Main Group Elements Questions and Answers

Q: The formula of Ammonium phosphomolybdate is

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.

Q: Which of the following metals are normally used in the preparation of dihydrogen in the laboratory?

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.

Q: Which of the following is/are examples of oxoacids ?

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.

Q: The molecular formula of Plaster of Paris is :

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.

Q: Chlorine is used as a bleaching agent. This bleaching property is due to

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.

Q: Which is the formula of 'Dead Burnt Plaster' ?

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.

Q: When carbon dioxide gas is bubbled through a brine solution, saturated with ammonia, which compound is formed ?

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.

Q: Hydrogen gas is used in

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.

Q: The chemical formula of bleaching powder is

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.

21.

The formula of Ammonium phosphomolybdate is

A.\((NH_{4})_{3}PO_{4}.12MoO_{3}\)
B.\((NH_{4})_{2}.12MoO_{4}\)
C.\((NH_{4})_{2}PO_{4}.12MoO_{3}\)
D.\((NH_{4})_{3}PO_{3}.12MoO_{3}\)
E.\((NH_{4})_{3}PO_{4}.2MoO_{3}\)

Exam Year(s)

KEAM - 2025,

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.

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22.

Which of the following metals are normally used in the preparation of dihydrogen in the laboratory?

A.Na and Ca
B.Ca and Ba
C.Fe and Na
D.Zn and Mg
E.Na and Ba

Exam Year(s)

KEAM - 2026,

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.

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23.

Which of the following is/are examples of oxoacids ?

A.Hypochlorous acid
B.Hydrochloric acid
C.Both (A) and (B)
D.None of these

Exam Year(s)

MPPSC Forest Service - 2022,

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.

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24.

The molecular formula of Plaster of Paris is :

A.\(\mathrm{2CaSO_{4} \cdot 2HO(s)}\)
B.\(\mathrm{2CaSO_{4} \cdot \tfrac{1}{2}H_{2}O(s)}\)
C.\(\mathrm{2CaSO_{4} \cdot 2HO(\mathit{l})}\)
D.\(\mathrm{3CaSO_{4} \cdot \tfrac{1}{3}H_{2}O(s)}\)

Exam Year(s)

MPPSC Forest Service - 2022,

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.

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25.

Chlorine is used as a bleaching agent. This bleaching property is due to

A.Reduction
B.Chlorination
C.Oxidation
D.Hydrogenation

Exam Year(s)

MPPSC Forest Service - 2022,

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.

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26.

Which is the formula of 'Dead Burnt Plaster' ?

A.Anhydrous magnesium sulphate
B.Anhydrous calcium sulphate
C.Hydrated calcium sulphate
D.Hydrated magnesium sulphate

Exam Year(s)

MPPSC Forest Service - 2022,

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.

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27.

When carbon dioxide gas is bubbled through a brine solution, saturated with ammonia, which compound is formed ?

A.\(\text{CaCO}_{3}\)
B.\(\text{Na}_{2}\text{CO}_{3}\)
C.\((\text{NH}_{4})_{2}\text{CO}_{3}\)
D.\(\text{NaHCO}_{3}\)

Exam Year(s)

MPPSC Forest Service - 2022,

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.

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28.

Chemical formula of plaster of Paris is

A.\(\mathrm{CaSO_{4} \cdot 2H_{2}O}\)
B.\(\mathrm{CaSO_{4} \cdot H_{2}O}\)
C.\(\mathrm{CaSO_{4} \cdot \frac{1}{2}H_{2}O}\)
D.\(\mathrm{CaSO_{4}}\)

Exam Year(s)

MPPSC Forest Service - 2023,

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.

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29.

Hydrogen gas is used in

A.preparation of ammonia
B.preparation of hydrochloric acid
C.preparation of methanol
D.All of the above

Exam Year(s)

MPPSC Forest Service - 2023,

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.

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30.

The chemical formula of bleaching powder is

A.\(MgCO_{3}\)
B.\(BaCO_{3}\)
C.\(Ca(OCl)_{2}\)
D.\(CaSO_{4}\)

Exam Year(s)

MPPSC Forest Service - 2023,

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.

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