnesium/aluminium) [1 Mark]

Answer: sodium.

Teacher's Note:
a) Metallic character decreases across a period from left to right as nuclear charge increases and atomic radius decreases.
b) Sodium is in Group 1, making it the most metallic element in Period 3.

(c) Write a balanced chemical equation for each of the following reactions: [5 Marks]
(i) Reduction of copper (II) oxide by hydrogen.

Answer:
CuO(s) + H₂(g) → Cu(s) + H₂O(g)

Teacher's Note:
a) Hydrogen acts as a reducing agent by removing oxygen from copper (II) oxide.
b) Ensure physical states are indicated correctly where required.

(ii) Action of dilute sulphuric acid on sodium hydroxide. [1 Mark]

Answer:
H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2H₂O(l)

Teacher's Note:
a) This is a classic neutralization reaction between an acid and a base.
b) Balance the sodium atoms by placing a coefficient of 2 before NaOH and water.

(iii) Action of dilute sulphuric acid on zinc sulphide. [1 Mark]

Answer:
ZnS + H₂SO₄ → ZnSO₄ + H₂S(g)

Teacher's Note:
a) Metal sulphides react with dilute acids to evolve hydrogen sulphide gas with a rotten egg smell.
b) The official key shows a typographical error in equation balancing with incorrect products; the correct reaction is ZnS + H₂SO₄ → ZnSO₄ + H₂S.

Teacher's Note:
a) Always write the correct formula for hydrogen sulphide as H₂S.
b) Double displacement reaction occurs yielding zinc sulphate and hydrogen sulphide gas.

(iv) Ammonium hydroxide is added to ferrous sulphate solution. [1 Mark]

Answer:
FeSO₄ + 2NH₄OH → (NH₄)₂SO₄ + Fe(OH)₂↓

Teacher's Note:
a) A dirty green precipitate of ferrous hydroxide is formed.
b) Ensure ammonium hydroxide is written correctly as NH₄OH.

(v) Chlorine gas is reacted with ethane. [1 Mark]

Answer:
C₂H₆ + Cl₂ → C₂H₅Cl + HCl

Teacher's Note:
a) This is a photochemical substitution reaction occurring in the presence of sunlight or UV light.
b) One hydrogen atom of ethane is replaced by a chlorine atom.

(d) State one observation for each of the following: [5 Marks]
(i) Concentrated nitric acid is reacted with sulphur.

Answer:
A deep red-orange or brown-coloured gas (nitrogen dioxide) with an irritating pungent odour is evolved, and sulphur is oxidized to sulphuric acid.

Teacher's Note:
a) Concentrated nitric acid is a powerful oxidizing agent.
b) Students must mention both the evolution of the brown gas and the colour change or dissolution of sulphur.

(ii) Ammonia gas is passed over heated copper (II) oxide. [1 Mark]

Answer:
The black copper (II) oxide turns reddish-brown as it is reduced to copper metal, and water droplets are formed on the cooler parts of the test tube along with the evolution of nitrogen gas.

Teacher's Note:
a) Ammonia acts as a reducing agent in this reaction.
b) The color change from black to reddish-brown is the key observation.

(iii) Copper sulphate solution is electrolysed using copper electrodes. [1 Mark]

Answer:
The blue colour of the copper sulphate solution remains unchanged, the cathode increases in size due to the deposition of pinkish-brown copper metal, and the copper anode decreases in mass/size.

Teacher's Note:
a) Since copper electrodes are active, copper ions discharged at the cathode are replenished by copper atoms dissolving from the anode.
b) Consequently, the concentration of copper ions in the electrolyte remains constant, keeping the blue color unchanged.

(iv) A small piece of zinc is added to dilute hydrochloric acid. [1 Mark]

Answer:
Effervescence occurs, and a colourless, odourless gas (hydrogen) is evolved rapidly, which burns with a pop sound when a burning splinter is brought near it.

