JEE Chemistry S Block Elements MCQs Set 18

Answer: (a) RbOH

Explanation:
1. Basic strength of alkali metal hydroxides increases down the group because the M-O bond becomes more ionic as the cation gets larger.
2. So RbOH is the strongest base here.

Teacher's Note:
1. Larger cation means weaker M-OH bond and easier release of OH^-.

Answer: (b) Mohr's salt

Explanation:
1. Mohr's salt, FeSO₄.(NH₄)₂SO₄.6H₂O, breaks into all its simple ions in solution, so it is a double salt.
2. Potassium ferricyanide and cuprammonium sulphate are complex salts.

Teacher's Note:
1. Double salts lose their identity in solution; complexes keep their complex ion.

Answer: (b) KNO₃

Explanation:
1. Nitre (saltpetre) is potassium nitrate, KNO₃.
2. Sodium nitrate is called Chile saltpetre.

Teacher's Note:
1. Do not confuse nitre with Chile saltpetre.

Answer: (d) Caustic soda

Explanation:
1. In the Nelson (diaphragm) cell, brine is electrolysed with a steel cathode separated by an asbestos diaphragm.
2. NaOH forms near the cathode and Cl₂ at the anode.

Teacher's Note:
1. Castner-Kellner uses a mercury cathode.

Answer: (a) NaHCO₃

Explanation:
1. HCO₃^- is the conjugate base of the weak acid H₂CO₃, so it hydrolyses to give a weakly basic solution.
2. NaHSO₄ is acidic, NaCl neutral, and NH₄HCO₃ is nearly neutral.

Teacher's Note:
1. Salt of strong base and weak acid is basic.

Answer: (a) Li

Explanation:
1. Lithium has the highest charge density, and the small Li⁺ stabilises the small N^3- ion in the lattice.
2. Li₃N forms by 6Li + N₂ gives 2Li₃N.

Teacher's Note:
1. This is one of the anomalous features of lithium.

Answer: (d) Rb

Explanation:
1. Densities (g/cm³): Li 0.53, Na 0.97, K 0.86, Rb 1.53.
2. Only Rb is denser than water (1.00).

Teacher's Note:
1. K is less dense than Na, the density irregularity.

Answer: (c) K⁺ and [HF₂]^-

Explanation:
1. The F^- ion forms a strong hydrogen bond with HF giving the bifluoride ion [F-H-F]^-.
2. So KHF₂ is K⁺ with [HF₂]^-.

Teacher's Note:
1. This is the strongest known hydrogen bond.

Answer: (b) Cs

Explanation:
1. Metallic character increases down the group as the ionisation energy decreases.
2. Cs has the lowest ionisation energy among these.

Teacher's Note:
1. Metallic character parallels ease of electron loss.

Answer: (d) Rb

Explanation:
1. Reactivity increases down the group.
2. Rb is the lowest in ionisation energy of the four, so it reacts most vigorously with water.

Teacher's Note:
1. Rb and Cs react explosively.

Answer: (b) Li and NH₃

Explanation:
1. A has the formula M₃N, so M is a monovalent metal.
2. Of the alkali metals only Li forms a nitride directly, giving Li₃N.
3. Li₃N + 3H₂O gives 3LiOH + NH₃; NH₃ forms the deep blue [Cu(NH₃)₄]²⁺ with CuSO₄.

Teacher's Note:
1. M₃N means M is +1; Al and Mg nitrides are AlN and Mg₃N₂.

Answer: (d) CaCO₃

Explanation:
1. CaCO₃ gives CaO + CO₂ on heating.
2. CaO + H₂O gives Ca(OH)₂ (Y), which with excess CO₂ gives soluble Ca(HCO₃)₂ (Z).
3. Boiling Z gives back CaCO₃ (X).

Teacher's Note:
1. Lime water turns milky then clear with CO₂.

Answer: (c) RbCl and BeCl₂

Explanation:
1. By Fajans' rules a larger cation and lower charge give more ionic character.
2. Rb⁺ is the largest cation, so RbCl is the most ionic; Be²⁺ is the smallest with high charge, so BeCl₂ is the most covalent.

Teacher's Note:
1. Polarising power is proportional to charge/size.

Answer: (d) Na₂SO₄.10H₂O

Explanation:
1. MgSO₄.7H₂O is Epsom salt, CuSO₄.5H₂O is blue vitriol and FeSO₄.7H₂O is green vitriol.
2. Glauber's salt is hydrated sodium sulphate.

Teacher's Note:
1. Learn the common names of hydrated sulphates.

Answer: (c) Sodium amide

Explanation:
1. 2Na + 2NH₃ gives 2NaNH₂ + H₂.
2. The product is sodamide (sodium amide).

Teacher's Note:
1. Sodium dissolved in liquid ammonia forms a blue solution.

Answer: (c) Alcohol

Explanation:
1. Benzene, kerosene and toluene are inert hydrocarbons.
2. Alcohols contain acidic O-H and react with sodium, giving sodium alkoxide and hydrogen.

Teacher's Note:
1. Always store in inert hydrocarbon liquids.

Answer: (c) HCOONa

Explanation:
1. CO + NaOH gives HCOONa (sodium formate).

Teacher's Note:
1. Formic acid is the anhydride-type partner of carbon monoxide.

Answer: (a) CO₂ and NH₃

Explanation:
1. CO₂ is regenerated by heating NaHCO₃ and limestone.
2. NH₃ is recovered from NH₄Cl by heating with Ca(OH)₂ (from CaO and water).
3. CaCl₂ is the waste by-product.

Teacher's Note:
1. The cheapness of the Solvay process comes from this recycling.

Answer: (b) They absorb moisture from air

Explanation:
1. NaOH is deliquescent: it takes up water vapour from the air and dissolves in it.
2. The liquid layer is a solution of NaOH in absorbed water.

Teacher's Note:
1. Deliquescence means absorbing moisture and dissolving.

Answer: (a) NaHSO₃

Explanation:
1. SO₂ is an acidic oxide, and with excess it forms bisulphite: Na₂CO₃ + 2SO₂ + H₂O gives 2NaHSO₃ + CO₂.
2. This reaction is used industrially to make NaHSO₃.

Teacher's Note:
1. With excess SO₂, the acid salt forms.

Answer: (b) 2CaSO₄.H₂O

Explanation:
1. At 393 K gypsum loses three-halves of its water.
2. The product is plaster of Paris, CaSO₄.1/2H₂O, written as 2CaSO₄.H₂O.

Teacher's Note:
1. Heating above 473 K gives anhydrous dead-burnt plaster.

Answer: (b) Electrolysis of fused beryllium chloride

Explanation:
1. BeCl₂ is covalent and a poor conductor, so NaCl is added to the molten BeCl₂.
2. Electrolysis then deposits Be at the cathode.

Teacher's Note:
1. Be is too reactive for carbon reduction.

Answer: (d) Heating the hydrate to red heat in an atmosphere of HCl gas

Explanation:
1. Simple heating of MgCl₂.6H₂O causes hydrolysis to Mg(OH)Cl and MgO.
2. Heating in a stream of dry HCl suppresses hydrolysis, leaving anhydrous MgCl₂.

Teacher's Note:
1. HCl shifts the hydrolysis equilibrium back.