ICSE Class 10 Chemistry Board Exam Question Paper 2018 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 :
(i) The salt solution which does not react with ammonium hydroxide is :
(A) Calcium Nitrate
(B) Zinc Nitrate
(C) Lead Nitrate
(D) Copper Nitrate [1 Mark]

Answer: (A) Calcium Nitrate

Calcium nitrate does not form a precipitate with ammonium hydroxide.

Teacher's Note:
a) Calcium ions are not precipitated by ammonium hydroxide solution.
b) Transition metal ions like zinc, lead, and copper form characteristic hydroxide precipitates with ammonium hydroxide.

(ii) The organic compound which undergoes substitution reaction is :
(A) \( \text{C}_2\text{H}_2 \)
(B) \( \text{C}_2\text{H}_4 \)
(C) \( \text{C}_{10}\text{H}_{18} \)
(D) \( \text{C}_2\text{H}_6 \) [1 Mark]

Answer: (D) \( \text{C}_2\text{H}_6 \)

Ethane (\( \text{C}_2\text{H}_6 \)) is a saturated hydrocarbon and undergoes substitution reactions.

Teacher's Note:
a) Saturated hydrocarbons (alkanes) typically undergo substitution reactions, whereas unsaturated hydrocarbons (alkenes and alkynes) undergo addition reactions.
b) Compounds A, B, and C are unsaturated hydrocarbons.

(iii) The electrolysis of acidified water is an example of :
(A) Reduction
(B) Oxidation
(C) Redox reaction
(D) Synthesis [1 Mark]

Answer: (C) Redox reaction

Water undergoes simultaneous oxidation at the anode and reduction at the cathode.

Teacher's Note:
a) Oxidation involves loss of electrons (at anode) and reduction involves gain of electrons (at cathode).
b) Since both processes occur concurrently, it is classified as a redox reaction.

(iv) The IUPAC name of dimethyl ether is :
(A) Ethoxy methane
(B) Methoxy methane
(C) Methoxy ethane
(D) Ethoxy ethane [1 Mark]

Answer: (B) Methoxy methane

The structure is \( \text{CH}_3 - \text{O} - \text{CH}_3 \), which consists of a methane root with a methoxy substituent.

Teacher's Note:
a) Ethers are named as alkoxy alkanes where the smaller alkyl group forms the alkoxy part.
b) Both groups are methyl, hence it is methoxymethane.

(v) The catalyst used in the contact process is :
(A) Copper
(B) Iron
(C) Vanadium pentoxide
(D) Manganese dioxide [1 Mark]

Answer: (C) Vanadium pentoxide

\( \text{V}_2\text{O}_5 \) is used as the catalyst in the contact process for the manufacture of sulphuric acid.

Teacher's Note:
a) Vanadium pentoxide promotes the oxidation of sulphur dioxide to sulphur trioxide.
b) Platinum can also be used, but vanadium pentoxide is preferred due to lower cost and higher resistance to poisoning.

(b) Give one word or a phrase for the following statements :
(i) The energy released when an electron is added to a neutral gaseous isolated atom to form a negatively charged ion. [1 Mark]

Answer: Electron affinity or electron gain enthalpy.

Teacher's Note:
a) Electron affinity measures the energy change when an electron is added to a gaseous atom.
b) It is typically expressed in kJ/mol.

(ii) Process of formation of ions from molecules which are not in ionic state. [1 Mark]

Answer: Ionization

Teacher's Note:
a) Ionization is distinct from dissociation, which involves separation of already existing ions.
b) Polar covalent compounds like HCl undergo ionization when dissolved in water.

(iii) The tendency of an element to form chains of identical atoms. [1 Mark]

Answer: Catenation

Teacher's Note:
a) Catenation is most prominent in carbon due to its small size and strong carbon-carbon covalent bonds.
b) It accounts for the vast number of organic compounds.

(iv) The property by which certain hydrated salts, when left exposed to atmosphere, lose their water of crystallization and crumble into powder. [1 Mark]

Answer: Efflorescence

Teacher's Note:
a) Efflorescent substances have a high aqueous vapour pressure compared to atmospheric humidity.
b) Example includes hydrated sodium carbonate (washing soda crystals).