Teacher's Note:
a) Active metals displace hydrogen from dilute mineral acids.
b) Always test the evolved gas with a burning splinter to confirm hydrogen.

(v) Lead nitrate is heated strongly in a test tube. [1 Mark]

Answer:
A yellow residue of lead (II) oxide is formed when hot, and a reddish-brown gas (nitrogen dioxide) along with a colourless gas that rekindles a glowing splinter (oxygen) is evolved.

Teacher's Note:
a) Nitrates of heavy metals decompose on heating to give metal oxide, nitrogen dioxide, and oxygen.
b) Note that lead oxide is reddish-brown when hot and yellow when cold.

(e) (i) Calculate: [5 Marks]
1. The number of moles in 12g of oxygen gas. [O = 16]

Answer:
Molar mass of O₂ = 16 × 2 = 32 g/mol
Number of moles = Mass / Molar mass = 12 / 32 = 0.375 moles.

Teacher's Note:
a) Oxygen gas exists as a diatomic molecule (O₂), so the molar mass is 32 g, not 16 g.
b) Always check whether the question asks for atoms or molecules when calculating moles.

2. The weight of 10²² atoms of carbon. [C = 12, Avogadro's No. = 6 × 10²³] [1 Mark]

Answer:
Weight = (Given number of atoms / Avogadro number) × Atomic mass
Weight = (10²² / (6 × 10²³)) × 12 = (12 / 60) = 0.2 g.

Teacher's Note:
a) Use the unitary method or mole formula: Moles = Number of particles / Avogadro number.
b) Multiply the number of moles by the atomic mass to get the mass in grams.

(ii) Molecular formula of a compound is C₆H₁₈O₃. Find its empirical formula. [2 Marks]

Answer:
The molecular formula is C₆H₁₈O₃.
Dividing the subscripts by the highest common factor (which is 3), we get the simplest ratio of atoms C:H:O = 2:6:1.
Therefore, the empirical formula is C₂H₆O.

Teacher's Note:
a) The empirical formula represents the simplest whole-number ratio of atoms of each element in a compound.
b) Divide all subscripts in the molecular formula by their greatest common divisor.

(f) (i) Give the IUPAC name of the following organic compounds: [5 Marks]
1. CH₃-C≡C-H

Answer:
Propyne (or Prop-1-yne)

Teacher's Note:
a) The compound contains 3 carbon atoms (prop-) and a triple bond (-yne).
b) Numbering starts from the end nearer to the triple bond.

2. CH₃-CHO [1 Mark]

Answer:
Ethanal

Teacher's Note:
a) The functional group is an aldehyde (-CHO) containing 2 carbon atoms.
b) The suffix for aldehydes is '-al'.

(ii) What is the special feature of the structure of ethyne? [1 Mark]

Answer:
In ethyne, the two carbon atoms are joined by a triple covalent bond, and the molecule has a linear shape due to sp hybridization.

Teacher's Note:
a) Each carbon atom is also attached to one hydrogen atom by a single covalent bond.
b) The bond angle in ethyne is 180 degrees.

(iii) Name the saturated hydrocarbon containing two carbon atoms. [1 Mark]

Answer:
Ethane (C₂H₆)

Teacher's Note:
a) Saturated hydrocarbons belong to the alkane family with single carbon-carbon bonds.
b) With two carbon atoms, the alkane is ethane.

(iv) Give the structural formula of acetic acid. [1 Mark]

Answer:
CH₃-COOH (Structure: Central carbon bonded to three hydrogens and the carbonyl carbon, which is double bonded to oxygen and single bonded to a hydroxyl group).

Teacher's Note:
a) Acetic acid is also known as ethanoic acid.
b) It contains a carboxyl functional group (-COOH).

(g) Give the appropriate term defined by the statements given below: [5 Marks]
(i) The formula that represents the simplest ratio of the various elements present in one molecule of the compound.

Answer: Empirical formula

Teacher's Note:
a) It gives only the relative number of each type of atom in a molecule.
b) It is contrasted with the molecular formula, which gives the exact number of atoms.

