Chemistry NCERT Exemplar Solutions Class 11th Chapter Nine: Overview, Questions, Preparation

(i) Industrial preparation: H₂O₂ is prepared by the auto-oxidation of 2- alkylanthraquinols

2-ethylanthraquinol ↔ H₂O₂⁺ oxidised product

2Fe₂+ (aq)+2H+ (aq)+H₂O₂ → Fe3 (aq)+2H₂O (l)

PbS (s)+4H₂O₂ (l) → PbSO4 (s)+4H₂O (l)

(ii) Reducing action of hydrogen peroxide 

2MnO₄^-+6H⁺+5H₂O₂→ 2Mn²⁺+8H₂O+5H₂

HOCl+H₂O₂ → H₃O⁺+Cl^-+O₂

Oxidising action of hydrogen peroxide 

2Fe²⁺+H₂O₂→ 2Fe³⁺+2OH^-

Mn²⁺+H₂O₂ → Mn⁴⁺+2OH^-

Acidic properties of H₂O₂

I₂+H₂O₂+2OH^-→ 2I^-+2H₂O+H₂

2MnO₄^-+3H₂O₂→ 2MnO₂+3O₂+2H₂O+2OH^-

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Atomic hydrogen is highly reactive, whereas molecular hydrogen is rather inert. Bond dissociation enthalpy determines the chemical behaviour of dihydrogen (and, for that matter, any molecule) to a great extent. For a single bond between two atoms of any element, the H-H bond dissociation enthalpy is the highest. As a result, molecular hydrogen reacts only with a few elements.

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D₂O can be prepared by prolonged electrolysis of water. Due to high molecular mass D₂O differs from water.

NaOH+D₂O → NaOD+HOD

HCl+D₂O → DCl+ HOD

NH₄Cl+D₂O → NH₃DCl+HOD

Physical Properties of H₂O and D₂O

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Hydrogen peroxide is produced by acidifying barium peroxide and eliminating surplus water by evaporation under low pressure. Water is used to extract it, then distillation under lower pressure concentrates it to around 30% (by mass). Careful distillation under low pressure can increase the concentration to 85%. To achieve pure H₂O₂, the residual water can be frozen out.

Uses of H₂O₂:

(i) As an antiseptic it is sold in the market as perhydrol.

(ii) It is used to manufacture chemicals like sodium perborate and per - carbonate. It is employed in the industries as a bleaching agent for textiles.

(iii) It is used in laboratories as an oxidising agent. Also used in rocket fuels.

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Mass of H₂O₂ = 68 g.

Mol. Mass of H₂O₂=34gmol^-1

∴1L of M solution of H₂O₂ will contain H₂O₂=34×5 g

∴2L of 5 M solution will contain H₂O₂=34×5×2=340g

or 200 mL of 5 solution will contain H₂O₂ = $\frac{340}{2000}$ $\times$  200 = 34 g

Now 2H₂O₂ → 2H₂O+O₂

           64 g  32g

Now 68 g of H₂O₂ on decomposition will give O₂=32g

∴34g of H₂O₂ on decomposition will give O₂ = $\frac{34}{68}$ $\times$    34 =16 g

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Since, a colourless liquid 'A' contains only hydrogen and oxygen and decomposes slowly on exposure to light but is stabilised by addition of urea, therefore, liquid A may be hydrogen peroxide. A is H₂O₂. 

Structure of H₂O₂ is given below.

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It is lithium hydride (LiH) because it has significant covalent character due to the smallest alkali metal, LiH is very stable. It is almost nonreactive towards oxygen and chlorine. It reacts with Al2Cl2 to form lithium aluminium hydride.

8LiH+ Al₂Cl₆ → 2LiAlH₄ → LiCl

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Sodium reacts with Dihydrogen to form sodium hydride which is a crystalline ionic solid.

2Na + H₂ → 2Na + H^-

It reacts with H₂O to produce H₂ gas

2NaH+2H₂O → 2NaOH+2H₂

Although Na⁺H^- does not conduct electricity in the solid state, the electrolysis of its melt produces H₂ at the anode and Na at the cathode.