(v) The process by which sulphide ore is concentrated. [1 Mark]

Answer: Froth floatation

Teacher's Note:
a) This process relies on the different wetting properties of ore and gangue particles with oil and water.
b) Sulphide ores are preferentially wetted by pine oil.

(c) Write a balanced chemical equation for each of the following :
(i) Action of concentrated sulphuric acid on carbon. [1 Mark]

Answer:

\( \text{C}_{(s)} + 2\text{H}_2\text{SO}_{4(conc.)} \rightarrow \text{CO}_{2(g)} + 2\text{SO}_{2(g)} + 2\text{H}_2\text{O}_{(l)} \)

Teacher's Note:
a) Concentrated sulphuric acid acts as a strong oxidizing agent in this reaction.
b) Non-metals like carbon are oxidized to carbon dioxide.

(ii) Reaction of sodium hydroxide solution with iron (III) chloride solution. [1 Mark]

Answer:

\( \text{FeCl}_{3(aq)} + 3\text{NaOH}_{(aq)} \rightarrow \text{Fe(OH)}_{3(s)} \downarrow + 3\text{NaCl}_{(aq)} \)

Teacher's Note:
a) A reddish-brown precipitate of iron (III) hydroxide is formed.
b) Ensure all reactant and product coefficients are balanced correctly.

(iii) Action of heat on aluminium hydroxide. [1 Mark]

Answer:

\( 2\text{Al(OH)}_{3(s)} \xrightarrow{\Delta} \text{Al}_2\text{O}_{3(s)} + 3\text{H}_2\text{O}_{(g)} \)

Teacher's Note:
a) Aluminium hydroxide decomposes on heating to form aluminium oxide (alumina) and steam.
b) Alumina is an important intermediate in the extraction of aluminium.

(iv) Reaction of zinc with potassium hydroxide solution. [1 Mark]

Answer:

\( \text{Zn} + 2\text{KOH} + 2\text{H}_2\text{O} \rightarrow \text{K}_2[\text{Zn}(\text{OH})_4] + \text{H}_{2}\uparrow \)

Teacher's Note:
a) Zinc is an amphoteric metal that reacts with alkalis to evolve hydrogen gas.
b) Potassium zincate is formed as the soluble salt product.

(v) Action of dilute hydrochloric acid on magnesium sulphite. [1 Mark]

Answer:

\( \text{MgSO}_3 + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2\text{O} + \text{SO}_2\uparrow \)

Teacher's Note:
a) Sulphites react with dilute acids to liberate sulphur dioxide gas with a suffocating smell.
b) Salt and water are formed alongside the gas.

(d) (i) Give the IUPAC name for each of the following :
1. \(\text{H} - \text{C} = \text{O}\)
\(\phantom{1. \text{H}}\)
\(\text{H}\)
2. \(\text{H} - \text{C} - \text{C} - \text{C} - \text{OH}\)
\(\phantom{2. }\text{H} \;\; \text{H} \;\; \text{H}\)
3. \(\text{H}_3\text{C} - \text{C} = \text{C} - \text{CH}_3\)
\(\phantom{3. \text{H}_3\text{C} -}\text{H} \;\; \text{H}\)
(ii) Write the structural formula of the two isomers of butane. [5 Marks]

Answer:
(i) IUPAC Names:
1. Methanal
2. Propan-1-ol (or Propanol)
3. But-2-ene
(ii) Structural formulae of butane isomers:
1. n-Butane: \( \text{CH}_3 - \text{CH}_2 - \text{CH}_2 - \text{CH}_3 \)
2. Iso-butane (2-methylpropane):
\( \begin{array}{c} \text{CH}_3 \\ | \\ \text{CH} - \text{CH}_3 \\ | \\ \text{CH}_3 \end{array} \)

Teacher's Note:
a) Ensure correct numbering of the carbon chain for alcohols and alkenes.
b) Isomers have the same molecular formula (\( \text{C}_4\text{H}_{10} \)) but different structural arrangements.

(e) State one relevant observation for each of the following :
(i) Lead nitrate solution is treated with sodium hydroxide solution drop wise till it is excess. [1 Mark]

Answer: A chalky white precipitate of lead hydroxide is formed which dissolves in excess sodium hydroxide to form a clear solution.

Teacher's Note:
a) Lead hydroxide is amphoteric in nature.
b) The resulting clear solution contains sodium plumbite.

