Main Group Elements Questions and Answers

Answer: C

Limestone is chemically calcium carbonate, \(CaCO_{3}\).

Limestone is a sedimentary rock made largely of calcium carbonate, often from the shells and skeletons of marine organisms. Chalk and marble are other forms of the same compound. Heating limestone gives quicklime (CaO) and carbon dioxide, which is the basis of the lime and cement industries; Madhya Pradesh has large limestone deposits around Katni, Satna and Rewa.

Sodium nitrate is Chile saltpetre, sodium sulphate is Glauber's salt (in its hydrated form), and ammonium chloride is sal ammoniac; none is limestone.

Answer: A

Bromine is a non-metal.

Bromine (Br) is a halogen in group 17 of the periodic table, along with fluorine, chlorine and iodine, all of which are non-metals. It is the only non-metallic element that is a liquid at room temperature: a dark red, volatile, corrosive liquid. The only metal that is liquid at room temperature is mercury, which is the source of the confusion.

Bromine is therefore neither a metal nor a solid metal, and as a pure element it is not an alloy, which is a mixture of metals.

Answer: A

The product is \(2\text{NaHCO}_{3}\).

Carbon dioxide passed through a solution of sodium carbonate converts it to sodium hydrogen carbonate (bicarbonate):

\[ \begin{aligned} &\text{Na}_{2}\text{CO}_{3} + \text{CO}_{2} + \text{H}_{2}\text{O} \\ &\rightarrow 2\text{NaHCO}_{3} \end{aligned} \]

The equation balances: 2 Na, 2 C, 2 H and 6 O on each side. \(\text{NaCO}_{3}\) is not a real compound, and adding an acidic gas such as carbon dioxide cannot turn a carbonate into the stronger base sodium hydroxide.

Answer: B

Water gas is CO + H₂.

It is made by passing steam over red-hot coke:

\[ \mathrm{C + H_{2}O \rightarrow CO + H_{2}} \]

The mixture of carbon monoxide and hydrogen (also called synthesis gas) is a fuel and a starting material for making methanol and hydrogen. CO + H₂O are the reactants of the water-gas shift reaction, whose products are CO₂ + H₂; steam alone is the reactant, not the gas.

Answer: C

Slaked lime is Ca(OH)₂, calcium hydroxide.

Quicklime (CaO) reacts vigorously with water, slaking, to give calcium hydroxide:

\[ \mathrm{CaO + H_{2}O \rightarrow Ca(OH)_{2}} \]

CaO is quicklime, CaCO₃ is limestone (chalk, marble), and Ca(HCO₃)₂ is calcium bicarbonate, which causes temporary hardness of water.

Answer: D

The answer is Trigonal bipyramid, the arrangement of the electron pairs around bromine in BrF₃.

Bromine has seven valence electrons; three bond to fluorines and the remaining four form two lone pairs, giving five electron pairs in all. Five pairs are arranged trigonal bipyramidally. The two lone pairs take equatorial positions, where they suffer the least repulsion, so the three fluorines (two axial, one equatorial) give a T-shaped molecule with F-Br-F angles near 86°.

No option offers T-shaped, so the question is answered by the electron-pair geometry. Square planar and octahedral need six pairs (as in BrF₅ or ICl₄^-), and tetrahedral needs four.

Answer: C

The F-N-F bond angle in NF₃ is about 102° 30′.

NF₃ is pyramidal, with three bond pairs and one lone pair on nitrogen. The lone pair compresses the angles below the tetrahedral 109° 28′. In NF₃ the compression is larger than in NH₃ because fluorine is more electronegative than nitrogen and pulls the bonding electrons away from nitrogen, so the bond pairs repel each other less and the lone pair pushes them closer together. The measured angle is about 102°.

107° 48′ is close to the angle in NH₃ (about 107°), where the bond pairs sit nearer nitrogen. 104° 27′ is the angle in water, and 109° 28′ the ideal tetrahedral angle.

Answer: C

The element present in dolomite but absent in limestone is Mg.

Limestone is mainly calcium carbonate, CaCO₃. Dolomite is calcium magnesium carbonate, CaMg(CO₃)₂, formed when magnesium replaces part of the calcium in a carbonate rock.

Carbon, calcium and oxygen occur in both minerals, since both are carbonates of calcium, so only magnesium distinguishes dolomite.

Answer: B

The hardest form of carbon is diamond.

In diamond each carbon atom is bonded covalently to four others in a rigid three-dimensional tetrahedral network, so there are no weak planes along which it can be scratched or cleaved easily. It stands at 10, the top of Mohs' scale.

Graphite has carbon in flat sheets held together only by weak forces, so the layers slide and it is soft enough to use as a lubricant and in pencils. Charcoal (printed 'Charcol') and coke are impure, porous, largely amorphous forms of carbon and are soft and brittle.

Answer: C

The order is GeCl₄ > SiCl₄ > CCl₄.

In a group, the covalent radius of the central atom grows as a new shell is added: C (about 77 pm), Si (about 117 pm), Ge (about 122 pm). The E-Cl bond lengthens with it, from about 177 pm in CCl₄ to about 202 pm in SiCl₄ and about 210 pm in GeCl₄.

Options A and B put SiCl₄ first, ignoring that germanium is larger than silicon, and option D places CCl₄, with the smallest central atom, above SiCl₄.