ICSE Class 10 Chemistry Board Exam Question Paper 2023 with Solutions

ICSE Class 10 Chemistry Board Exam Question Paper with Solutions

SECTION A (40 Marks)

(Attempt all questions from this Section.)

Question 1

Choose the correct answers to the questions from the given options. [15]
(Do not copy the questions, write the correct answers only.)

(i) An element in period 3, whose electron affinity is zero: [1 Mark]
(a) Neon
(b) Sulphur
(c) Sodium
(d) Argon

Answer: (d) Argon

Argon belongs to period 3. It is an inert element and stable. It neither requires gain nor loss. It has its octet complete and hence it has zero electron affinity.

Teacher's Note:
a) Noble gases have a stable octet (or duet in case of helium) configuration, making their electron affinity zero.
b) Students often confuse electron affinity with electronegativity or ionization potential.

(ii) An element with the largest atomic radius among the following is: [1 Mark]
(a) Carbon
(b) Nitrogen
(c) Lithium
(d) Beryllium

Answer: (c) Lithium

For the elements belonging to one period, an increase in atomic number results in a decrease in atomic radius due to increasing effective nuclear charge. Lithium lies on the extreme left of Period 2.

Teacher's Note:
a) Atomic size decreases across a period from left to right and increases down a group.
b) Lithium is an alkali metal in Period 2 and has the largest size among the given options.

(iii) The compound that is not an ore of aluminium: [1 Mark]
(a) Cryolite
(b) Corundum
(c) Fluorspar
(d) Bauxite

Answer: (c) Fluorspar

Fluorspar is calcium fluoride having the chemical formula CaF₂, whereas bauxite, corundum, and cryolite are ores/minerals of aluminium.

Teacher's Note:
a) Bauxite (Al₂O₃.2H₂O), Cryolite (Na₃AlF₆), and Corundum (Al₂O₃) are principal ores/compounds of aluminium.
b) Fluorspar is used as a flux in the extraction of aluminium but is not an aluminium ore.

(iv) The vapour density of CH₃OH is (At. Wt. C-12, H=1, O=16) [1 Mark]
(a) 32
(b) 18
(c) 16
(d) 34

Answer: (c) 16

Molecular mass of methyl alcohol = (1 × 12 + 3 × 1 + 16 × 1 + 1 × 1) = 32. Vapour Density = Molecular mass / 2 = 32 / 2 = 16.

Teacher's Note:
a) Always calculate the relative molecular mass first by summing up the atomic weights of all constituent atoms.
b) Remember the standard relation: Vapour Density = Molecular Mass ÷ 2.

(v) Which of the following reactions takes place at the anode during the electroplating of an article with silver? [1 Mark]
(a) Ag - 1e^- → Ag¹⁺
(b) Ag + 1e^- → Ag^1-
(c) Ag - 1e^- → Ag
(d) None of the above

Answer: (a) Ag - 1e^- → Ag¹⁺

In electroplating with silver, oxidation takes place at the anode where silver atoms lose electrons to form silver ions: Ag - 1e^- → Ag⁺.

Teacher's Note:
a) Oxidation always occurs at the anode (loss of electrons).
b) Reduction always occurs at the cathode (gain of electrons).

(vi) The metallic hydroxide which forms a deep inky blue solution with excess ammonium hydroxide solution is: [1 Mark]
(a) Fe(OH)₂
(b) Cu(OH)₂
(c) Ca(OH)₂
(d) Fe(OH)₃

Answer: (b) Cu(OH)₂

Copper (II) hydroxide reacts with a solution of excess ammonium hydroxide to form a deep inky blue solution due to the formation of tetraamminecopper(II) complex ions.

Teacher's Note:
a) Transition metal ions like Cu²⁺ form characteristic colored complexes with excess reagents.
b) Learn color reactions of cations with NaOH and NH₄OH thoroughly as they are frequently tested.

(vii) An example of a cyclic organic compound is: [1 Mark]
(a) Propene
(b) Pentene
(c) Butene
(d) Benzene

Answer: (d) Benzene

Benzene is a cyclic unsaturated hydrocarbon with a ring structure containing alternating single and double bonds (C₆H₆).