(ii) At the anode, when molten lead bromide is electrolyzed using graphite electrodes. [1 Mark]

Answer: Reddish-brown fumes of bromine gas are evolved at the anode.

Teacher's Note:
a) Bromide ions migrate to the anode and undergo oxidation by losing electrons.
b) Bromine gas is characterized by its distinct reddish-brown colour.

(iii) Lead nitrate solution is mixed with dilute hydrochloric acid and heated. [1 Mark]

Answer: A white precipitate of lead chloride is formed which dissolves on heating and reappears on cooling.

Teacher's Note:
a) Lead chloride is sparingly soluble in cold water but soluble in hot water.
b) This property is used in the identification of lead ions.

(iv) Anhydrous calcium chloride is exposed to air for some time. [1 Mark]

Answer: It absorbs moisture from the atmosphere and dissolves in it to form a solution (deliquescence).

Teacher's Note:
a) Deliquescent substances absorb enough water from the air to form a liquid solution.
b) Anhydrous calcium chloride is a powerful drying agent.

(v) Barium chloride solution is slowly added to sodium sulphate solution. [1 Mark]

Answer: A dense white precipitate of barium sulphate is formed immediately, which is insoluble in dilute hydrochloric acid.

Teacher's Note:
a) This is a classic precipitation reaction used to test for sulphate ions.
b) The white precipitate does not dissolve upon addition of mineral acids.

(f) Give a reason for each of the following :
(i) Ionic compounds have a high melting point. [1 Mark]

Answer: Ionic compounds consist of oppositely charged ions held together by strong electrostatic forces of attraction, requiring a large amount of thermal energy to break.

Teacher's Note:
a) Strong inter-ionic forces result in stable crystal lattices.
b) Consequently, considerable energy is needed to separate the ions into the molten state.

(ii) Inert gases do not form ions. [1 Mark]

Answer: Inert gases possess a stable, completely filled valence electron shell configuration (duet or octet), making them energetically stable and chemically unreactive.

Teacher's Note:
a) They have zero electron affinity and very high ionization potential.
b) Thus, they neither lose nor gain electrons easily.

(iii) Ionisation potential increases across a period, from left to right. [1 Mark]

Answer: Across a period, atomic size decreases while nuclear charge increases, resulting in a stronger attraction between the nucleus and valence electrons.

Teacher's Note:
a) Smaller atomic radius means valence electrons are closer to the nucleus.
b) More energy is therefore required to remove an electron.

(iv) Alkali metals are good reducing agents. [1 Mark]

Answer: Alkali metals have large atomic sizes and a single valence electron which is easily lost, allowing them to act as electron donors.

Teacher's Note:
a) Low ionization energy facilitates the easy loss of valence electrons.
b) Donating electrons defines reducing chemical behaviour.

(v) Conductivity of dilute hydrochloric acid is greater than that of acetic acid. [1 Mark]

Answer: Hydrochloric acid is a strong electrolyte that dissociates completely in water, whereas acetic acid is a weak electrolyte that dissociates only partially.

Teacher's Note:
a) Higher concentration of free mobile ions leads to greater electrical conductivity.
b) HCl provides far more ions per mole than \( \text{CH}_3\text{COOH} \).

(g) Name the gas that is produced in each of the following cases :
(i) Sulphur is oxidized by concentrated nitric acid.
(ii) Action of dilute hydrochloric acid on sodium sulphide.
(iii) Action of cold and dilute nitric acid on copper.
(iv) At the anode during the electrolysis of acidified water.
(v) Reaction of ethanol and sodium. [5 Marks]

Answer:
(i) Sulphur dioxide (\( \text{SO}_2 \))
(ii) Hydrogen sulphide (\( \text{H}_2\text{S} \))
(iii) Nitric oxide (\( \text{NO} \)) [Note: The official key states \( \text{NO}_2 \), but dilute nitric acid on copper produces nitric oxide (\( \text{NO} \)), which turns brown on reacting with air.]
(iv) Oxygen (\( \text{O}_2 \))
(v) Hydrogen (\( \text{H}_2 \))

Teacher's Note:
a) Identify the reacting species and the specific oxidizing or reducing nature of the acid.
b) Memorize characteristic gas colours and odours for identification questions.