                                      At cathode  At anode

Na + H^- (l) →  2Na (l)       +      H₂ (g)

Their melts conduct electricity and on electrolysis liberate Dihydrogen gas at anode, which confirms the existence of H^-ion.

The Physical Properties of Hydrogen are-

• Hydrogen is a gas with no color and no odor 
• Hydrogen has the least density of all gases. 
• Hydrogen is viewed as the clean fuel of the future which is developed from water and returned back to water when oxidized.
• Hydrogen is present in almost all molecules in living things. 
• It remains connected with carbon and oxygen atoms. 
• Hydrogen is the most plentiful element in the whole universe. It is present as a gas in the atmosphere in 1 part per million volume. 
• It is spotless, non-toxic, and safe to produce from various sources.
• It is named an energy career because it stores energy that is first created somewhere else.

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Water-gas shift reaction: The water-gas shift reaction is a chemical reaction in which carbon monoxide reacts with steam in the presence of a catalyst to form carbon dioxide and hydrogen.

CO+H₂+H₂O → CO₂+2H₂

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These are formed by many d-block and/block elements. However, the metals of group 7, 8 and 9 do not form hydride. Unlike saline hydrides, they are almost always non- stoichiometric, being deficient in hydrogen. For example, LaH 2.87, YbH 2.55, TiH 1.5-1.8, ZrH 1.3-1.75. Dihydrogen forms molecular compounds with most of the jp-block elements.

Most familiar examples are CH₄, NH₃, H₂O and HF.

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NaH belongs to Saline hydrides and B₂H₆ and H₂O belong to molecular hydrides.

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Ice has a highly ordered 3D hydrogen bonded structure. Each oxygen atom is surrounded tetrahedrally by four other oxygen atoms at a distance of 276 pm.

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(i) PBS (s)+4H₂O₂ (aq) → PbSO₄ (s)+4H₂O (l)

(ii) CO (g)+2H₂ (g) → CH₃OH (l)

(i) During winter, the temperature of lake water keeps on decreasing. Since cold water is heavier, it moves towards the bottom.

(ii) Density of ice is less than that of water that is why it floats on the surface of water.

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Self ionization of water is known as autoprotolysis of water.

H₂O (l)     +      H₂O (l)             →        H₂O₂ (aq)         +          OH^- (aq)

Acid                Base                          Conjugate acid         Conjugate acid

Significance of autoprotolysis: Due to autoprotolysis, water has the ability to act as an acid as well as a base. Thus, it behaves as an amphoteric substance.

H₂O (l)+NH₃ (aq) ↔ OH^- (aq)+NH₄⁺ (aq)

H₂O (l)+H₂ (aq) ↔ H₃O⁺ (aq)+HS^- (aq)

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Water demineralization using organic or synthetic ion exchange resins (ion exchange resins):

a. Synthetic resins are insoluble polymeric solids that contain a large hydrocarbon network with reactive acidic or basic groups.   

These are better than Zeolite since they can remove all kinds of cations and anions from water. Demineralised or deionised water is the product of this process.

b. There are two categories of these:

Resins for cation exchange: Acidic groups such as COOH or SO₃H can be found in them. Resin-H+-can be used to represent them.

Mg²⁺            +      2H-resin  →          Mg (resin)₂+       2H⁺

In hard water       Cation exchanger

Ca²⁺  +     2H-resin           →Ca (resin)₂+2H⁺

In hard water  cation exchanger

Anion exchange resins: They have basic groups such as OH- or NH₂.

They may be represented as resin—OH^- or resin. 

SO₄^2-    +   2OH-resin  →   SO₄- (resin)₂+    2OH^-

Hard water  anion exchanger 

Cl^-       +       OH-resin  →   Cl- resin +OH

Hard water  anion exchanger 

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(a) Electron deficient hydrides are those that lack sufficient number of electrons to form typical covalent bonds. Group 13 hydrides, for example (BH₃, AlH₃, etc.).