(ii) The substance that releases hydronium ion as the only positive ion when dissolved in water. [1 Mark]

Answer: Acid

Teacher's Note:
a) Hydronium ions (H₃O⁺) are formed when hydrogen ions released by acids combine with water molecules.
b) This is the Arrhenius definition of an acid.

(iii) The process by which certain ores, specially carbonates, are converted to oxides in the absence of air. [1 Mark]

Answer: Calcination

Teacher's Note:
a) Calcination is carried out below the melting point of the ore in the absence or limited supply of air.
b) It expels moisture and volatile impurities while converting carbonate ores to oxides.

(iv) The covalent bond in which the electrons are shared equally between the combining atoms. [1 Mark]

Answer: Non-polar covalent bond

Teacher's Note:
a) Non-polar covalent bonds are formed between atoms of similar electronegativities.
b) Examples include diatomic molecules like Cl₂, H₂, and O₂.

(h) Arrange the following according to the instructions given in brackets: [5 Marks]
(i) K, Pb, Ca, Zn (In the increasing order of reactivity)

Answer: Pb < Zn < Ca < K

Teacher's Note:
a) Lead is the least reactive among the given metals, while potassium is the most reactive.
b) Recall the standard reactivity series of metals.

(ii) Mg²⁺, Cu²⁺, Na¹⁺, H¹⁺ (In the order of preferential discharge at the cathode) [1 Mark]

Answer: Cu²⁺ > H¹⁺ > Mg²⁺ > Na¹⁺ (or in order of increasing difficulty: Cu²⁺, H¹⁺, Mg²⁺, Na¹⁺ as per electrochemical series)

Teacher's Note:
a) Ions lower in the electrochemical series are discharged preferentially at the respective electrodes.
b) Copper ions are discharged most readily, followed by hydrogen ions.

(iii) Li, K, Na, H (In the decreasing order of their ionisation potential) [1 Mark]

Answer: H > Li > Na > K

Teacher's Note:
a) Ionisation potential decreases down a group as atomic size increases.
b) Hydrogen has a smaller size and higher ionization potential than alkali metals.

(iv) F, B, N, O (In the increasing order of electron affinity) [1 Mark]

Answer: B < N < O < F

Teacher's Note:
a) Electron affinity generally increases across a period from left to right.
b) Halogens have the highest electron affinities in their respective periods.

(v) Methane, ethane, ethene, ethyne (In the increasing order of molecular weight) [1 Mark]

Answer: Methane < Ethyne < Ethene < Ethane

Teacher's Note:
a) Calculate molecular weights: Methane (CH₄ = 16), Ethyne (C₂H₂ = 26), Ethene (C₂H₄ = 28), Ethane (C₂H₆ = 30).
b) Note that ethyne is lighter than ethene and ethane.

SECTION II (40 Marks)

Attempt any four questions from this Section

Question 2

(a) Draw the electron dot structure of [3 Marks]
(i) Nitrogen molecule [N = 7]
(ii) Sodium chloride [Na = 11, Cl = 17]
(iii) Ammonium ion [N = 7, H = 1]

Answer:
(i) Nitrogen molecule: :N:::N: (sharing 3 pairs of electrons between two nitrogen atoms, forming a triple covalent bond).
(ii) Sodium chloride: Na⁺ [ :Cl: ]^- (transfer of 1 electron from sodium to chlorine).
(iii) Ammonium ion: A central nitrogen bonded with four hydrogen atoms by covalent bonds, carrying an overall positive charge enclosing the structure.

Teacher's Note:
a) Use dots and crosses to represent valence electrons clearly in Lewis dot structures.
b) Show brackets and charges for ionic species like sodium chloride and ammonium ions.

(b) The pH values of three solutions A, B and C are given in the table. Answer the following questions: [3 Marks]

(i) Which solution will have no effect on litmus solution?
(ii) Which solution will liberate CO₂ when reacted with sodium carbonate?
(iii) Which solution will turn red litmus solution blue?