Teacher's Note:
a) Cyclic compounds contain a closed ring of carbon atoms.
b) Propene, pentene, and butene are straight-chain or branched aliphatic (acyclic) alkenes.

(viii) In the laboratory preparation, HCl gas is dried by passing through: [1 Mark]
(a) dilute nitric acid
(b) concentrated sulphuric acid
(c) dilute sulphuric acid
(d) acidified water

Answer: (b) concentrated sulphuric acid

Hydrogen chloride gas is passed through concentrated sulphuric acid because of its strong hygroscopic (drying) property to remove moisture.

Teacher's Note:
a) Concentrated H₂SO₄ is a non-volatile acid and a powerful drying agent that does not react with HCl gas.
b) Basic drying agents like quicklime cannot be used as they would react with acidic HCl gas.

(ix) The nitrate which on thermal decomposition leaves behind a residue which is yellow when hot and white when cold: [1 Mark]
(a) Lead nitrate
(b) Ammonium nitrate
(c) Copper nitrate
(d) Zinc nitrate

Answer: (d) Zinc nitrate

Upon thermal decomposition, zinc nitrate produces zinc oxide, which is yellow when hot and white when cold, along with nitrogen dioxide and oxygen.

Teacher's Note:
a) Zinc oxide exhibits thermochromism (color change with temperature).
b) Lead oxide (PbO) is reddish-brown when hot and yellow when cold, which is a common point of confusion.

(x) The salt formed when concentrated sulphuric acid reacts with KNO₃ above 200°C: [1 Mark]
(a) K₂SO₄
(b) K₂SO₃
(c) KHSO₄
(d) KHSO₃

Answer: (a) K₂SO₄

Reaction above 200°C: 2KNO₃ + H₂SO₄ → K₂SO₄ + 2HNO₃, yielding normal potassium sulphate.

Teacher's Note:
a) Below 200°C, an acid salt (potassium hydrogen sulphate, KHSO₄) is formed.
b) Above 200°C, higher temperature permits formation of the normal salt and prevents damage to glass apparatus.

(xi) The property exhibited by concentrated sulphuric acid when it is used to prepare hydrogen chloride gas from potassium chloride: [1 Mark]
(a) Dehydrating property
(b) Drying property
(c) Oxidizing property
(d) Non-volatile acid property

Answer: (d) Non-volatile acid property

Concentrated sulphuric acid has a high boiling point (non-volatile nature) and is used to displace more volatile acids like HCl from their salts.

Teacher's Note:
a) A less volatile acid displaces a more volatile acid from its salt.
b) Since H₂SO₄ has a high boiling point, it readily displaces gaseous HCl.

(xii) The hydrocarbon formed when sodium propanoate and soda lime are heated together: [1 Mark]
(a) Methane
(b) Ethane
(c) Ethene
(d) Propane

Answer: (b) Ethane

Decarboxylation of sodium propanoate with soda lime yields ethane (CH₃-CH₃), containing one carbon atom less than the parent salt.

Teacher's Note:
a) Decarboxylation removes a carbon atom in the form of Na₂CO₃.
b) Sodium propanoate (C₂H₅COONa) thus produces ethane, having 2 carbon atoms.

(xiii) The acid which does not form acid salt by a basic radical: [1 Mark]
(a) H₂CO₃
(b) H₃PO₄
(c) H₂SO₄
(d) CH₃COOH

Answer: (d) CH₃COOH

Acetic acid is a monoprotic (monobasic) weak acid containing only one replaceable hydrogen ion per molecule, hence it forms only normal salts and not acid salts.

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Teacher's Note:
a) Only polybasic acids (acids with two or more replaceable hydrogen ions) can form acid salts.
b) Acetic acid has only one ionizable hydrogen atom in its carboxyl group.

(xiv) The general formula of hydrocarbons with single covalent bonds is: [1 Mark]
(a) C_nH_2n+2
(b) C_nH_2n
(c) C_nH_2n-2
(d) C_nH_2n-6

Answer: (a) C_nH_2n+2

Hydrocarbons with single covalent bonds are alkanes, and their general formula is C_nH_2n+2.

Teacher's Note:
a) Alkanes are saturated hydrocarbons containing only carbon-carbon single bonds.
b) C_nH_2n represents alkenes and C_nH_2n-2 represents alkynes.

