WBCS Prelims Chemistry Questions and Answers (Previous Year Paper)

Answer: (C) Hydrogen bonding

Explanation: Hydrogen bonding occurs when hydrogen is covalently bonded to a small, highly electronegative atom such as F, O or N. The hydrogen then carries a strong partial positive charge and is attracted to a lone pair on a neighbouring molecule. In H₂O and HF, fluorine and oxygen are small and highly electronegative, so hydrogen bonds form. Sulphur in H₂S is larger and much less electronegative, so the S–H bond is only weakly polar and H₂S has no hydrogen bonding. This is why H₂O is a liquid at room temperature while H₂S is a gas.

Hydrogen Bonding:

Answer: (C) 44

Explanation: The molecular weight of MCl₄ is 154, and four chlorine atoms contribute 4 × 35.5 = 142. So the atomic weight of M is 154 − 142 = 12, which is carbon (CCl₄). M shows a valency of 4, so its oxide is MO₂ (CO₂ for carbon). Its molecular weight is 12 + 2 × 16 = 44.

Valency and Oxide Formula:

Answer: (B) 2n

Explanation: The number of molecules is proportional to the number of moles. 14 g of N₂ is 14/28 = 0.5 mole, which contains n molecules, so one mole contains 2n molecules. The molar mass of NO₂ is 14 + 32 = 46 g/mol, so 46 g of NO₂ is exactly 1 mole. It therefore contains 2n molecules.

Mole Concept:

Answer: (C) low pressure and high temperature

Explanation: Real gases deviate from ideal behaviour because their molecules have a finite volume and attract one another. At low pressure the molecules are far apart, so their own volume is negligible compared with the volume of the container. At high temperature the molecules move fast, so their kinetic energy overcomes the weak intermolecular attractions. Under these conditions the two assumptions of the ideal gas model hold, and the gas obeys PV = nRT closely.

Ideal vs Real Gas:

Answer: (D) All of the above

Explanation: The natural radioactive series (uranium, thorium and actinium) consist of heavy elements. Many lighter elements outside these series also have radioactive isotopes, which are mostly artificial. Fluorine has F-18, chlorine has Cl-36 and iodine has I-131. All three therefore qualify.

Radioactive Isotopes of Common Elements:

Answer: (B) Aluminium metal

Explanation: The thermit reaction is Fe₂O₃ + 2Al → Al₂O₃ + 2Fe + heat. Aluminium is more reactive than iron, so it takes oxygen away from iron oxide. A substance that removes oxygen from another (and itself gets oxidised) is the reducing agent. Aluminium is therefore the reducing agent and iron oxide is reduced to molten iron, which fuses the broken railway track.

Thermit Reaction:

Answer: (B) 10

Explanation: Mass number = protons + neutrons, so the number of protons is 27 − 14 = 13. A neutral atom has 13 electrons. The ion carries a charge of +3, which means it has lost three electrons, so it has 13 − 3 = 10 electrons. This ion is Al³⁺, which has the neon configuration 1s² 2s² 2p⁶.

Atomic Structure of Al and Al³⁺:

Answer: (B) released.

Explanation: A liquid drop has surface energy because molecules at the surface have fewer neighbours and so are at a higher energy than those inside. When two drops merge, the total surface area of the single drop is smaller than the combined area of the two separate drops. The surface energy therefore decreases, and the difference is released, usually as heat. The reverse process, breaking a big drop into smaller ones, needs energy to be supplied.

Surface Tension and Surface Energy:

Answer: (B) 2n

Explanation:
The number of molecules is proportional to the number of moles. The molar mass of water is 18 g/mol, so 9 g of water is 9/18 = 0.5 mole, which contains n molecules. The molar mass of CO₂ is 12 + 32 = 44 g/mol, so 44 g of CO₂ is 1 mole. That is twice the number of moles in the water sample, so the number of molecules is 2n.

Mole Concept:

Answer: (D) All of the above

Explanation: Water is a polar solvent and dissolves substances that can form hydrogen bonds with it or are polar. Glucose has several –OH groups, urea has –NH₂ and C=O groups, and acetone has a C=O group, so each can hydrogen bond with water. All three therefore dissolve in water, acetone being completely miscible.

Solubility in Water:

Answer: (D) All of the above

Explanation: In this reaction magnesium combines with oxygen to form a single product, MgO, so it is a combination reaction. Magnesium gains oxygen, so it is oxidised, which makes it an oxidation reaction. Energy is given out as heat and light, so it is also exothermic. All three descriptions apply to the same reaction.