Answer:
(i) Solution C
(ii) Solution B
(iii) Solution A

Teacher's Note:
a) Neutral solutions with a pH of 7, such as solution C, do not affect litmus.
b) Acidic solutions liberate carbon dioxide from carbonates, basic solutions turn red litmus blue.

(c) Study the extract of the periodic table given below and answer the questions that follow. Give the letter corresponding to the element in question. DO NOT repeat an element. [4 Marks]

[Figure: Outline of a periodic table extract showing positions of elements A, B, C, D, E, F, G where A is in group 1/2, B is below A, C, D, E are in a period, G and F are halogens and noble gas respectively]

(i) Which element forms an electrovalent compound with G?
(ii) The ion of which element will migrate towards the cathode during electrolysis?
(iii) Which non-metallic element has the valency of 2?
(iv) Which is an inert gas?

Answer:
(i) Element B
(ii) Element B
(iii) Element E
(iv) Element F

Teacher's Note:
a) Metals form electrovalent compounds with non-metals (halogens like G).
b) Cations migrate to the cathode; metals form positively charged ions.

Question 3

(a) Name the particles present in [3 Marks]
(i) Strong electrolyte
(ii) Non-electrolyte
(iii) Weak electrolyte

Answer:
(i) Mainly free mobile ions and very few or no molecules.
(ii) Only molecules, no ions.
(iii) Both molecules and a small percentage of ions.

Teacher's Note:
a) Degree of dissociation determines whether an electrolyte is strong or weak.
b) Non-electrolytes are covalent compounds that do not ionize in water.

(b) Distinguish between the following pairs of compounds using the reagent given in the brackets. [3 Marks]
(i) Manganese dioxide and copper (II) oxide (using concentrated HCl)
(ii) Ferrous sulphate solution and ferric sulphate solution (using sodium hydroxide solution)
(iii) Dilute hydrochloric acid and dilute sulphuric acid (using lead nitrate solution)

Answer:
(i) Manganese dioxide reacts with concentrated HCl on heating to evolve greenish-yellow chlorine gas, whereas copper (II) oxide reacts to form a green/blue solution of copper chloride without evolving chlorine gas.
(ii) Ferrous sulphate gives a dirty green precipitate with sodium hydroxide, while ferric sulphate gives a reddish-brown precipitate.
(iii) Dilute hydrochloric acid gives a white precipitate of lead chloride with lead nitrate solution which is soluble in hot water, whereas dilute sulphuric acid gives a heavy white precipitate of lead sulphate which is insoluble in all acids.

Teacher's Note:
a) Analytical chemistry questions require specific observations such as gas evolution or precipitate color.
b) Mention reagents and exact color changes clearly.

(c) Choose the method of preparation of the following salts from the methods given in the list: [4 Marks]
[List: A. Neutralisation B. Precipitation C. Direct combination D. Substitution]
(i) Lead chloride
(ii) Iron (II) sulphate
(iii) Sodium nitrate
(iv) Iron (III) chloride

Answer:
(i) Lead chloride: B. Precipitation
(ii) Iron (II) sulphate: D. Substitution
(iii) Sodium nitrate: A. Neutralisation
(iv) Iron (III) chloride: C. Direct combination

Teacher's Note:
a) Insoluble salts are prepared by precipitation (ionic synthesis).
b) Direct combination involves direct synthesis from constituent elements, such as iron reacting with chlorine gas to form iron (III) chloride.

Question 4

(a) Complete the following equations: [3 Marks]
(i) S + conc. HNO₃ →
(ii) C + conc. H₂SO₄ →
(iii) Cu + dil. HNO₃ →

Answer:
(i) S + 6HNO₃(conc.) → H₂SO₄ + 6NO₂ + 2H₂O
(ii) C + 2H₂SO₄(conc.) → CO₂ + 2SO₂ + 2H₂O
(iii) 3Cu + 8HNO₃(dil.) → 3Cu(NO₃)₂ + 4H₂O + 2NO

Teacher's Note:
a) Concentrated nitric acid and sulphuric acid act as powerful oxidizing agents.
b) Ensure equations are fully balanced with proper state symbols or coefficients.