(xv) The indicator which changes to pink colour in an alkaline solution is: [1 Mark]
(a) Blue Litmus
(b) Methyl Orange
(c) Red Litmus
(d) Phenolphthalein

Answer: (d) Phenolphthalein

Phenolphthalein imparts a pink color in alkaline solutions and is colorless in acidic solutions.

Teacher's Note:
a) Methyl orange turns pink/red in acidic medium and yellow in basic medium.
b) Litmus turns red in acid and blue in alkali.

Question 2

(i) Match the Column A with Column B: [5 Marks]

Answer:
(a) Sodium Chloride - 2. Has high melting and boiling point
(b) Methane - 3. A greenhouse gas
(c) Hydrogen chloride gas - 4. Has low melting and boiling point
(d) Oxidation reaction - 5. Zn - 2e^- → Zn²⁺
(e) Water - 1. Has two shared pair electrons

Teacher's Note:
a) Match each chemical property or bonding characteristic accurately with its known physical or chemical descriptor.
b) Water molecule (H-O-H) contains two covalent bonds, which represent two shared pairs of electrons.

(ii) The following sketch illustrates the process of conversion of Alumina to Aluminium: Study the diagram and answer the following: [4 Marks]

[Figure: Hall-Heroult cell showing carbon anodes Y, carbon lining acting as cathode Z, electrolyte mixture, and powdered coke layer X on top]

(a) Name the constituent of the electrolyte mixture which has a divalent metal in it.
(b) Name the powdered substance 'X' sprinkled on the surface of the electrolyte mixture.
(c) What is the name of the process?
(d) Write the reactions taking place at the electrodes 'Y' (anode) and 'Z' (cathode) respectively.

Answer:
(a) Fluorspar (CaF₂), which contains the divalent metal calcium (Ca²⁺).
(b) Powdered coke.
(c) Hall-Heroult's process.
(d) Reactions:
At cathode (Z): Al³⁺ (melt) + 3e^- → Al(l)
At anode (Y): 2O^2- - 4e^- → O₂; C + O₂ → CO₂.

Teacher's Note:
a) The electrolyte is a molten mixture of alumina (20%), cryolite (60%), and fluorspar (20%).
b) Anode needs periodic replacement because it is continuously oxidized by oxygen liberated at high temperatures.

(iii) Fill in the blanks with the choices given in the brackets: [5 Marks]
(a) Metals are good ___________ [oxidizing agents/reducing agents]
(b) Non-polar covalent compounds are _______________ [good/bad] conductors of heat and electricity.
(c) Higher the pH value of a solution, the more _________ [acidic / alkaline] it is.
(d) _______________ [Silver chloride / Lead chloride] is a white precipitate that is soluble in excess of Ammonium hydroxide solution.
(e) Conversion of ethene to ethane is an example of _______________ [hydration / hydrogenation]

Answer:
(a) reducing agents
(b) bad
(c) alkaline
(d) Silver chloride
(e) hydrogenation

Teacher's Note:
a) Metals readily lose electrons, acting as reducing agents.
b) Silver chloride dissolves in excess NH₄OH forming a soluble complex, distinguishing it from lead chloride.

(iv) State the terms/process for the following: [5 Marks]
(a) The energy released when an atom in the gaseous state accepts an electron to form an anion.
(b) Tendency of an element to form chains of identical atoms.
(c) The name of the process by which Ammonia is manufactured on a large scale.
(d) A type of salt formed by partial replacement of hydroxyl radicals with an acid radical.
(e) The ratio of the mass of a certain volume of gas to the same volume of hydrogen measured under the same conditions of temperature and pressure.

Answer:
(a) Electron Affinity (or Electron Gain Enthalpy)
(b) Catenation
(c) Haber's process
(d) Basic salt
(e) Vapour Density

Teacher's Note:
a) Ensure precise terminology is used for definitions in chemistry.
b) Basic salts contain replaceable hydroxyl groups in addition to normal anions and cations.