(h) Fill up the blanks with the correct choice given in brackets.
(i) Ionic or electrovalent compounds do not conduct electricity in their _______ state. (fused/solid)
(ii) Electrolysis of aqueous sodium chloride solution will form ________ at the cathode. (Hydrogen gas / Sodium metal)
(iii) Dry hydrogen chloride gas can be collected by ________ displacement of air. (downward / upward)
(iv) The most common ore of iron is ________. (Calcium / Haematite)
(v) The salt prepared by the method of direct combination is _______. (iron (II) chloride / iron (III) chloride) [5 Marks]

Answer:
(i) Solid
(ii) Hydrogen gas
(iii) Downward
(iv) Haematite
(v) Iron (III) chloride

Teacher's Note:
a) Ionic compounds conduct electricity only in molten or aqueous states due to free mobile ions.
b) Direct combination of iron and chlorine gas yields iron (III) chloride (\( \text{FeCl}_3 \)) due to the oxidizing nature of chlorine.

SECTION - II (40 Marks)

Attempt any four questions from this Section

Question 2

(a) (i) What do you understand by a lone pair of electrons?
(ii) Draw the electron dot diagram of Hydronium ion (\( \text{H} = 1; \text{O} = 8 \)) [3 Marks]

Answer:
(i) Lone pair of electrons are valence shell electrons that do not participate in chemical bonding and remain non-bonded.
(ii) Hydronium ion (\( [\text{H}_3\text{O}]^+ \)) electron dot diagram:

[Figure: Showing oxygen atom bonded to three hydrogen atoms with single covalent bonds, with one lone pair remaining on oxygen, and a positive charge outside the square brackets enclosing the ion]

Teacher's Note:
a) A coordinate covalent bond is formed when water donates a lone pair to a hydrogen ion (\( \text{H}^+ \)).
b) Clearly show brackets and the positive charge for polyatomic ions.

(b) In Period 3 of the Periodic Table, element B is placed to the left of element A.
On the basis of this information, choose the correct word from the brackets to complete the following statements:
(i) The element B would have (lower / higher) metallic character than A.
(ii) The element A would probably have (lesser / higher) electron affinity than B.
(iii) The element A would have (greater / smaller) atomic size than B. [3 Marks]

Answer:
(i) Higher
(ii) Higher
(iii) Smaller

Teacher's Note:
a) Metallic character decreases across a period from left to right.
b) Atomic size decreases and electron affinity increases from left to right across a period.

(c) Copy and complete the following table which refers to the conversion of ions to neutral particles.
Conversion | Ionic equation | Oxidation / Reduction
Chloride ion to chlorine molecule | (i) ________ | (ii) ________
Lead (II) ion to lead | (iii) ________ | (iv) ________ [4 Marks]

Answer:

Teacher's Note:
a) Loss of electrons is oxidation (increase in oxidation state or discharge of anions at anode).
b) Gain of electrons is reduction (decrease in oxidation state or discharge of cations at cathode).

Question 3

(a) (i) Write the balanced chemical equation to prepare ammonia gas in the laboratory by using an alkali.
(ii) State why concentrated sulphuric acid is not used for drying ammonia gas.
(iii) Why is ammonia gas not collected over water ? [3 Marks]

Answer:
(i) \( \text{Ca(OH)}_2 + 2\text{NH}_4\text{Cl} \rightarrow \text{CaCl}_2 + 2\text{H}_2\text{O} + 2\text{NH}_3\uparrow \)
(ii) Ammonia is basic in nature and reacts chemically with concentrated sulphuric acid to form ammonium sulphate salt.
(iii) 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) Quicklime (\text{CaO}) is used instead for drying ammonia gas.
b) Ammonia is collected by downward displacement of air because it is lighter than air.

(b) (i) Name the acid used for the preparation of hydrogen chloride gas in the laboratory. Why is this particular acid preferred to other acids?
(ii) Write the balanced chemical equation for the laboratory preparation of hydrogen chloride gas. [3 Marks]

Answer:
(i) Concentrated sulphuric acid (\( \text{H}_2\text{SO}_4 \)). It is preferred because it is non-volatile and has strong dehydrating and high boiling point properties.
(ii) \( \text{NaCl}_{(s)} + \text{H}_2\text{SO}_{4(aq)} \xrightarrow{\lt 200^{\circ}\text{C}} \text{NaHSO}_{4(aq)} + \text{HCl}_{(g)} \)

Teacher's Note:
a) Volatile acids like nitric acid cannot be used because they would decompose.
b) The temperature is maintained below \( 200^{\circ}\text{C} \) to save fuel and prevent damage to the glass apparatus.