Reaction Types:

Answer: (B) Magnesium

Explanation: Newlands’ law of octaves arranged the elements in order of increasing atomic mass, and every eighth element resembled the first, like the eighth note in a musical octave. Beryllium (Be) is the second element of the second group of the octave table, and its eighth element from it is magnesium (Mg). Both are alkaline earth metals with two valence electrons and similar properties, so beryllium resembles magnesium.

Newlands’ Octaves:

Answer: (C) sodium hydrogen carbonate and tartaric acid

Explanation: Baking powder is a mixture of sodium hydrogen carbonate (sodium bicarbonate, NaHCO₃) and a mild edible acid such as tartaric acid. When it is heated or moistened, the acid reacts with the bicarbonate and releases carbon dioxide, which makes bread and cakes rise and become soft and spongy. The acid also neutralises the bitter sodium carbonate that would otherwise form when NaHCO₃ is heated alone.

Baking Soda and Related Compounds:

Answer: (B) Yellow precipitation of lead iodide is formed.

Explanation: When potassium iodide solution is added to lead nitrate solution, a double displacement reaction takes place: Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃. Lead iodide is insoluble in water, so it separates out as a bright yellow precipitate. Potassium nitrate stays dissolved. On heating, the precipitate dissolves, and on cooling it reappears as shiny golden-yellow crystals, known as the “golden rain” effect.

Precipitation Reactions:

Answer: (D) Chlorofluorocarbon

Explanation: Refrigerators use a refrigerant, a volatile fluid that absorbs heat when it evaporates inside the cooling coils and releases it when it is compressed and condensed outside. Chlorofluorocarbons (CFCs) such as Freon-12 (CCl₂F₂) were widely used because they are non-toxic, non-flammable and easily liquefied. They have since been phased out because they deplete the ozone layer, and are being replaced by HFCs and hydrocarbons such as isobutane.

Common Refrigerants:

Answer: (A) F⁻

Explanation: Isoelectronic species have the same number of electrons. The O²⁻ ion has 8 + 2 = 10 electrons. F⁻ has 9 + 1 = 10 electrons, so it is isoelectronic with O²⁻. Cl⁻ has 18 electrons, Li⁺ has 2 and K⁺ has 18, so none of these match. O²⁻ and F⁻ both have the neon configuration 1s² 2s² 2p⁶.

Isoelectronic Species:

Answer: (A) Radioactivity

Explanation: The curie (Ci) is the older unit for the activity of a radioactive source, that is, the number of nuclei disintegrating per second. One curie equals 3.7 × 10¹⁰ disintegrations per second, which is roughly the activity of 1 g of radium-226. The SI unit of radioactivity is the becquerel (Bq), equal to one disintegration per second.

Radiation Units:

Answer: (C) Copper and Tin

Explanation: Bronze is an alloy made mainly of copper (about 88 to 90%) and tin (about 10 to 12%). Adding tin makes the metal harder, stronger and more resistant to corrosion than pure copper. Because of these properties bronze is used for statues, medals, bells, coins and ship fittings.

Common Alloys:

Answer: (B) 32°F

Explanation: Pure water freezes at 0 °C under normal atmospheric pressure. Converting to the Fahrenheit scale with F = (9/5) × C + 32 gives F = (9/5) × 0 + 32 = 32 °F. So the freezing point of pure water is 32 °F, while its boiling point is 212 °F.

Temperature Scales:

Answer: (D) Butane (C₄H₁₀)

Explanation: LPG (liquefied petroleum gas) is a mixture of hydrocarbons that are gases at room temperature but easily liquefied under moderate pressure. Its main constituents are butane and propane, with butane forming the larger share in the usual domestic cylinder mix (roughly 60 to 70% butane and 30 to 40% propane in India). A small amount of ethyl mercaptan, a foul-smelling compound, is added so that leaks can be detected by smell.

LPG and Related Fuels:

Answer: (B) C¹⁴

Explanation: Radiocarbon dating uses carbon-14, a radioactive isotope of carbon with a half-life of about 5,730 years. Living organisms keep taking in carbon, so the C-14 to C-12 ratio in their bodies matches that of the atmosphere. After death no fresh carbon is taken in, and C-14 decays steadily. Measuring the remaining C-14 gives the age of organic remains such as wood, bone, charcoal and cloth, up to about 50,000 years old.