(v) (a) Give the structural formula of the following organic compounds: [3 Marks]
1. 2-chlorobutane
2. Methanal
3. But-2-yne
(b) Give the IUPAC name of the following organic compounds: [2 Marks]
1. [Figure: structural formula showing H-C(=O)-OH]
2. [Figure: structural formula showing H₃C-CH₂-CH(OH)-CH₃]

Answer:
(a) 1. 2-chlorobutane: CH₃-CH(Cl)-CH₂-CH₃
2. Methanal: H-CHO
3. But-2-yne: CH₃-C≡C-CH₃
(b) 1. Ethanoic acid
2. Butan-2-ol (or 2-butanol)

Teacher's Note:
a) Draw clear carbon skeletons with all hydrogen bonds and functional groups clearly indicated.
b) IUPAC naming must follow lowest locant rule for functional groups and unsaturation.

SECTION B (40 Marks)
(Attempt any four questions from this Section.)

Question 3

(i) Identify the cation in each of the following cases: [2 Marks]
(a) Ammonium hydroxide solution when added to Solution B gives a white precipitate which does not dissolve in excess of ammonium hydroxide solution.
(b) Sodium hydroxide solution when added to Solution C gives a white precipitate which is insoluble in excess of sodium hydroxide solution.

Answer:
(a) Cation is Lead (Pb²⁺).
(b) Cation is Calcium (Ca²⁺).

Teacher's Note:
a) Lead salts give a chalky white precipitate of Pb(OH)₂ with NH₄OH and NaOH, both insoluble in excess.
b) Calcium salts give a white precipitate with NaOH which is insoluble in excess NaOH.

(ii) Fill in the blanks by choosing the correct answer from the brackets: [2 Marks]
(a) During electrolysis, the compound _______________ in its molten state liberates reddish brown fumes at the anode. [NaCl / PbBr₂]
(b) The ion which could be discharged most readily during electrolysis is [Fe²⁺ / Cu²⁺]

Answer:
(a) PbBr₂
(b) Cu²⁺

Teacher's Note:
a) Molten lead bromide on electrolysis gives reddish-brown bromine vapors at the anode.
b) Copper ions are lower in the electrochemical series of cations and are discharged preferentially over iron ions.

(iii) Arrange the following as per the instruction given in the brackets: [3 Marks]
(a) Al, K, Mg, Ca (decreasing order of its reactivity)
(b) N, Be, O, C (increasing order of non-metallic character)
(c) P, Si, F, Be (decreasing order of valence electrons)

Answer:
(a) K > Ca > Mg > Al
(b) Be < C < N < O
(c) F > P > Si > Be

Teacher's Note:
a) Reactivity of metals decreases down the electrochemical series.
b) Non-metallic character increases across a period from left to right.

(iv) Complete and balance the following equations: [3 Marks]
(a) NH₄Cl + Ca(OH)₂ →
(b) CuSO₄ + NH₄OH →
(c) Cu + Conc. HNO₃ →

Answer:
(a) 2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2H₂O + 2NH₃↑
(b) CuSO₄ + 2NH₄OH → Cu(OH)₂↓ + (NH₄)₂SO₄
(c) Cu + 4HNO₃ (conc.) → Cu(NO₃)₂ + 2H₂O + 2NO₂↑

Teacher's Note:
a) Ensure all chemical equations are balanced with proper physical state arrows (↑ for gas, ↓ for precipitate).
b) Concentrated nitric acid oxidizes copper to copper nitrate and produces brown nitrogen dioxide gas.

Question 4

(i) State a relevant reason for the following: [2 Marks]
(a) Hydrogen chloride gas cannot be dried over quick lime.
(b) Ammonia gas is not collected over water.

Answer:
(a) Hydrogen chloride is acidic in nature while quick lime (CaO) is basic. They react together to form a salt and water.
(b) Ammonia gas is extremely soluble in water (1 volume of water dissolves about 700 volumes of ammonia), hence it cannot be collected over water.

Teacher's Note:
a) Drying agents must be chemically inert to the gas being dried.
b) Highly soluble gases like HCl and NH₃ are collected by upward delivery (downward displacement of air).

(ii) Identify the alloy in each case from the given composition: [2 Marks]
(a) aluminium, magnesium, manganese, copper
(b) iron, nickel, chromium, carbon

Answer:
(a) Duralumin
(b) Stainless steel

Teacher's Note:
a) Duralumin is lightweight and strong, used in aircraft construction.
b) Stainless steel resists corrosion due to chromium and nickel content.

