JEE Chemistry Hydrogen MCQs Set 12

Answer: (d) 3.4%

Explanation:
1. 2H₂O₂ → 2H₂O + O₂: 68 g of H₂O₂ gives 22.4 L of O₂ at STP.
2. 11.2 L of O₂ comes from 68 × 11.2/22.4 = 34 g of H₂O₂, which is present in 1 L (about 1000 g) of solution.
3. Percentage w/w = 34/1000 × 100 = 3.4%.

Teacher's Note:
1. Volume strength of 1 mL of solution means the volume of O₂ at STP given by 1 mL of H₂O₂ on decomposition.

Answer: (c) protium, deuterium and tritium

Explanation:
1. Hydrogen has three isotopes with mass numbers 1, 2 and 3.
2. They are protium (¹H), deuterium (²H) and tritium (³H).

Teacher's Note:
1. All three isotopes have one proton; they differ in the number of neutrons (0, 1, 2).

Answer: (c) zinc

Explanation:
1. Zinc is amphoteric: Zn + 2HCl → ZnCl₂ + H₂.
2. With alkali, Zn + 2NaOH → Na₂ZnO₂ + H₂.

Teacher's Note:
1. Amphoteric metals such as Zn and Al react with both acids and alkalis to liberate H₂.

Answer: (a) (1), (2), (3) and (4)

Explanation:
1. Saline hydrides react with water: MH + H₂O → MOH + H₂.
2. 3LiAlH₄ + 4BF₃ → 2B₂H₆ + 3LiF + 3AlF₃.
3. PH₃ has a lone pair (electron rich) while CH₄ has exactly enough electrons for its bonds (electron precise); HF and CH₄ are discrete molecular hydrides. All four are correct.

Teacher's Note:
1. Hydrides are classed as electron deficient, precise or rich according to the electrons available for bonding.

Answer: (d) Ca(HCO₃)₂

Explanation:
1. Temporary hardness is due to the bicarbonates of calcium and magnesium.
2. It is removed by boiling, which precipitates the insoluble carbonate or hydroxide.

Teacher's Note:
1. Chlorides and sulphates of Ca and Mg cause permanent hardness.

Answer: (a) Mg(HCO₃)₂ and Mg(OH)₂

Explanation:
1. Temporary hardness arises from Ca(HCO₃)₂ or Mg(HCO₃)₂.
2. On boiling, Ca(HCO₃)₂ gives insoluble CaCO₃, but Mg(HCO₃)₂ gives the still less soluble Mg(OH)₂: Mg(HCO₃)₂ → Mg(OH)₂ + 2CO₂.
3. Of the options, only Mg(HCO₃)₂ → Mg(OH)₂ is correct.

Teacher's Note:
1. Magnesium bicarbonate gives the hydroxide on boiling because Mg(OH)₂ is less soluble than MgCO₃.

Answer: (a) oxidising and reducing agent in both acidic and basic medium

Explanation:
1. H₂O₂ is an oxidising agent in acidic medium (H₂O₂ + 2H⁺ + 2e⁻ → 2H₂O) and in basic medium (H₂O₂ + 2e⁻ → 2OH⁻).
2. It is also a reducing agent in both media, as in H₂O₂ + 2OH⁻ → O₂ + 2H₂O + 2e⁻.

Teacher's Note:
1. Oxygen in H₂O₂ is at -1, between 0 and -2, so it can be oxidised or reduced.

Answer: (d) 3 and 1

Explanation:
1. Hydrogen has three isotopes: protium, deuterium and tritium.
2. Only tritium (n/p = 2) is radioactive, so the answer is 3 and 1.

Teacher's Note:
1. Nuclei with a high neutron to proton ratio are unstable and radioactive.

Answer: (c) saline hydride

Explanation:
1. Hydrogen combines with strongly electropositive s-block metals to form ionic hydrides, Na⁺H⁻.
2. Such ionic hydrides are called saline hydrides.

Teacher's Note:
1. Except Be and Mg, the s-block metals form saline hydrides.

Answer: (a) I, II and III only

Explanation:
1. Statements I, II and III are correct: H₂ burns to water, its storage is bulky, and it is stored in alloys such as NaNi₅ as interstitial hydrides.
2. Statement IV is wrong because the energy per gram is about 142 kJ for dihydrogen and about 50 kJ for LPG, not the other way round.

Teacher's Note:
1. Hydrogen has the highest energy per gram of any common fuel but is difficult to store.

Answer: (c) 11.2

Explanation:
1. Volume strength = 11.2 × molarity.
2. For 1 M H₂O₂ the volume strength is 11.2 (1 L of solution gives 11.2 L of O₂ at STP; 1 mol of H₂O₂ gives 0.5 mol of O₂).

Teacher's Note:
1. Volume strength = 11.2 × M = 5.6 × N.

Answer: (c) H₂O and (H₂O + O₂)

Explanation:
1. In acidic medium H₂O₂ acts as an oxidising agent: 2[Fe(CN)₆]⁴⁻ + H₂O₂ + 2H⁺ → 2[Fe(CN)₆]³⁻ + 2H₂O, so the other product is H₂O.
2. In alkaline medium H₂O₂ acts as a reducing agent: 2[Fe(CN)₆]³⁻ + H₂O₂ + 2OH⁻ → 2[Fe(CN)₆]⁴⁻ + O₂ + 2H₂O, so the other products are H₂O and O₂.

Teacher's Note:
1. H₂O₂ oxidises in acidic medium and reduces in basic medium in this pair of reactions.

Answer: (b) There is extensive intramolecular hydrogen bonding in the condensed phase

Explanation:
1. Water is amphoteric, heavy water ice is denser than normal water, and water is oxidised to O₂ in photosynthesis.
2. The hydrogen bonding in water in the condensed phase is intermolecular, not intramolecular, so statement (b) is false.

Teacher's Note:
1. Water molecules are linked to each other by intermolecular hydrogen bonds.

Answer: (a) It can act only as an oxidising agent

Explanation:
1. Oxygen in H₂O₂ is at -1, so it can be oxidised to 0 or reduced to -2.
2. Hence H₂O₂ is both an oxidising and a reducing agent, so (a) is incorrect; the other statements are correct because light, dust and alkali from glass catalyse its decomposition.

Teacher's Note:
1. H₂O₂ is stored in dark plastic or wax lined bottles because it decomposes easily.

Answer: (d) Reaction of salts like hydrides with water

Explanation:
1. Hydrides such as CaH₂ react with water to give H₂ of high purity: CaH₂ + 2H₂O → Ca(OH)₂ + 2H₂.
2. The other processes give gas mixtures or need a purification step.

Teacher's Note:
1. Ionic hydrides are an instant source of pure hydrogen.

Answer: (d) CO is oxidised to CO₂ with steam in the presence of a catalyst followed by absorption of CO₂ in alkali

Explanation:
1. In the water gas shift reaction CO + H₂O → CO₂ + H₂ over a catalyst, CO is converted into CO₂.
2. The CO₂ is then absorbed in alkali (for example KOH, giving K₂CO₃), leaving pure hydrogen.

Teacher's Note:
1. The water gas shift reaction is the industrial route to extra H₂.

Answer: (c) Saturation of water with CaSO₄

Explanation:
1. Hardness is caused by soluble salts of calcium and magnesium.
2. CaSO₄ is soluble enough to cause permanent hardness, whereas CaCO₃ and MgCO₃ are practically insoluble in pure water and Na₂SO₄ does not contain Ca or Mg.

Teacher's Note:
1. Soluble Ca and Mg sulphates or chlorides give permanent hardness.