(c) For the preparation of hydrochloric acid in the laboratory:
(i) Why is direct absorption of hydrogen chloride gas in water not feasible?
(ii) What arrangement is done to dissolve hydrogen chloride gas in water? [2 Marks]

Answer:
(i) Direct absorption is not feasible because hydrogen chloride is extremely soluble in water, causing rapid suction of water back into the generator apparatus, leading to an explosion.
(ii) An inverted funnel arrangement is connected to the delivery tube; the rim of the funnel just touches the water surface, preventing back-suction and providing a large surface area for absorption.

Teacher's Note:
a) Back-suction occurs when the rate of dissolution exceeds the rate of gas production.
b) The funnel arrangement ensures safe and continuous absorption of HCl gas.

(d) For the electro-refining of copper :
(i) What is the cathode made up of?
(ii) Write the reaction that takes place at the anode. [2 Marks]

Answer:
(i) A thin strip of pure copper metal.
(ii) Anode reaction (Oxidation):
\( \text{Cu} - 2e^- \rightarrow \text{Cu}^{2+} \) (or \( \text{Cu} \rightarrow \text{Cu}^{2+} + 2e^- \))

Teacher's Note:
a) Impure copper serves as the anode and dissolves into the electrolyte as copper ions.
b) Pure copper gets deposited at the cathode.

Question 4

(a) The percentage composition of a gas is:
Nitrogen 82.35%, Hydrogen 17.64%.
Find the empirical formula of the gas. [N = 14, H = 1] [3 Marks]

Answer:
Moles of Nitrogen = \( \frac{82.35}{14} = 5.88 \)
Moles of Hydrogen = \( \frac{17.64}{1} = 17.64 \)
Simplest mole ratio = \( 5.88 : 17.64 \) = \( 1 : 3 \)
Therefore, the empirical formula is \( \text{NH}_3 \).

Teacher's Note:
a) Divide the percentage of each element by its relative atomic mass to get relative moles.
b) Divide all mole values by the smallest mole value to obtain the simplest whole-number ratio.

(b) Aluminum carbide reacts with water according to the following equation :
\( \text{Al}_4\text{C}_3 + 12\text{H}_2\text{O} \rightarrow 4\text{Al(OH)}_3 + 3\text{CH}_4 \)
(i) What mass of aluminum hydroxide is formed from 12g of aluminum carbide?
(ii) What volume of methane at s.t.p. is obtained from 12g of aluminum carbide?
[Relatively molecular weight of \( \text{Al}_4\text{C}_3 = 144 \); \( \text{Al(OH)}_3 = 78 \)] [3 Marks]

Answer:
From the balanced equation:
\( 144\text{ g} \) of \( \text{Al}_4\text{C}_3 \) produces \( 4 \times 78 = 312\text{ g} \) of \( \text{Al(OH)}_3 \) and \( 3 \times 22.4\text{ litres} \) (or \( 67,200\text{ cc} \)) of methane at s.t.p.
(i) Mass of \( \text{Al(OH)}_3 \) formed from \( 12\text{ g} \) of \( \text{Al}_4\text{C}_3 \) = \( \frac{312}{144} \times 12 = 26\text{ g} \).
(ii) Volume of methane at s.t.p. = \( \frac{3 \times 22400}{144} \times 12 = 5600\text{ cc} \) (or \( 5.6\text{ litres} \)).

Teacher's Note:
a) Use stoichiometric mass-mass and mass-volume relationships derived from the balanced equation.
b) Ensure units for volume are consistent (cc or litres).

(c) (i) If 150 cc of gas A contains X molecules, how many molecules of gas B will be present in 75 cc of B? The gases A and B are under the same conditions of temperature and pressure.
(ii) Name the law on which the above problem is based. [2 Marks]

Answer:
(i) Since 150 cc of gas contains \( X \) molecules, 75 cc of gas B (half the volume) will contain \( \frac{X}{2} \) molecules.
(ii) Avogadro's Law.