(iii) Solve the following numerical problem: [3 Marks]
Ethane burns in oxygen according to the chemical equation:
2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O
If 80 ml of ethane is burnt in 300 ml of oxygen, find the composition of the resultant gaseous mixture when measured at room temperature.

Answer:
According to Gay-Lussac's Law:
2 volumes of C₂H₆ react with 7 volumes of O₂.
80 ml of C₂H₆ requires (7/2) × 80 = 280 ml of O₂.
Volume of O₂ supplied = 300 ml.
Volume of unreacted O₂ = 300 - 280 = 20 ml.
Volume of CO₂ produced = (4/2) × 80 = 160 ml.
(Water condenses to liquid at room temperature).
Resultant gaseous mixture contains: 160 ml of CO₂ and 20 ml of unreacted O₂ (Total = 180 ml).

Teacher's Note:
a) Apply stoichiometry of gaseous volumes directly using reaction coefficients.
b) Water formed is in liquid state at room temperature and is excluded from the gaseous mixture volume.

(iv) The following questions are pertaining to the laboratory preparation of Ammonia gas from Magnesium nitride: [3 Marks]
(a) Write a balanced chemical equation for its preparation.
(b) Why is this method seldom used?
(c) How do you identify the gas formed?

Answer:
(a) Mg₃N₂ + 6H₂O → 3Mg(OH)₂ + 2NH₃↑
(b) This method is expensive because magnesium nitride is a costly starting material.
(c) Identification: When a glass rod dipped in concentrated HCl is brought near the gas, dense white fumes of ammonium chloride are formed.

Teacher's Note:
a) Metal nitrides react with water to liberate ammonia gas.
b) The characteristic pungent smell and basic nature turning moist red litmus blue also confirm ammonia.

Question 5

(i) Write one use of the following alloys: [2 Marks]
(a) Bronze
(b) Fuse metal

Answer:
(a) Bronze is used for making statues, coins, and medals.
(b) Fuse metal is used for electrical fuses in domestic wiring to protect appliances from heavy current surges.

Teacher's Note:
a) Bronze is an alloy of copper and tin.
b) Fuse metal has a low melting point so it melts easily when excessive current passes through the circuit.

(ii) Draw the electron dot structure for the following: [2 Marks]
(a) Ammonium ion
(b) A molecule of nitrogen
[At. No.: N=7, H=1]

Answer:
(a) Ammonium ion ([NH₄]⁺): Nitrogen shares 3 electrons with 3 hydrogen atoms and donates its lone pair to a proton (H⁺) via coordinate bonding, enclosed in square brackets with a positive charge.
(b) Nitrogen molecule (N₂): Two nitrogen atoms share three pairs of electrons forming a triple covalent bond between them, with a lone pair on each nitrogen atom.

Teacher's Note:
a) Show all valence shell electrons clearly as dots or crosses.
b) Indicate coordinate covalent bonds with an arrow or show the complete octet/duet satisfaction.

(iii) Give a balanced chemical equation for the following conversions with conditions: [3 Marks]
(a) Ethene from ethanol
(b) Ethyne from calcium carbide
(c) Monochloromethane from methane

Answer:
(a) C₂H₅OH + Conc. H₂SO₄ (160 - 170°C) → C₂H₄ + H₂O
(b) CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂↑
(c) CH₄ + Cl₂ + Diffused sunlight → CH₃Cl + HCl

Teacher's Note:
a) Concentrated sulphuric acid acts as a dehydrating agent in the preparation of ethene.
b) Substitution reactions of alkanes require diffused sunlight or UV light.

(iv) Study the following observations and name the anions present in each of the reactions: [3 Marks]
(a) When a crystalline solid 'P' is warmed with concentrated H₂SO₄ and copper turnings, a reddish-brown gas is released.
(b) When few drops of dilute sulphuric acid are added to Salt 'R' and heated, a colourless gas is released which turns moist lead acetate paper silvery black.
(c) When few drops of barium nitrate solution are added to the salt solution 'Q', a white precipitate is formed which is insoluble in HCl.

