Mock Paper 3 — 40 MCQ

Answer: C — Protons and electrons. Isotopes are atoms of the same element with the same atomic number (same number of protons) but different mass numbers (different numbers of neutrons). Since atoms are neutral, protons = electrons. So isotopes have the same number of protons AND electrons, but different numbers of neutrons. Example: ¹²C and ¹⁴C both have 6 protons and 6 electrons, but 6 and 8 neutrons respectively.

Answer: A — Period 3, Group VI. The number of electron shells = Period number. 2,8,6 has 3 shells → Period 3. The number of outer-shell electrons = Group number (for Groups I–VII). 6 outer electrons → Group VI. This element is sulfur (S), atomic number 16.

Answer: B — 11 protons, 10 electrons. Sodium (Na) has atomic number 11 → 11 protons (unchanged by ion formation). Na⁺ has lost 1 electron, so 11 − 1 = 10 electrons. The positive charge comes from having more protons than electrons. Proton number is always fixed for an element — only electrons change when ions form.

Answer: C — 35.5. Relative atomic mass = (mass × abundance) summed for all isotopes. = (35 × 75/100) + (37 × 25/100) = 26.25 + 9.25 = 35.5. The Ar of Cl is 35.5, which is why the periodic table shows a non-integer value — it reflects the weighted average of naturally occurring isotopes.

Answer: B. Ionic compounds form giant ionic lattices — a regular 3D arrangement of oppositely charged ions held by strong electrostatic forces. This gives them: high melting/boiling points (much energy needed to break many ionic bonds), ability to conduct when molten or in solution (ions free to move), but NOT when solid (ions fixed in lattice). Examples: NaCl, MgO.

Answer: A. Oxygen in H₂O has 2 bonding pairs (O–H bonds) and 2 lone pairs. Lone pair–lone pair repulsion > lone pair–bonding pair repulsion > bonding pair–bonding pair repulsion. The two lone pairs push the H–O–H angle down to approximately 104.5°, giving a bent/V-shape. Compare with NH₃ (1 lone pair, 3 bonding pairs) which is trigonal pyramidal at ~107°.

Answer: B. In metallic bonding, metal atoms release their outer electrons into a "sea" of delocalised electrons. These electrons are free to move throughout the metallic lattice when a potential difference is applied, carrying charge and making metals good conductors. The positive metal ions remain fixed in a regular lattice. This also explains why metals are good heat conductors — the mobile electrons transfer kinetic energy.

Answer: C — 196 g. Mr of H₂SO₄ = (2×1) + 32 + (4×16) = 2 + 32 + 64 = 98 g/mol. Mass = moles × Mr = 2 × 98 = 196 g. Remember: moles = mass/Mr, so mass = moles × Mr.

Answer: C — 35%. Mr of NH₄NO₃ = 14 + 4 + 14 + 48 = 80 g/mol. There are 2 nitrogen atoms: 2 × 14 = 28 g of N per mole. % N = (28/80) × 100 = 35%. NH₄NO₃ is widely used as a nitrogen fertiliser because of this high N content.

Answer: B — 36 g. Mr of H₂ = 2, so 4 g = 4/2 = 2 moles of H₂. From equation: 2 mol H₂ → 2 mol H₂O (1:1 ratio). So 2 mol H₂ produces 2 mol H₂O. Mr of H₂O = 18. Mass = 2 × 18 = 36 g. Always: moles H₂ → use ratio → moles H₂O → mass H₂O.

Answer: B — Strongly acidic with high H⁺ concentration. The pH scale runs from 0–14: pH 7 = neutral, pH < 7 = acidic, pH > 7 = alkaline. pH 2 is strongly acidic. A pH of 2 means [H⁺] = 0.01 mol/dm³, which is 10× more concentrated than pH 3. Strong acids like HCl fully dissociate to give high H⁺ concentrations.

Answer: B — K₂CO₃ + H₂SO₄. Salt name = metal ion from base + acid anion. Potassium sulfate contains K⁺ (from potassium base) and SO₄²⁻ (from sulfuric acid). K₂CO₃ + H₂SO₄ → K₂SO₄ + H₂O + CO₂. Option A gives KCl (from HCl). The acid determines the anion: HCl → chloride, H₂SO₄ → sulfate, HNO₃ → nitrate.