Teacher's Note:
a) Avogadro's law states that equal volumes of all gases under the same conditions of temperature and pressure contain equal numbers of molecules.
b) Volume is directly proportional to the number of molecules.

(d) Name the main component of the following alloys:
(i) Brass
(ii) Duralumin [2 Marks]

Answer:
(i) Brass: Copper and Zinc
(ii) Duralumin: Aluminium, Copper, Magnesium, and Manganese (Copper is also accepted as a main component).

Teacher's Note:
a) Brass is an alloy of copper (approx 60-80%) and zinc.
b) Duralumin is a lightweight, high-strength alloy primarily made of aluminium.

Question 5

(a) Complete the following table which relates to the homologous series of hydrocarbons.
General Formula | IUPAC name of the homologous series | Characteristic bond type | IUPAC name of the first member of the series
\( \text{C}_n\text{H}_{2n-2} \) | (A) _________ | (B) _________ | (C) _________
\( \text{C}_n\text{H}_{2n+2} \) | (D) _________ | (E) _________ | (F) _________ [4 Marks]

Answer:

Teacher's Note:
a) Homologous series share a general formula and similar chemical properties.
b) The first member of alkynes is ethyne (\(\text{C}_2\text{H}_2\)), as \(n=1\) is not possible for a triple bond between carbons.

(b) (i) Name the most common ore of the metal aluminum from which the metal is extracted. Write the chemical formula of the ore.
(ii) Name the process by which impure ore of aluminum gets purified by using concentrated solution of an alkali.
(iii) Write the equation for the formation of aluminum at the cathode during the electrolysis of alumina. [3 Marks]

Answer:
(i) Bauxite; Chemical formula: \( \text{Al}_2\text{O}_3 \cdot 2\text{H}_2\text{O} \)
(ii) Bayer's process (or Hall's process)
(iii) Cathode reaction: \( \text{Al}^{3+} + 3e^- \rightarrow \text{Al} \) (or \( \text{Al}^{3+} + 3e^- \rightarrow \text{Al}_{(s)} \))

Teacher's Note:
a) Bauxite is hydrated aluminium oxide.
b) Aluminium ions gain three electrons at the carbon cathode to form molten aluminium metal.

(c) (i) What do you understand by electroplating?
(ii) Give two reasons why electroplating is done. [3 Marks]

Answer:
(i) Electroplating is the process of depositing a thin layer of a superior metal over another metallic object using electrical current and electrolysis.
(ii) Two reasons for electroplating are:
1. To protect the base metal from corrosion or rusting.
2. To enhance the aesthetic appearance of the object.

Teacher's Note:
a) The article to be electroplated is always made the cathode in the electrolytic cell.
b) The electrolyte must contain ions of the metal being plated.

Question 6

(a) A compound X (having vinegar like smell) when treated with ethanol in the presence of the acid Z, gives a compound Y which has a fruity smell.
The reaction is:
\( \text{C}_2\text{H}_5\text{OH} + \text{X} \xrightarrow{\text{Z}} \text{Y} + \text{H}_2\text{O} \)
(i) Identify Y and Z.
(ii) Write the structural formula of X.
(iii) Name the above reaction. [3 Marks]

Answer:
(i) Y is Ethyl ethanoate (ester); Z is concentrated Sulphuric acid (\( \text{H}_2\text{SO}_4 \)).
(ii) Structural formula of X (Acetic acid / Ethanoic acid):
\( \begin{array}{c} \text{H} \\ | \\ \text{H} - \text{C} - \text{C} = \text{O} \\ | \;\;\; \backslash \\ \text{H} \;\;\; \text{OH} \end{array} \)
(iii) Esterification

Teacher's Note:
a) Vinegar-like smell indicates ethanoic acid (\(\text{CH}_3\text{COOH}\)).
b) Concentrated sulphuric acid acts as a dehydrating agent in esterification.