Answer:
(a) Nitrate anion (NO₃^-)
(b) Sulphide anion (S^2-)
(c) Sulphate anion (SO₄^2-)

Teacher's Note:
a) Nitrate salts with conc. H₂SO₄ and copper turnings give reddish-brown NO₂ gas.
b) Sulphides react with dilute acids to give H₂S gas which turns lead acetate paper black due to formation of PbS.

Question 6

(i) Define / State: [2 Marks]
(a) Electronegativity
(b) Gay-Lussac's Law of combining volumes

Answer:
(a) Electronegativity is the tendency of an atom in a molecule to attract the shared pair of electrons towards itself.
(b) Gay-Lussac's Law of combining volumes states that when gases react together, they do so in volumes which bear a simple whole number ratio to one another and to the volume of the gaseous products, provided all volumes are measured under the same conditions of temperature and pressure.

Teacher's Note:
a) Electronegativity is a dimensionless property relative to fluorine on the Pauling scale.
b) Mention temperature and pressure conditions when stating gas laws.

(ii) The Empirical formula of an organic compound is CHCl₂. If its relative molecular mass is 168, what is its molecular formula? (At. Wt. C=12, H=1, Cl=35.5) [3 Marks]

Answer:
Empirical formula mass of CHCl₂ = 12 + 1 + (35.5 × 2) = 13 + 71 = 84 amu.
Multiplication factor (n) = Molecular mass / Empirical formula mass = 168 / 84 = 2.
Molecular formula = (Empirical formula)_n = (CHCl₂)₂ = C₂H₂Cl₄.

Teacher's Note:
a) Always calculate empirical formula mass first by summing atomic masses.
b) Multiply each subscript in the empirical formula by 'n' to obtain the molecular formula.

(iii) Choose the substances given in the box below to answer the following questions: [3 Marks]
[ Iron, Magnesium sulphite, Zinc, Sodium sulphide, Lead, Ferric chloride, Copper, Ferrous sulphate ]
(a) The metal that will not produce hydrogen gas when reacted with dilute acids.
(b) The compound that will produce sulphur dioxide gas when reacted with dilute HCl.
(c) The solution of this compound produces dirty green precipitate with NaOH.

Answer:
(a) Copper
(b) Magnesium sulphite
(c) Ferrous sulphate

Teacher's Note:
a) Metals below hydrogen in the reactivity series (like copper) do not displace hydrogen from dilute acids.
b) Ferrous salts give a dirty green precipitate of Fe(OH)₂ with sodium hydroxide.

(iv) State one relevant observation for each of the following: [3 Marks]
(a) To the copper nitrate solution, initially few drops of sodium hydroxide solution is added and then added in excess.
(b) Burning of ammonia in excess of oxygen.
(c) Dry ammonia gas is passed over heated PbO.

Answer:
(a) A pale blue precipitate is formed which is insoluble in excess of sodium hydroxide solution.
(b) Ammonia burns in excess oxygen with a greenish-yellow flame to produce nitrogen gas and water vapour.
(c) Yellow lead(II) oxide is reduced to greyish metallic lead, and nitrogen gas is liberated.

Teacher's Note:
a) Transition metal hydroxides like Cu(OH)₂ are insoluble in excess NaOH.
b) Ammonia acts as a reducing agent when passed over heated metallic oxides like PbO.

Question 7

(i) Name the following: [2 Marks]
(a) Organic compounds with same molecular formula but different structural formula.
(b) Group of organic compounds where the successive members follow a regular structural pattern, successive compounds differ by a '-CH₂-' group.

Answer:
(a) Isomers
(b) Homologous series

Teacher's Note:
a) Isomerism is a key feature of organic compounds leading to different chemical and physical properties.
b) Members of a homologous series share the same general formula and similar chemical properties.

(ii) Give reason for the following: [2 Marks]
(a) Ionisation potential decreases down a group.
(b) Ionic compounds do not conduct electricity in solid state.

Answer:
(a) Down a group, atomic size increases and the valence electrons are farther from the nucleus, resulting in lesser electrostatic attraction and easier removal of electrons.

(b) In the solid state, ions in ionic compounds are held in fixed positions by strong electrostatic forces of attraction and cannot move freely.

Teacher's Note:
a) Ionization potential is inversely proportional to atomic size.
b) Ionic compounds conduct electricity only in molten or aqueous states where ions are free to migrate.