(b) Electron precise: Electron precise hydrides are those that have the exact number of electrons required to form covalent bonds. Group 14 hydrides, for example (CH₄, SiH₄, GeH₄, SnH₄, PbH₄ etc.). They are tetrahedral in shape.

(c) Electron rich hydrides: Electron rich hydrides are those that have more electrons than are required to create conventional covalent bonds. Hydrides of groups 15 to 17. (NH₃, PH₃, H₂O, H₂S, H₂Se, H₂Te, HF etc.).

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Heavy water can be made by electrolyzing water repeatedly or as a by-product in the fertiliser industry. It's a deuterium compound that's used to make other deuterium compounds.

Physical Properties of H₂O and D₂O

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Peroxodisulphate, obtained by electrolytic oxidation of acidified sulphate solutions at high current density, on hydrolysis yields hydrogen peroxide.

2HSO₄^- (aq) → HO₃SOOSO₃ 2HSO₄^- (aq)+2H⁺ (aq)+H₂O₂ (aq)

This method is now used for the laboratory preparation Of D₂O₂.

K₂S₂O₈ (s)+2D₂O (l) → 2KDSO₄ (aq)+D₂O₂ (l)

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2H₂O₂ → 2H₂O+O₂

5 volume H₂O₂ means IL of H₂O₂ will give 5L of O₂ at STP.

On the basis of the equation it is clear that 22.7 L of O₂ is produced by 68 g of H₂O₂.

5L of O₂ is produced by = $\frac{68g\times 51}{22.71}$  = $\frac{3400}{227}$ = g of H₂O₂=14.9g

i.e. 15 g H₂O₂ dissolved in 1 L solution will give 5 L oxygen or 1.5 g H₂O₂/100 mL solution will give 500 mL oxygen. Thus 15 g/L or 1.5% solution is known as the 5V solution of H₂O₂.

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In H₂O₂ oxygen is in -1 oxidation state which has tendency to become -2 that is why it is a better oxidising agent than water.

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D₂O has greater intermolecular force of attraction. This is because D₂O exhibits higher values for melting point, enthalpy of vaporization, and viscosity compared to H₂O, indicating stronger intermolecular interactions needed to overcome these properties. 

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Dihydrogen reacts with dioxygen to form water. The reaction is highly exothermic.

H₂ (g) + O₂ (g) → H₂O (l)

? H = -285.9KJmol^-1

The isotope of hydrogen which has one proton and one neutron is deuterium. When it reacts with O₂ it forms Deuterium oxide (D₂O). Deuterium reacts in a similar way but the reactivity will be lesser than hydrogen because of high bond dissociation enthalpy of D₂ as compared to hydrogen.

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Because of hydrogen bonding HF is liquid whereas HCl is gas as there is lack of hydrogen bonding in HCl.

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The compound is water which undergoes self ionization.

The autoprotolysis (self-ionization) of water takes place as follows:

H₂O (l)     +      H₂O (l)             →        H₃O⁺ (aq)         +          OH^- (aq)

Acid                Base                         Conjugate acid           Conjugate acid

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(i) The given compound is H₂O₂.

(ii) Textiles, paper pulp, leather, oils, fats, and other products use it as a bleaching agent. In both acidic and alkaline media, it works as an oxidising and reducing agent. It is therefore kept in dark wax-lined glass or plastic containers. As a stabiliser, urea might be used. It is kept away from dust since dust can cause the compound to decompose explosively.

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Hydrogen has electronic configuration 1s¹. On the one hand, its electronic configuration is similar to that of alkali metals in the first group of the periodic table, which have an outer electronic configuration (ns¹). As a result, hydrogen resembles alkali metals that lose one electron to become unipositive ions.

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Because of ionization enthalpy, hydrogen resembles more with halogens, is 520 kJ mol^-1, F is 1680 kJ mol^-1and that of His 1312 kJ mol^-1. Like halogens, it forms a diatomic molecule, combining with elements to form hydrides and a large number of covalent compounds.