Radioisotopes Used in Dating and Other Applications:

Answer: (D) Solid CO₂

Explanation: Dry ice is the solid form of carbon dioxide. It is called “dry” because it does not melt into a liquid at normal atmospheric pressure. Instead it sublimes directly from solid to gas at −78.5 °C, leaving no wet residue. Because it is much colder than ordinary ice, it is used as a refrigerant for storing ice cream, vaccines and other perishable goods.

Dry Ice and Related Substances:

Answer: (D) Sn and Pb

Explanation: Solder is an alloy of tin (Sn) and lead (Pb), commonly about 60% tin and 40% lead. It has a much lower melting point (around 183 °C for the eutectic mix) than either parent metal, so it melts easily and, on cooling, forms a firm, conducting joint between metal parts. It is widely used in electrical and plumbing work.

Solder and Related Alloys:

Answer: (A) Sulphur

Explanation: When an onion is chopped, its cells break and an enzyme (alliinase) acts on sulphur-containing compounds, forming a volatile sulphur compound called syn-propanethial-S-oxide. This gas reaches the eyes and dissolves in the moisture of the tear film, forming mild sulphur-containing acids that irritate the nerve endings of the cornea. The eyes then produce tears to wash the irritant away.

Sulphur Compounds in Food and Everyday Life:

Answer: (D) allotropes

Explanation: Allotropes are different physical forms of the same element in the same physical state. Graphite, diamond and charcoal (amorphous carbon) are all made only of carbon atoms, but the atoms are arranged differently. In diamond each carbon is bonded to four others in a rigid three-dimensional network, while in graphite the atoms lie in flat hexagonal layers. This difference in structure gives them very different properties.

Allotropes of Carbon and Other Elements:

Answer: (B) oxidizing

Explanation: Potassium permanganate (KMnO₄) is a strong oxidising agent. When added to water, it oxidises and destroys bacteria, other microorganisms and organic impurities, which purifies the water. It is also used to disinfect wells and drinking water supplies. The purple colour of the solution fades as Mn(VII) is reduced, which indicates that oxidation has taken place.

Potassium Permanganate:

Answer: (C) Henri Becquerel

Explanation: Natural radioactivity was discovered in 1896 by the French physicist Henri Becquerel. He found that uranium salts emitted penetrating rays that darkened a photographic plate even without exposure to light. Marie Curie and Pierre Curie later studied the phenomenon in detail, coined the term “radioactivity” and discovered polonium and radium. Becquerel shared the 1903 Nobel Prize in Physics with them.

Discoverers in Radioactivity and Nuclear Science:

Answer: (D) NaNO₃

Explanation: A fertilizer is a substance added to soil to supply essential plant nutrients such as nitrogen, phosphorus and potassium. Sodium nitrate (NaNO₃), also called Chile saltpetre, supplies nitrogen in the nitrate form, which plants absorb directly. N₂ and O₂ are atmospheric gases that plants cannot use in this form, and P₄ is white phosphorus, a poisonous, reactive element, so none of these is a fertilizer.

Common Fertilizers:

Answer: (B) nuclear fusion

Explanation: A hydrogen bomb works on nuclear fusion, in which light nuclei of hydrogen isotopes (deuterium and tritium) combine to form helium with a huge release of energy. Fusion needs temperatures of millions of degrees, so an atomic (fission) bomb is used as a trigger to supply the heat. The same process powers the Sun and other stars.

Fission vs Fusion:

Answer: (A) nitrogen

Explanation: Electric bulbs are filled with an inert or unreactive gas at low pressure, mainly nitrogen mixed with a little argon. The hot tungsten filament would burn out quickly in oxygen, so reactive gases are avoided. An inert atmosphere also reduces evaporation of the filament, which makes the bulb last longer. Among the given options, nitrogen is the correct choice.

Gases Used in Bulbs and Other Devices:

Answer: (B) propane and butane

Explanation: LPG (liquefied petroleum gas) is a mixture of hydrocarbons, mainly propane (C₃H₈) and butane (C₄H₁₀), which are gases at room temperature but liquefy easily under moderate pressure. This makes it convenient to store and transport in cylinders. A small amount of ethyl mercaptan, a foul-smelling compound, is added so that leaks can be detected by smell.

LPG and Related Fuels:

Answer: (B) Sulphur

Explanation: Hair setting (permanent waving or straightening) works by breaking and reforming the disulphide (–S–S–) bonds between keratin protein chains in hair. A reducing agent first breaks these sulphur bridges so the hair can be reshaped, and an oxidising agent then re-forms them in the new position. Sulphur-containing compounds are therefore the key to the process. The same disulphide cross-links give hair its natural strength and curl.