(iii) Calculate: [3 Marks]
(a) The percentage of phosphorus in the fertilizer super phosphate Ca(H₂PO₄)₂ correct to 1 decimal point. [At. Wt. H=1, P=31, O=16, Ca=40]
(b) Write the empirical formula of C₈H₁₈.

Answer:
(a) Molecular mass of Ca(H₂PO₄)₂ = 40 + [1 × 2 + 31 + (16 × 4)] × 2 = 40 + [2 + 31 + 64] × 2 = 40 + 97 × 2 = 40 + 194 = 234 amu.
Mass of phosphorus in one molecule = 31 × 2 = 62 amu.
Percentage of Phosphorus = (62 / 234) × 100 = 26.495% = 26.5%.
(b) Empirical formula of C₈H₁₈: Divide subscripts by the highest common factor (2), giving C₄H₉.

Teacher's Note:
a) Percentage composition = (Total atomic weight of element / Molecular mass of compound) × 100.
b) Empirical formula expresses the simplest whole-number ratio of atoms in a compound.

(iv) Answer the following questions with reference to electrorefining of copper: [3 Marks]
(a) What is the anode made of?
(b) What do you observe at the cathode?
(c) Write the reaction taking place at the cathode.

Answer:
(a) Impure copper.
(b) The cathode becomes thicker due to the continuous deposition of pure copper.
(c) Reaction at cathode: Cu²⁺ (aq) + 2e^- → Cu (s).

Teacher's Note:
a) During electrorefining, impure copper acts as anode and pure copper strip acts as cathode.
b) Acidified copper sulphate solution is used as the electrolyte.

Question 8

(i) Arrange the following according to the instructions given in brackets: [2 Marks]
(a) C₂H₂, C₃H₆, CH₄, C₂H₄ (In the increasing order of the molecular weight)
(b) Cu²⁺, Na⁺, Zn²⁺, Ag⁺ (The order of Preferential discharge at the cathode)

Answer:
(a) CH₄ < C₂H₂ < C₂H₄ < C₃H₆
(b) Na⁺ < Zn²⁺ < Cu²⁺ < Ag⁺

Teacher's Note:
a) Molecular weights: CH₄ (16), C₂H₂ (26), C₂H₄ (28), C₃H₆ (42).
b) Cations lower in the electrochemical series are discharged preferentially (easier to reduce).

(ii) Differentiate between the following pairs based on the criteria given in the brackets: [2 Marks]
(a) Cane sugar and hydrated copper sulphate [using concentrated H₂SO₄]
(b) Sulphuric acid and hydrochloric acid [type of salts formed]

Answer:

Teacher's Note:
a) Conc. H₂SO₄ is a powerful dehydrating agent removing elements of water from organic compounds.
b) Dibasic or polybasic acids form acid salts, whereas monobasic acids form only normal salts.

(iii) Convert the following reactions into a balanced chemical equation: [3 Marks]
(a) Ammonia to nitric oxide using oxygen and platinum catalyst.
(b) Sodium hydroxide to sodium sulphate using sulphuric acid.
(c) Ferrous sulphide to hydrogen sulphide using hydrochloric acid.

Answer:
(a) 4NH₃ + 5O₂ → (Pt, 800°C) → 4NO + 6H₂O
(b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
(c) FeS + 2HCl → FeCl₂ + H₂S↑

Teacher's Note:
a) Catalytic oxidation of ammonia is the first step in Ostwald's process for manufacture of nitric acid.
b) Metal sulphides react with dilute acids to evolve foul-smelling hydrogen sulphide gas.

(iv) Choose the answer from the list which fits in the description: [3 Marks]
[ CCl₄, PbO, NaCl, CuO, NH₄Cl ]
(a) A compound which undergoes thermal dissociation.
(b) An amphoteric oxide.
(c) A compound which is a non-electrolyte.

Answer:
(a) NH₄Cl (Ammonium chloride)
(b) PbO (Lead(II) oxide)
(c) CCl₄ (Carbon tetrachloride)

Teacher's Note:
a) Thermal dissociation is a reversible decomposition reaction upon heating (e.g., NH₄Cl ↔ NH₃ + HCl).
b) Amphoteric oxides react with both acids and bases to form salt and water.