Chemistry: previous-year questions

Answer: (A) Proton + Neutron

Nearly all the mass of an atom lies in its nucleus, which contains protons and neutrons; the mass number is $A = Z + N$ (protons plus neutrons). An electron is about $\frac{1}{1836}$ as heavy as a proton, so its contribution is ignored, which rules out options B, C and D. Related facts: the atomic number $Z$ (number of protons) fixes the identity of an element; isotopes have the same $Z$ but different $A$; isobars have the same $A$ but different $Z$; isotones have the same number of neutrons. Discoveries: electron, J.J. Thomson (1897); proton, Rutherford; neutron, James Chadwick (1932); nucleus, Rutherford's gold-foil experiment (1911).

Answer: (C) Hydrogen bonding

A hydrogen bond forms when hydrogen is attached to a small, highly electronegative atom: fluorine, oxygen or nitrogen. Water and HF therefore have hydrogen bonding between their molecules, but $\mathrm{H_2S}$ does not, because sulphur is larger and much less electronegative. This explains why water boils at $100^\circ$C while $\mathrm{H_2S}$ is a gas, why HF has a far higher boiling point than HCl, and why ice is lighter than water (an open cage-like structure). All three molecules have covalent bonds within the molecule, so A does not distinguish them, and none is ionic or metallic. Hydrogen bonds also hold together the two strands of DNA and give proteins their shape.

Answer: (C) 44

Molecular weight of $\mathrm{MCl_4} = M + 4 \times 35.5 = 154$, so the atomic weight of M is $154 - 142 = 12$, i.e. carbon, and the chloride is carbon tetrachloride. The formula $\mathrm{MCl_4}$ shows that M has valency 4, so its oxide is $\mathrm{MO_2}$, with molecular weight $12 + 2 \times 16 = 44$ (carbon dioxide). The other options are the weights of familiar oxides and act as traps: 64 is $\mathrm{SO_2}$, 72 is FeO and 40 is MgO. Two-step method for such questions: first find the atomic weight from the given compound, then use the valency to write the formula of the compound asked.

Answer: (B) $2n$

Equal numbers of moles contain equal numbers of molecules. 14 g of $\mathrm{N_2}$ (molar mass 28) is $\frac{14}{28} = 0.5$ mole and contains $n$ molecules, so 1 mole contains $2n$ molecules. 46 g of $\mathrm{NO_2}$ (molar mass $14 + 32 = 46$) is exactly 1 mole and therefore contains $2n$ molecules. Option A assumes equal masses mean equal molecules, the usual mistake. One mole of any substance contains Avogadro's number of particles, $6.022 \times 10^{23}$, and one mole of any gas occupies 22.4 litres at STP.

Answer: (D) All of the above

All three halogens have radioactive isotopes although none belongs to the natural radioactive decay series (the uranium, thorium and actinium series), so PSC's answer is 'All of the above'. Fluorine-18 is the positron emitter used in PET scans; iodine-131 is used to diagnose and treat thyroid disorders and iodine-129 is a long-lived fission product; chlorine-36 (half-life about 3 lakh years) is used to date old groundwater. In fact almost every element has at least one radioisotope, for example carbon-14 (radiocarbon dating), potassium-40, phosphorus-32, cobalt-60 (cancer radiotherapy) and tritium, which is why choosing iodine alone would be incomplete. The halogens are fluorine, chlorine, bromine, iodine and astatine, the last having no stable isotope at all.

Answer: (C) free delocalized electrons.

In graphite each carbon uses three electrons for bonds within its layer, and the fourth electron is delocalised across the layer, free to carry current. A tetrahedral arrangement describes diamond, which does not conduct, and localised bonds or electrostatic bonding do not supply mobile charge carriers.

Answer: (A) Starch

Starch is a natural polymer of glucose made by plants. Teflon, neoprene and nylon are all synthetic polymers made in factories, so B, C and D are wrong.

Answer: (D) 60

The best-known fullerene, buckminsterfullerene, is $C_{60}$, a cage of 60 carbon atoms arranged like a football. The other numbers do not describe this molecule.

Answer: (B) acetic acid

Vinegar is about 5–8% acetic (ethanoic) acid in water, made by bacterial oxidation of ethyl alcohol. Ethyl alcohol is the starting material, not vinegar itself; formic acid occurs in ant stings, and vanadium is a metal.

Answer: (C) Gallium

Gallium is a metal (it melts in the hand at about 30 °C). Boron, silicon and germanium have properties between metals and non-metals and are classed as metalloids.

Answer: (C) Both

Chlorine is the standard disinfectant for public water supplies, and potassium permanganate is widely used to disinfect well water in rural areas. Since both are used, (C) is correct; NO$_2$ is a pollutant, not a disinfectant.

Answer: (B) s < p < d < f

Within one shell of a many-electron atom, orbitals with higher azimuthal quantum number l penetrate less towards the nucleus, so energy rises in the order s < p < d < f within a shell. The other orders put higher-l orbitals below lower-l ones, which is wrong.

Answer: (A) Brine

Brine is a solution of common salt in water, so evaporating it leaves solid salt behind. Bromine, mercury and alcohol are pure liquids that simply vaporise completely.

Answer: (A) CF$_4$

CF$_4$ is tetrahedral, with all F–C–F angles 109.5°. PF$_5$ is trigonal bipyramidal (axial–equatorial angles of 90°), and SF$_6$ and SiF$_6^{2-}$ are octahedral with 90° angles.

Answer: (A) Black

Copper(II) sulphide precipitates as a black solid when H$_2$S is passed through a copper salt solution, a standard test in group II analysis. Blue is the colour of hydrated copper sulphate (CuSO$_4$), and yellow sulphides are those of cadmium or arsenic.

Answer: (B) Electronic

Ultraviolet and visible photons have enough energy to move electrons between molecular orbitals, so UV spectroscopy studies electronic transitions. Rotational transitions are studied with microwaves, vibrational ones with infrared, and nuclear spin transitions with radio waves in NMR.

Answer: (A) reduction of lead.

In PbO + H$_2$ → Pb + H$_2$O, hydrogen removes oxygen from lead oxide, so lead is reduced from +2 to metallic lead while hydrogen is oxidised. No chlorine is present to form PbCl$_2$, and PbO is the reactant, not the product.

Answer: (A) Stoichiometry

Stoichiometry, the calculation of quantities in compounds and reactions, rests on the law of definite proportions: a compound always contains its elements in a fixed ratio by mass. Electrometry, photometry and spectrometry are measurement techniques, not branches based on this law.

Answer: (A) 7.5 – 8.5

Sea water is mildly alkaline because of dissolved carbonates and bicarbonates, with pH usually between about 7.5 and 8.5 (surface average about 8.1). Values of 9.5 and above would be strongly alkaline.

Answer: (A) Anthracite

Anthracite has the highest carbon content (over 85–90%) and calorific value, and burns with little smoke, so it is the best quality coal. Bituminous coal comes next, then lignite, and peat is the lowest grade.