Sulphur in Biology and Everyday Use:

Answer: (A) Urea

Explanation: The nitrogen content of a fertilizer is found from the ratio of the mass of nitrogen to the molar mass of the compound. Urea, CO(NH₂)₂, has a molar mass of 60 and contains 28 g of nitrogen, so it is about 46.7% nitrogen. Ammonium nitrate (NH₄NO₃, molar mass 80) has 28 g of nitrogen, which is 35%. Potassium nitrate (KNO₃, molar mass 101) has about 13.9%, and ammonium phosphate has still less. Urea therefore has the highest nitrogen content.

Nitrogen Content of Fertilizers:

Answer: (A) Lemon juice

Explanation: The pH scale measures acidity: values below 7 are acidic, 7 is neutral and values above 7 are basic. Lemon juice contains citric acid and has a pH of about 2 to 2.5, so it is acidic. Lime water is calcium hydroxide, a base with pH of about 12. Human blood is slightly basic at about 7.4, and antacids are bases used to neutralise excess stomach acid.

pH of Common Substances:

Answer: (C) basic

Explanation: Toothpaste is mildly basic, with a pH of about 8 to 9. Tooth decay begins when bacteria in the mouth break down leftover sugar and produce acids, which attack the calcium phosphate (enamel) of the teeth. A basic toothpaste neutralises these acids and so protects the enamel from decay. Common ingredients such as calcium carbonate and sodium bicarbonate also contribute to the basic nature.

Oral pH and Dental Care:

Answer: (B) Limestone

Explanation: Cement is made mainly from limestone (calcium carbonate), which supplies the lime (CaO), together with clay that supplies silica, alumina and iron oxide. The raw materials are ground, mixed and heated in a rotary kiln at about 1400 to 1500 °C to form clinker. A small amount of gypsum (about 2 to 3%) is then added to the ground clinker to slow down the setting. Limestone is therefore the main ingredient.

Cement:

Answer: (A) Copper & Zinc

Explanation: Brass is an alloy of copper (about 60 to 70%) and zinc (about 30 to 40%). Zinc makes the metal harder and stronger than pure copper while keeping it malleable and easy to cast. Brass has a golden-yellow colour and resists corrosion, so it is used for utensils, musical instruments, taps, locks and decorative items.

Brass and Related Alloys:

Answer: (A) increase

Explanation: Adding a non-volatile solute such as salt to pure water lowers its vapour pressure. The solution must therefore be heated to a higher temperature before its vapour pressure equals the atmospheric pressure, so the boiling point rises. This is called elevation of boiling point, and it is a colligative property because it depends on the number of solute particles and not on their nature. NaCl dissociates into two ions, so its effect is about twice that of a non-dissociating solute at the same concentration.

Colligative Properties:

Answer: (D) oxidation

Explanation: Chlorine bleaches only in the presence of moisture. It reacts with water to form hypochlorous acid: Cl₂ + H₂O → HCl + HOCl. HOCl then decomposes to give nascent oxygen: HOCl → HCl + [O]. This nascent oxygen oxidises coloured substances to colourless products, so the bleaching is permanent and works by oxidation. Dry chlorine does not bleach.

Bleaching Agents:

Answer: (B) Dilute acetic acid

Explanation: Vinegar is a dilute solution of acetic acid (ethanoic acid, CH₃COOH) in water, usually about 5 to 8% acetic acid by volume. It is made by the fermentation of sugar or alcohol-containing liquids such as fruit juice, sugarcane juice or wine, in which bacteria of the genus Acetobacter oxidise ethanol to acetic acid. Its sour taste and pH of about 2.5 to 3 come from this acid.

Vinegar and Common Chemical Names:

Answer: (A) Polyvinyl Chloride

Explanation: PVC stands for polyvinyl chloride, a thermoplastic polymer made by the addition polymerisation of the monomer vinyl chloride (CH₂=CHCl). It is strong, durable, water-resistant and a good electrical insulator. Because of these properties it is widely used for pipes, cable insulation, raincoats, flooring and bottles.

Common Polymers:

Answer: (B) Gypsum

Explanation: Plaster of Paris is made by heating gypsum (CaSO₄·2H₂O) to about 120 to 130 °C (373 K) in a kiln. At this temperature gypsum loses three-quarters of its water of crystallisation and forms calcium sulphate hemihydrate: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O. When mixed with water it sets into a hard solid mass of gypsum again, which is why it is used for casts and moulds. Heating above 200 °C gives anhydrous CaSO₄ (dead burnt plaster), which does not set.