Answer: C — Precipitation. Insoluble salts are made by precipitation: mixing two soluble solutions whose ions combine to form the insoluble salt. BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s)↓ + 2NaCl(aq). The BaSO₄ precipitates out. Filter, wash with distilled water, dry. You cannot dissolve BaSO₄ and evaporate because it is insoluble. Titration is used for soluble salt preparation from acid + alkali.

Answer: B. HCl is a strong acid — it fully (100%) dissociates into H⁺ and Cl⁻. CH₃COOH is a weak acid — it only partially dissociates (≈1% at 0.1 mol/dm³). Same concentration but HCl has much more H⁺ → lower pH. Same concentration of acid neutralises the same amount of alkali (same number of moles), but HCl reacts more vigorously with metals, giving a faster rate.

Answer: B — Endothermic. Thermal decomposition reactions require a continuous supply of heat → endothermic. Energy is absorbed from the surroundings to break the bonds in CaCO₃ (the energy input to break bonds exceeds the energy released forming CaO and CO₂). The temperature of the reaction vessel would fall if the heat supply were removed. Endothermic: ΔH is positive; reactants are at lower energy than products.

Answer: B — Forming covalent bonds. Bond breaking is always endothermic (absorbs energy). Bond forming is always exothermic (releases energy). A reaction is exothermic overall if energy released forming new bonds > energy absorbed breaking old bonds. Options C and D are not always exothermic: some ionic compounds absorb heat on dissolving (endothermic dissolution), and evaporation always absorbs energy.

Answer: C. Activation energy (Ea) is the minimum energy that colliding particles must possess for a reaction to occur — it is the energy required to start breaking the bonds in the reactants. On an energy profile, it is the energy difference between the reactants and the top of the energy "hill" (transition state). A catalyst lowers Ea by providing an alternative reaction pathway. ΔH is the difference between reactant and product energy levels.

Answer: C — Volume of CO₂ per unit time. Rate = change in quantity / time. Measuring the volume of CO₂ produced at regular time intervals gives a direct measurement of the reaction rate. Alternatively, loss in mass (as CO₂ escapes) can be measured. Temperature and mass of marble are independent variables, not measurements of rate. The faster CO₂ is produced, the steeper the gradient on a volume-vs-time graph.

Answer: C. Increasing pressure compresses the gas — molecules are forced into a smaller volume, so they are closer together. This increases the number of collisions per unit time (collision frequency), leading to more successful collisions per second and a faster rate. Pressure does not change activation energy or give particles more energy (that would require a temperature increase).

Answer: C. A heterogeneous catalyst is in a different physical state from the reactants. Example: iron catalyst (solid) in the Haber process with N₂ and H₂ gases. Reactants adsorb onto the catalyst surface, where bonds are weakened and reaction occurs more easily. The catalyst is not consumed — it can be recovered unchanged. It lowers (not increases) activation energy. A homogeneous catalyst is in the same state as the reactants.

Answer: C — Reactant(s) used up. When a limiting reagent is completely consumed, the reaction stops. No more product forms → the volume graph becomes flat. The final volume reached is the same regardless of how fast the reaction was — changing concentration or temperature changes the gradient (speed) but not the final volume (total product), as long as the same amount of limiting reagent is used.

Answer: D — Potassium. The reactivity series (most to least reactive): K, Na, Ca, Mg, Al, Zn, Fe, Sn, Pb, Cu, Ag, Au. Only metals above Mg react with cold water (K, Na, Ca). K reacts explosively, Na vigorously, Ca slowly. Mg reacts very slowly with cold water but quickly with steam. Zn and Fe react with steam only. Cu does not react with water at all.

Answer: B. In pure metals, layers of atoms of the same size can slide over one another easily (making the metal malleable/ductile). In an alloy, atoms of a different size (e.g. Cr in Fe) distort the regular lattice — the layers can no longer slide as easily, making the alloy harder and stronger. This is why alloys like bronze, brass, and stainless steel are more useful than pure metals for structural applications.