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In sodium the last shell electron is in 3s₁ after losing that electron, it can acquire the configuration of Ne. Whereas in hydrogen, the electron is in s-orbital.

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Hydrogen economy (Hydrogen as fuel):

(i) The electricity cannot be stored in automobiles. It is not possible to store and transport nuclear energy. Hydrogen is an alternative source of energy and hence called the 'hydrogen economy". Hydrogen has some advantages as fuel.

(ii) Available in abundance in combined form as water.

(iii) On combustion produces H₂O. Hence, pollution free.

(iv) H₂O₂ fuel cells give more power.

(v) Excellent reducing agent. Therefore, it can be used as a substitute of carbon in reduction for processes in industry.

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Heavy water can be used as a moderator in nuclear reactors and in exchange reactions for the study of reaction mechanisms. It can be prepared by exhaustive electrolysis of water or as a by-product in some fertilizer industries.

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H₂O₂ structure in gas phase, dihedral angle is 111.50. H₂O₂ structure in solid phase at 110 k, dihedral angle is 90.20.

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(i) Oxidising action in acidic medium:

2Fe²⁺ (aq)+2H⁺ (aq)+H₂O₂ (aq) → 2Fe³⁺ (aq)+2H₂O (l)

PbS (s)+4H₂O₂ (aq) → PbSO₄ (s)+4H₂O (l)

(ii) Reducing action in acidic medium:

2MnO₄^-+6H⁺+5H₂O₂ → 2Mn²⁺+8H₂O+5O₂

HOCl+H₂O₂ → H₃O⁺+Cl^-+H₂O

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H₂O₂ decomposes slowly on exposure to light.

2H₂O₂→ 2H₂O+2O₂

In daily life it is used as a hair bleach and as a mild disinfectant. As an antiseptic it is sold in the market as perhydrol.

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Water molecules are polar because in the gas phase water is a bent molecule with a bond angle of 104.5°, O-H bond length of 95.7 pm. Oxygen is more elsectronegative  (EN = 3.5 ) than hydrogen (EN= 2.1 ) hence, O-H bond is polar . In the water molecule, two polar O-H nonds area present which are held together at an angle of 104.5 . Due to the resultant of these two dipoles, water molecule is polar and has an dipole moment of 1.84 Debye.

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Water has a significantly higher boiling point than hydrogen sulphide because of the presence of strong hydrogen bonds between water molecules, which are absent in hydrogen sulphide due to the lower electronegativity of sulfur compared to oxygen; this means more energy is required to separate water molecules, resulting in a higher boiling point. 

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Hydrogen peroxide cannot be concentrated by heating as it can cause bumps on heating. It can be extracted with water and concentrated to ~30% (by mass) by distillation under reduced pressure. It can be further concentrated to ~85% by careful distillation under low pressure. The remaining water can be frozen out to obtain pure H₂O₂.

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Hydrogen peroxide is stored in wax lined bottles because decomposes slowly on exposure to light.

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With soap, hard water generates scum/precipitate. Soap containing sodium stearate (C1₁₇H₃₅COONa) interacts with hard water to form calcium and magnesium stearate (Ca/Mg stearate). As a result, it is unsuitable for laundering. Because of the deposition of salts in the form of scale, it is also hazardous to boilers.

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Because H₂SO₄ can activate the decomposition of H₂O₂.

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In water hybridisation of oxygen is sp³. Angle should be109^o (approx.) but due to LP - LP repulsion bond angle reduces to 104.5°.

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2F₂ (g)+2H₂O → 4H⁺ (aq)+4F^- (aq)+O₂ (g)

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H₂O (l)+NH₃ (aq) → OH^- (aq)+NH₄⁺ (aq)

H₂O (l)+H₂S (aq) → H₃O⁺ (aq)+HS^- (aq)

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option (ii)

Its tendency to gain a single electron in its valence shell to attain stable electronic configuration.

Like halogens (with ns²np⁵ configuration belonging to the seventeenth group of the periodic table), it is short by one electron to the corresponding noble gas configuration, helium (ls²).