Plaster of Paris and Gypsum:

Answer: (A) Lindlar catalyst

Explanation: The conversion of an alkyne (CH₃–C≡CH) to an alkene (CH₃–CH=CH₂) needs partial hydrogenation, which must stop at the alkene stage. Lindlar catalyst is palladium deposited on calcium carbonate and poisoned with lead acetate and quinoline. The poisoning lowers the activity of palladium, so hydrogen adds only once across the triple bond and the alkene is not reduced further to an alkane. With alkynes such as 2-butyne, it gives the cis (Z) alkene.

Reduction of Alkynes:

Answer: (A) Mg / dry ether / N₂ atmosphere

Explanation: Alkyl bromides react with magnesium metal to form Grignard reagents (RMgBr). The Grignard reagent is a very strong base and nucleophile, so even traces of water, alcohol or any acidic hydrogen destroy it by converting it to the alkane (RMgBr + H₂O → RH + Mg(OH)Br). The reaction is therefore carried out in dry ether, which also stabilises the reagent by coordinating to magnesium. An inert nitrogen atmosphere excludes moisture and oxygen, which would otherwise oxidise the reagent.

Grignard Reagent:

Answer: (A) MeO–CH₂–Cl

Explanation: The rate of an SN1 reaction depends on how stable the carbocation formed in the slow step is. Loss of chloride from MeO–CH₂–Cl gives the CH₃–O⁺=CH₂ cation (an oxocarbenium ion). The lone pair on oxygen is donated into the empty p orbital of carbon by resonance, which stabilises the cation very strongly. This lowers the activation energy, so this halide ionises fastest and reacts most rapidly by the SN1 path.

Carbocation Stability and SN1 Rate:

Answer: (A) Cs (Cesium)

Explanation: Electropositivity is the tendency of an atom to lose electrons and form a positive ion. It increases down a group because the outermost electron lies farther from the nucleus and is shielded by more inner shells, so the ionisation energy falls. Among the given elements, Cs is the lowest in the alkali metal group, has the lowest ionisation energy and loses its valence electron most easily. It is therefore the most electropositive of the options. Francium is theoretically more electropositive, but it is extremely rare and radioactive.

Electropositivity and Related Trends:

Answer: (D) see-saw

Explanation: Sulphur in SF₄ has 6 valence electrons, 4 of which form bonds with fluorine, leaving one lone pair. The total of 5 electron pairs gives a trigonal bipyramidal arrangement (sp³d hybridisation) as per VSEPR theory. The lone pair occupies an equatorial position, where it has the least repulsion, so the shape of the molecule is see-saw (distorted tetrahedron).

Shapes of Molecules with Lone Pairs:

Answer: (A) Delta G = -ve

Explanation: The Gibbs free energy change is given by ΔG = ΔH − TΔS. A process is spontaneous at constant temperature and pressure only when ΔG is negative. A negative ΔH or a positive ΔS favours spontaneity, but neither alone guarantees it, because the result depends on their combination and on the temperature. ΔG is the single criterion that accounts for both factors, so ΔG < 0 is the definite condition for spontaneity.

Spontaneity and Gibbs Energy:

Answer: (D) Lead

Explanation: Lead is a dense metal with a high atomic number, so it absorbs gamma rays and X-rays very effectively. Its high density means many electrons and nuclei lie in the path of the radiation, which reduces its intensity to a safe level. Lead is also cheap, soft and easy to shape into sheets, aprons and containers. For these reasons it is the standard shielding material against nuclear radiation.

Radiation Shielding:

Answer: (B) Nitrous oxide

Explanation: Nitrous oxide (N₂O) is popularly called laughing gas because inhaling a small amount produces mild euphoria and sometimes uncontrollable laughter. It was discovered by Joseph Priestley in 1772, and its anaesthetic and mood-altering effects were noted by Humphry Davy. It is a colourless, sweet-smelling gas that is used as a mild anaesthetic in dentistry and as a propellant in whipped cream dispensers.

Oxides of Nitrogen:

Answer: (B) Methyl isocyanate

Explanation: The Bhopal gas tragedy occurred on the night of 2 to 3 December 1984 at the Union Carbide pesticide plant in Bhopal, Madhya Pradesh. Water entered a storage tank holding methyl isocyanate (MIC, CH₃NCO), triggering a violent exothermic reaction that released toxic MIC vapour into the surrounding area. MIC is highly toxic and irritating to the eyes and lungs, and exposure caused thousands of deaths and long-term illness. It was used in the manufacture of the pesticide carbaryl (Sevin).