Answer: B. Gold is at the very bottom of the reactivity series — it is extremely unreactive. It does not react with oxygen, water, or dilute acids under normal conditions, so it is stable as the free element in nature. Very unreactive metals (Pt, Au, Ag to some extent) are found as native elements. Highly reactive metals (Na, K, Ca) are always found as compounds (oxides, chlorides, etc.).

Answer: B — Anode; it dissolves. In copper purification: impure copper = anode (+), pure copper = cathode (−), CuSO₄ solution = electrolyte. At anode: Cu → Cu²⁺ + 2e⁻ (impure copper oxidised/dissolves). At cathode: Cu²⁺ + 2e⁻ → Cu (pure copper deposited). Impurities (Zn, Fe) also dissolve but Cu²⁺ is preferentially deposited. Precious metal impurities (Ag, Au) fall as anode sludge.

Answer: B — +3. In Fe₂O₃: oxygen is always −2. Total charge from 3 oxygen atoms = 3 × (−2) = −6. The compound is neutral so 2 Fe atoms must contribute +6 total. Oxidation state of Fe = +6 ÷ 2 = +3. This is iron(III) oxide. Compare with FeO where Fe is +2 (iron(II) oxide). Iron commonly exists in +2 and +3 states — the Roman numeral in the name tells you the oxidation state.

Answer: B. Cl₂ + 2e⁻ → 2Cl⁻ (Cl₂ GAINS electrons → REDUCED). 2Br⁻ → Br₂ + 2e⁻ (Br⁻ LOSES electrons → OXIDISED). OIL RIG. Cl₂ is the oxidising agent (it causes Br⁻ to be oxidised while itself being reduced). This is a halogen displacement reaction — Cl₂ is more reactive than Br₂, so Cl₂ displaces Br⁻ from its salt. The solution turns orange-brown as Br₂ forms.

Answer: B — C₃H₈ (propane). Alkanes have the general formula CₙH₂ₙ₊₂. C₃H₈: n=3, 2(3)+2=8 ✓. Check others: C₂H₄ = C₂H₂(2)+0 → this fits CₙH₂ₙ (alkene formula), it is ethene. C₂H₅OH is ethanol (alcohol). CH₃COOH is ethanoic acid. Alkanes are saturated (only C–C and C–H single bonds), chemically unreactive except for combustion and substitution in UV light.

Answer: C — Carbon dioxide. Fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Conditions: yeast (enzyme), warm temperature (~30°C), absence of oxygen (anaerobic). CO₂ is the other product — it causes the dough to rise in bread-making and is responsible for bubbles in alcoholic drinks. Fermentation is an alternative to cracking for producing ethanol, though it gives a lower concentration (≈15% max before yeast is killed).

Answer: B — Shorter alkanes and alkenes. Cracking breaks C–C bonds in long-chain alkanes using high temperature and a catalyst (or high temperature alone — thermal cracking). Products are shorter alkane chains AND alkenes (which contain C=C double bonds). Example: C₁₅H₃₂ → C₈H₁₈ + C₅H₁₀ + C₂H₄. The alkenes produced are valuable as monomers for polymerisation (e.g. ethene → poly(ethene)). Cracking converts less useful heavy fractions into more valuable petrol/naphtha fractions.

Answer: C. The greenhouse effect: solar (short-wave) radiation passes through CO₂ to warm the Earth's surface. The Earth re-emits longer-wave infrared (heat) radiation. CO₂ (and CH₄, H₂O, N₂O) absorb this IR and re-radiate it in all directions, including back toward Earth — trapping heat. This natural effect keeps Earth warm enough for life; the problem is enhanced greenhouse effect from burning fossil fuels increasing CO₂ levels. CFCs (not CO₂) damage the ozone layer.