(b) Write a balanced chemical equation for the preparation of [3 Marks]
(i) Ethene from bromoethane
(ii) Ethyne using calcium carbide
(iii) Methane from sodium acetate

Answer:
(i) C₂H₅Br + KOH(alc., hot) → C₂H₄↑ + KBr + H₂O
(ii) CaC₂ + 2H₂O → C₂H₂↑ + Ca(OH)₂
(iii) CH₃COONa + NaOH →[CaO, Heat]→ CH₄↑ + Na₂CO₃

Teacher's Note:
a) Preparation of alkanes, alkenes, and alkynes are standard syllabus reactions.
b) Mention reagents like soda lime and alcoholic potassium hydroxide explicitly above the reaction arrow.

(c) Name the following organic compounds: [4 Marks]
(i) The compound with 3 carbon atoms whose functional group is carboxyl.
(ii) The first homologue whose general formula is C_nH_2n.
(iii) The compound that reacts with acetic acid to form ethyl ethanoate.
(iv) The compound formed by complete chlorination of ethyne.

Answer:
(i) Propanoic acid
(ii) Ethene
(iii) Ethanol
(iv) 1,1,2,2 - tetrachloroethane (Acetylene tetrachloride)

Teacher's Note:
a) Esters are formed by the reaction of carboxylic acids with alcohols.
b) Complete chlorination of ethyne involves addition of two moles of chlorine across the triple bond.

Question 5

(a) Give the chemical formula of [3 Marks]
(i) Bauxite
(ii) Cryolite
(iii) Sodium aluminate

Answer:
(i) Al₂O₃·2H₂O
(ii) Na₃AlF₆
(iii) NaAlO₂

Teacher's Note:
a) Bauxite is the chief ore of aluminium.
b) Cryolite acts as a solvent and lowers the fusion temperature in the Hall-Heroult process.

(b) Answer the following question based on the extraction of aluminium from alumina by Hall-Heroult's process: [3 Marks]
(i) What is the function of cryolite used along with alumina as the electrolyte?
(ii) Why is powdered coke sprinkled on top of the electrolyte?
(iii) Name the electrode from which aluminium is collected.

Answer:
(i) Cryolite lowers the fusion temperature of purified alumina from 2050°C to about 950°C and increases electrical conductivity.
(ii) Powdered coke is sprinkled to reduce heat loss by radiation and prevent burning of the carbon anodes.
(iii) Cathode (carbon lining of the electrolytic cell).

Teacher's Note:
a) Alumina is a poor conductor of electricity in solid state and has a very high melting point.
b) Molten aluminium collects at the bottom of the tank near the carbon cathode and is tapped off periodically.

(c) Match the alloys given in column I to the uses given in column II. [4 Marks]

Answer:
(i) Duralumin - C. Aircraft body
(ii) Solder - A. Electrical fuse
(iii) Brass - D. Decorative articles
(iv) Stainless steel - B. Surgical instruments

Teacher's Note:
a) Alloys are homogeneous mixtures of metals or a metal and a non-metal.
b) Solder has a low melting point, making it suitable for soldering and electrical fuses.

Question 6

(a) Identify the substances underlined: [3 Marks]
(i) The catalyst used to oxidise ammonia.
(ii) The organic compound which when solidified forms an ice-like mass.
(iii) The dilute acid which is an oxidising agent.

Answer:
(i) Platinum
(ii) Glacial acetic acid (pure acetic acid)
(iii) Dilute nitric acid

Teacher's Note:
a) Platinum is used in Ostwald's process for catalytic oxidation of ammonia.
b) Pure acetic acid freezes at 16.6°C to form ice-like crystals, hence called glacial acetic acid.