Major Industrial and Chemical Disasters:

Answer: (D) Magnesium

Explanation: Chlorophyll, the green pigment of plants, has a porphyrin ring with a magnesium ion (Mg²⁺) at its centre, held by four nitrogen atoms. This magnesium atom is essential for absorbing light energy during photosynthesis. Without magnesium, plants cannot make chlorophyll and their leaves turn yellow (chlorosis).

Metals in Biological Molecules:

Answer: (C) Hydrogen

Explanation: Vanaspati ghee is made by hydrogenation of vegetable oils. Liquid vegetable oils contain unsaturated fatty acids with C=C double bonds. Hydrogen gas is passed through the oil at about 150 to 200 °C in the presence of finely divided nickel as a catalyst, which adds hydrogen across the double bonds and converts them into single bonds. The oil becomes saturated and changes from liquid to a semi-solid fat that resembles ghee, with a longer shelf life.

Hydrogenation of Oils:

Answer: (C) Chlorofluoro carbon

Explanation: Chlorofluorocarbons (CFCs) are stable compounds that rise slowly into the stratosphere. There, ultraviolet radiation breaks the C–Cl bond and releases chlorine free radicals. A chlorine radical reacts with ozone (Cl• + O₃ → ClO• + O₂), and the ClO• then reacts with an oxygen atom to regenerate Cl• (ClO• + O → Cl• + O₂). Because the chlorine is regenerated, a single chlorine atom can destroy thousands of ozone molecules. This thins the ozone layer and creates the Antarctic ozone hole.

Ozone Depletion:

Answer: (C) Poly Vinyl Chloride

Explanation: PVC stands for polyvinyl chloride, a thermoplastic made by the addition polymerisation of vinyl chloride (CH₂=CHCl). It is strong, durable, water-resistant and a good electrical insulator. It is widely used for pipes, cable insulation, raincoats, flooring and bottles.

PVC and Related Polymers:

Answer: (B) Na⁺

Explanation: Isoelectronic species have the same number of electrons. Mg has 12 electrons, so Mg²⁺ has 12 − 2 = 10 electrons, the neon configuration. Na⁺ has 11 − 1 = 10 electrons, so it is isoelectronic with Mg²⁺. Ca²⁺ has 18 electrons, Zn²⁺ has 28 and Cu⁺ has 28, so none of these match.

Isoelectronic Species:

Answer: (B) sp² sp

Explanation: The question does not show the molecule, so it is read as the standard version of this question: CH₂=C=CH₂ (allene, propadiene), in which C₁ and C₃ are the terminal carbons and C₂ is the central carbon. Here C₂ forms two double bonds, so it has 2 sigma bonds and no lone pair, giving sp hybridisation. C₃ has one double bond and two C–H bonds, so it has 3 sigma bonds and is sp² hybridised. The pair C₂ and C₃ is therefore sp and sp², which is listed in option (B) as “sp² sp” (C₃ then C₂ order reversed). Note that the given text is incomplete, so check the original paper for the structure.

Hybridisation:

Answer: (C) HF

Explanation: Hydrofluoric acid reacts with the silica (SiO₂) and silicates in glass to form silicon tetrafluoride, a gas, and fluorosilicic acid: SiO₂ + 4HF → SiF₄ + 2H₂O. This dissolves the glass, so HF cannot be stored in glass vessels. It is kept in plastic (polythene or Teflon) or wax-coated bottles. This property is used for etching glass.

Hydrogen Halides:

Answer: (A) 2

Explanation: Molarity is moles of solute per litre of solution. For 100 mL of a 0.5 M solution, moles of NaOH needed = 0.5 × 0.1 = 0.05 mole. The molar mass of NaOH is 23 + 16 + 1 = 40 g/mol, so the mass required is 0.05 × 40 = 2 g.

Solution Concentration:

Answer: (A) Acetyl salicylic acid

Explanation: Aspirin is the common name of acetylsalicylic acid (2-acetoxybenzoic acid, C₉H₈O₄). It is prepared by acetylating the –OH group of salicylic acid with acetic anhydride. It acts as a pain reliever (analgesic), reduces fever (antipyretic) and lowers inflammation. It also prevents blood clotting, so low doses are used to reduce the risk of heart attacks.

Common Drugs and Chemical Names: