Main Group Elements Questions and Answers

Answer: A

The formula is \((NH_{4})_{3}PO_{4}.12MoO_{3}\).

Ammonium phosphomolybdate is the canary-yellow precipitate formed when a phosphate is warmed with nitric acid and ammonium molybdate; it is the confirmatory test for phosphorus. Its formula contains one phosphate, three ammonium ions and twelve \(MoO_{3}\) units.

The other formulas change the number of ammonium ions (2), the phosphorus species (\(PO_{3}\)), the molybdenum count (2), or omit phosphorus entirely.

Answer: D

The metals used are Zn and Mg.

In the laboratory, dihydrogen is usually prepared by the action of dilute acid on granulated zinc, \(\mathrm{Zn + 2HCl \rightarrow ZnCl_2 + H_2}\); magnesium reacts the same way. Both react at a steady, controllable rate.

Sodium, calcium and barium react with water and acids so violently that the reaction is hazardous and hard to control. Iron reacts only slowly with dilute acid, and pairing it with sodium in option 3 brings back the hazard.

Answer: A

The oxoacid is hypochlorous acid.

An oxoacid contains oxygen, with at least one hydrogen atom bonded to oxygen that can be released as a proton. Hypochlorous acid, HOCl, has its acidic hydrogen on an oxygen atom bonded to chlorine, so it is an oxoacid.

Hydrochloric acid, HCl, contains no oxygen; it is a hydracid (binary acid). So the options including it are wrong, and 'None of these' is ruled out by HOCl.

Answer: B

The answer the key accepts is \(\mathrm{2CaSO_{4} \cdot \tfrac{1}{2}H_{2}O(s)}\).

Plaster of Paris is calcium sulphate hemihydrate, \(\mathrm{CaSO_{4} \cdot \tfrac{1}{2}H_{2}O}\), that is one water molecule for every two units of calcium sulphate, also written \(\mathrm{(CaSO_{4})_{2} \cdot H_{2}O}\). It is made by heating gypsum, \(\mathrm{CaSO_{4} \cdot 2H_{2}O}\), to about 373 K.

As printed, the keyed option combines the doubled sulphate with the half molecule of water, but it is the only option describing the hemihydrate. The options with '2HO' are not valid formulas, and the one with a third of a water molecule is not plaster of Paris.

Answer: C

The bleaching action of chlorine is due to oxidation.

In the presence of moisture, chlorine forms hypochlorous acid, which releases nascent oxygen: Cl₂ + H₂O → HCl + HOCl, and HOCl → HCl + [O]. The nascent oxygen oxidises coloured substances to colourless ones, so the bleaching is permanent.

Reduction is how sulphur dioxide bleaches (temporarily). 'Chlorination' and hydrogenation are not the mechanism of chlorine's bleaching action.

Answer: B

Dead burnt plaster is anhydrous calcium sulphate.

Gypsum is CaSO₄·2H₂O. Heated to about 393 K it becomes plaster of Paris, CaSO₄·½H₂O. Heated above about 473 K it loses all its water and becomes anhydrous CaSO₄, called dead burnt plaster because it no longer sets when mixed with water.

Hydrated calcium sulphate is gypsum or plaster of Paris, not the dead burnt form, and the magnesium sulphates are different compounds altogether.

Answer: D

The compound formed is NaHCO₃.

This is the Solvay (ammonia-soda) process. CO₂ bubbled through brine saturated with ammonia gives ammonium bicarbonate, which reacts with sodium chloride: NaCl + NH₃ + CO₂ + H₂O → NaHCO₃ + NH₄Cl. Sodium bicarbonate, being sparingly soluble, precipitates.

Na₂CO₃ is obtained only afterwards, by heating the bicarbonate. CaCO₃ is the limestone used to make CO₂, and ammonium carbonate is not the product that separates.

Answer: C

Plaster of Paris is \(\mathrm{CaSO_4 \cdot \tfrac{1}{2}H_2O}\), calcium sulphate hemihydrate.

It is made by heating gypsum to about 373 K (100 °C), which drives off three-quarters of its water of crystallisation:

\[ \begin{aligned} \mathrm{CaSO_4 \cdot 2H_2O} &\rightarrow \mathrm{CaSO_4 \cdot \tfrac{1}{2}H_2O} \\ &\quad + \mathrm{1\tfrac{1}{2}H_2O} \end{aligned} \]

When mixed with water it takes the water back and sets hard as gypsum again, which is why it is used for casts and moulds.

\(\mathrm{CaSO_4 \cdot 2H_2O}\) is gypsum itself, the anhydrous \(\mathrm{CaSO_4}\) is dead-burnt plaster formed by overheating, and the monohydrate is not the composition of plaster of Paris.

Answer: D

Hydrogen is used in all of the above.

It is the feedstock for ammonia in the Haber process, \(\mathrm{N_2 + 3H_2 \rightleftharpoons 2NH_3}\); it burns in chlorine to give hydrogen chloride, which dissolves in water as hydrochloric acid; and it combines with carbon monoxide over a catalyst to give methanol, \(\mathrm{CO + 2H_2 \rightarrow CH_3OH}\).

Since hydrogen is used in all three preparations, any single option is incomplete.

Answer: C

The formula given for bleaching powder is \(Ca(OCl)_{2}\).

Bleaching powder is made by passing chlorine over dry slaked lime, and is often written \(CaOCl_{2}\) (calcium oxychloride). Its active bleaching and disinfecting component is calcium hypochlorite, \(Ca(OCl)_{2}\), which releases chlorine, and this is the formula the key accepts.

\(MgCO_{3}\) is magnesium carbonate, \(BaCO_{3}\) barium carbonate and \(CaSO_{4}\) calcium sulphate (the basis of gypsum and plaster of Paris); none is a bleaching agent.