(b) Copper sulphate solution reacts with sodium hydroxide solution to form a precipitate of copper hydroxide according to the equation: [3 Marks]
2NaOH + CuSO₄ → Na₂SO₄ + Cu(OH)₂↓
(i) What mass of copper hydroxide is precipitated by using 200 gm of sodium hydroxide? [H = 1, O = 16, Na = 23, S = 32, Cu = 64]
(ii) What is the colour of the precipitate formed?

Answer:
(i) Molar mass of 2NaOH = 2 × (23 + 16 + 1) = 80 g
Molar mass of Cu(OH)₂ = 64 + 2 × (16 + 1) = 98 g
From the equation, 80 g of NaOH gives 98 g of Cu(OH)₂.
Therefore, 200 g of NaOH will precipitate = (98 / 80) × 200 = 245 g of copper hydroxide.
(ii) Pale blue (or light blue) precipitate.

Teacher's Note:
a) Use stoichiometric mass-mass relationships based on balanced chemical equations.
b) Double check molecular masses: NaOH = 40, Cu(OH)₂ = 98.

(c) Find the empirical formula and the molecular formula of an organic compound from the data given below: [4 Marks]
C = 75.92%, H = 6.32% and N = 17.76%
The vapour density of the compound is 39.5.
[C = 12, H = 1, N = 14]

Answer:
Calculation of Empirical Formula:
- C: (75.92 / 12) = 6.32 / 1.26 = 5
- H: (6.32 / 1) = 6.32 / 1.26 = 5
- N: (17.76 / 14) = 1.27 / 1.26 = 1
Empirical formula = C₅H₅N.
Empirical formula mass = (5 × 12) + (5 × 1) + 14 = 60 + 5 + 14 = 79 g.
Molecular mass = 2 × Vapour density = 2 × 39.5 = 79 g.
Since Molecular mass / Empirical formula mass = 79 / 79 = 1,
Molecular formula = C₅H₅N.

Teacher's Note:
a) Always determine atomic ratios by dividing percentage composition by atomic weights, then divide by the smallest ratio to find the simplest whole-number ratio.
b) Molecular mass is always twice the vapour density.

Question 7

(a) Name the gas evolved in each of the following cases. [3 Marks]
(i) Alumina undergoes electrolytic reduction.
(ii) Ethene undergoes hydrogenation reaction.
(iii) Ammonia reacts with heated copper oxide.

Answer:
(i) Carbon dioxide (at the anode)
(ii) Ethane
(iii) Nitrogen

Teacher's Note:
a) Carbon dioxide is evolved at the carbon anodes due to the reaction of oxygen with carbon.
b) Hydrogenation of ethene adds hydrogen to form ethane gas.

(b) Study the flow chart given and give balanced equations to represent the reactions A, B and C: [3 Marks]

[Figure: Flow chart showing Mg₃N₂ reacting with A to give NH₃, which reacts with B to give NH₄Cl, and NH₄Cl reacts with C to give back NH₃]

Answer:
Reaction A: Mg₃N₂ + 6H₂O → 3Mg(OH)₂ + 2NH₃
Reaction B: NH₃ + HCl → NH₄Cl
Reaction C: 2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2H₂O + 2NH₃

Teacher's Note:
a) Metal nitrides react with water to liberate ammonia gas.
b) Ammonium salts react with alkalis like calcium hydroxide on heating to produce ammonia.

(c) Copy and complete the following table which refers to the industrial method for the preparation of ammonia and sulphuric acid: [4 Marks]

Answer:
(i) Haber's Process
(ii) N₂(g) + 3H₂(g) →[Iron oxide and K₂O/Al₂O₃]→ 2NH₃(g)
(iii) Contact Process
(iv) 2SO₂(g) + O₂(g) →[V₂O₅, 450°C]→ 2SO₃(g)

Teacher's Note:
a) Industrial preparation methods require specific catalysts, temperature conditions, and balanced equations.
b) Mention promoters like molybdenum or potassium oxide along with iron catalyst where applicable.