Answer: B. Acid rain: SO₂ (from burning sulfur-containing fossil fuels, volcanoes) + H₂O → H₂SO₃/H₂SO₄. NOₓ (from high-temperature combustion in engines/lightning) + H₂O → HNO₃. Normal rain is slightly acidic (pH ~5.6) due to CO₂, but acid rain has pH 4–5 or lower. Effects: damages buildings (especially limestone/marble), kills aquatic life, leaches nutrients from soil. Prevention: catalytic converters, flue gas desulfurisation, renewable energy.

Answer: B — A compromise. The forward reaction (N₂ + 3H₂ → 2NH₃) is exothermic. By Le Chatelier's principle: lower temperature favours the forward reaction (more NH₃). BUT at low temperature, the rate is too slow. 450°C is a compromise: yield is only ~15% but the rate is fast enough to be economically viable. The unreacted N₂ and H₂ are recycled. High pressure (200 atm) increases yield (fewer moles of gas on the right) AND rate.

Answer: C — Al³⁺. Both Al³⁺ and Zn²⁺ form white precipitates with NaOH that dissolve in excess (amphoteric hydroxides). However, the standard O-Level test distinguishes them: Al(OH)₃ dissolves in excess NaOH (forming aluminate, Al(OH)₄⁻). Zn(OH)₂ also dissolves in excess NaOH (forming zincate). To distinguish: use NH₃ solution — Zn(OH)₂ dissolves in excess NH₃ but Al(OH)₃ does not. Fe²⁺ → green ppt, Fe³⁺ → brown ppt, Cu²⁺ → blue ppt (none dissolve in excess NaOH).

Answer: C — Limewater, which turns milky. CO₃²⁻ + 2H⁺ → CO₂ + H₂O. The CO₂ produced is passed through limewater [Ca(OH)₂ solution]: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. The white CaCO₃ precipitate makes the limewater turn milky. This is the standard test for CO₂. Bromine water tests for alkenes (decolourised). Acidified KMnO₄ tests for reducing agents/alkenes. Damp red litmus turning blue tests for NH₃.

Answer: B — Chlorine. Chlorine (Cl₂) first turns damp red litmus blue (it dissolves in water to form HCl + HOCl; the HOCl is alkaline briefly) then bleaches it completely white. This bleaching is the definitive test for Cl₂. Cl₂ dissolves in water: Cl₂ + H₂O ⇌ HCl + HOCl. HOCl (hypochlorous acid) is the bleaching agent. Ammonia turns damp red litmus blue but does NOT bleach. H₂ causes squeaky pop, O₂ relights glowing splint.

Answer: B — Reactivity increases; melting point decreases. Down Group I: more electron shells → outer electron further from nucleus → shielded by more inner shells → weaker nuclear attraction → outer electron lost more easily → higher reactivity. Melting/boiling points DECREASE down Group I (weaker metallic bonding as atomic radius increases). Li: mp 181°C, Na: 98°C, K: 63°C. K reacts explosively with water; Li reacts gently — reactivity increases down the group.

Answer: B. Halogens react by gaining an electron (oxidising agents). Going UP Group VII (towards fluorine): smaller atomic radius → outer shell closer to nucleus → less electron shielding → stronger attraction for an incoming electron → electron gained more easily → higher reactivity. F is the most reactive non-metal known. Reactivity of halogens DECREASES down the group: F > Cl > Br > I.

Answer: B — Full outer electron shells. Noble gases (He: 2 electrons; Ne, Ar, Kr, Xe: 8 outer electrons) have complete outer electron shells. They have no tendency to gain, lose, or share electrons — therefore do not form bonds under normal conditions. This stability makes them inert. Uses rely on this inertness: He in balloons (non-flammable), Ar in welding (prevents oxidation), Ne in advertising signs (glows when current flows).

Answer: B — Coloured compounds and catalytic activity. Transition metals form coloured compounds (Cu²⁺ = blue, Fe³⁺ = yellow/brown, Fe²⁺ = green, Cr³⁺ = green, MnO₄⁻ = purple) because of incomplete d-subshells. They also act as catalysts (Fe in Haber process, Pt/Pd/Rh in catalytic converters, MnO₂ decomposing H₂O₂). Group I metals form mostly white/colourless compounds and are not typically catalysts. Both groups are solids (except Hg), conduct electricity, and have metallic bonding.