(b) Ethane burns in oxygen to form \( \text{CO}_2 \) and \( \text{H}_2\text{O} \) according to the equation:
\( 2\text{C}_2\text{H}_6 + 7\text{O}_2 \rightarrow 4\text{CO}_2 + 6\text{H}_2\text{O} \)
If 1250 cc of oxygen is burnt with 300 cc of ethane.
Calculate:
(i) the volume of \( \text{CO}_2 \) formed.
(ii) the volume of unused \( \text{O}_2 \) [4 Marks]

Answer:
From the balanced equation, 2 volumes of \( \text{C}_2\text{H}_6 \) require 7 volumes of \( \text{O}_2 \).
For 300 cc of ethane, oxygen required = \( \frac{7}{2} \times 300 = 1050\text{ cc} \).
Since 1250 cc of \( \text{O}_2 \) is available, ethane is the limiting reactant.
(i) Volume of \( \text{CO}_2 \) formed = \( \frac{4}{2} \times 300 = 600\text{ cc} \).
(ii) Unused \( \text{O}_2 \) = \( 1250\text{ cc} - 1050\text{ cc} = 200\text{ cc} \).

Teacher's Note:
a) Always determine the limiting reagent first in gas stoichiometry problems.
b) Gay-Lussac's Law of combining volumes applies directly to gaseous reactions.

(c) Three solutions P, Q and R have pH value of 3.5, 5.2 and 12.2 respectively. Which one of these is a:
(i) Weak acid?
(ii) Strong alkali? [2 Marks]

Answer:
(i) Q (having pH 5.2)
(ii) R (having pH 12.2)

Teacher's Note:
a) pH values close to 7 (like 5.2) represent weak acids, while lower values (like 3.5) represent stronger acids.
b) pH values well above 7 (like 12.2) indicate strong alkalis.

Question 7

(a) Give a chemical test to distinguish between the following pairs of chemicals:
(i) Lead nitrate solution and Zinc nitrate solution
(ii) Sodium chloride solution and Sodium nitrate solution [2 Marks]

Answer:
(i) Pass hydrogen sulphide gas through both solutions. Lead nitrate gives a black precipitate of lead sulphide (\text{PbS}), whereas zinc nitrate does not give a precipitate in neutral or acidic solution.
(ii) Add silver nitrate solution followed by dilute nitric acid to both. Sodium chloride gives a curdy white precipitate of silver chloride (\text{AgCl}) soluble in ammonium hydroxide, whereas sodium nitrate does not react.

Teacher's Note:
a) Analytical chemistry tests rely on characteristic precipitation reactions.
b) Always mention the reagent used and the specific observation for each chemical.

(b) Write a balanced equation for the preparation of each of the following salts:
(i) Copper sulphate from Copper carbonate.
(ii) Zinc carbonate from Zinc sulphate. [2 Marks]

Answer:
(i) \( \text{CuCO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{CuSO}_4 + \text{CO}_2\uparrow + \text{H}_2\text{O} \)
(ii) \( \text{ZnSO}_4 + \text{Na}_2\text{CO}_3 \rightarrow \text{Na}_2\text{SO}_4 + \text{ZnCO}_3\downarrow \)

Teacher's Note:
a) Insoluble salts like zinc carbonate are prepared by precipitation (double displacement).
b) Soluble salts like copper sulphate are prepared by the action of an acid on an insoluble carbonate.

(c) (i) What is the type of salt formed when the reactants are heated at a suitable temperature for the preparation of Nitric acid?
(ii) State why for the preparation of Nitric acid, the complete apparatus is made up of glass. [3 Marks]

Answer:
(i) An acid salt (specifically sodium hydrogensulphate / bisulphate) is formed when reactants are heated below \( 200^{\circ}\text{C} \).
(ii) The apparatus is made of glass because nitric acid vapour is highly corrosive and would attack and corrode metal parts.

Teacher's Note:
a) Glass is inert to the action of nitric acid and hydrogen chloride vapours.
b) All-glass retorts or round-bottom flasks are standard for laboratory preparation.

(d) Which property of sulphuric acid is shown by the reaction of concentrated sulphuric acid with:
(i) Ethanol?
(ii) Carbon? [3 Marks]

Answer:
(i) Dehydrating property (removes water molecule from ethanol to form ethene).
(ii) Oxidizing property (oxidizes carbon to carbon dioxide).

Teacher's Note:
a) Concentrated sulphuric acid has a strong affinity for water, making it an excellent dehydrating agent.
b) Its oxidizing action is demonstrated when reacting with non-metals by producing sulphur dioxide.