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option (iv)

Loss of an electron from a hydrogen atom results in a nucleus of very small size as compared to other atoms or ions. Due to its small size it cannot exist free.

Loss of the electron from hydrogen atom results in nucleus size (H⁺) of 1.5 × 10^-3. This is extremely small as compared to normal atomic and ionic sizes of 50 to 200 pm. As a consequence, H⁺ does not exist freely and is always associated with other atoms or molecules.

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Option (ii)

LiH < NaH < KH < RbH < CsH

In ionic hydride as it is formed by the s-Block element, down the group, electropositive character increases.

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Option (iv)

CH₄

These hydrides have the required number of electrons to write their conventional Lewis structures.

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Option (iii)

Tritium

The tritium concentration is about one atom per 10¹⁸ atoms of protium. Of these isotopes, only tritium is radioactive and emits low energy beta particles.

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Option (ii)

An oxidizing agent in (A) and reducing agent in (B)

H₂O₂ acts as an oxidising agent in A as it oxidized iodine from -1 to 0, while in B it acts as reducing agent as it reduced chlorine from +1 to -1

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Option (ii)

BaO₂ -8H₂O

Hydrogen peroxide is produced by acidifying barium peroxide and eliminating excess water by evaporation under low pressure.

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Option (iii)

2I^- + 2H⁺ + H₂O₂ → I₂+ 2H₂O

As the oxidation state of iodine changes from -1 to 0, this shows peroxide is oxidising in nature.

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Option (ii)

I₂ + H₂O₂ + 2OH^–→ 2I^-+ 2H₂O + O₂

As iodine is reduced from 0 to -1.

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Option (iii)

Both an oxidizing and a reducing agent

In both acidic and alkaline medium hydrogen peroxide can act as oxidizing and reducing agent.

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Option (iii)

CO (g)+H₂O (g) → CO₂ (g)+H₂ (g)

The production of dihydrogen can be increased by reacting carbon monoxide of syngas mixtures with steam in the presence of iron chromate as catalyst.

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Option (iv)

Sodium sulphate and hydrogen peroxide.

Na₂O₂+dil H₂SO₄ → Na₂SO₄ → Na₂SO₄+H₂O₂

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Option (ii)

Sulphuric acid

2HSO₄^- (aq) → HO₃SOOSO₃H (aq) → 2HSO₄^- (aq)+2H⁺ (aq) + H₂O₂ (aq)

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Option (iv)

CO (g)+2H₂ (g) → CH₃OH (l)

The reaction shown in equation (iv) shows the synthesis of methanol from water gas.

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Option (i)

Ca²⁺

Water becomes 'hard' when it contains calcium and magnesium salts in the form of hydrogen carbonate, chloride, and sulphate.

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Option (iii)

Na₆P₆O₁₈

Sodium hexametaphosphate (Na₆P₆O₁₈), commercially called ‘calgon’, is used to remove hardness from water.

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Option (i)

Groups 7,8, 9

Explanation: The metals belonging to group 7, 8 and 9 do not have tendency to form hydrides.

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Option (i)

Group 6

From group 6, only chromium forms CrH.

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Option (iii) and (iv)

H⁺ does not exist freely and is always associated with other atoms or molecules. It has an extremely high ionisation enthalpy and does not have metallic properties under normal conditions, unlike alkali metals.

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Option (i) and (ii)

The combination of CO and H₂ is known as water gas. This mixture of CO and H₂ is also known as synthesis gas or 'syngas' since it is used to make methanol and other hydrocarbons.

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Option (i) and (iii)

Heavy water is utilised as a moderator in nuclear reactors and exchange reactions, and it is more associated with water due to its higher mass.

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Option (i) and (ii)

When an electron is removed from a hydrogen atom, the nucleus decreases to 1.5 × 10^-3 pm in size. When compared to usual atomic and ionic sizes of 50 to 200 pm, this is exceedingly small. As a result, H+ does not exist independently and is always bound to other atoms or molecules.

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Option (iii) and (iv)

The presence of extensive hydrogen bonding between water molecules gives it remarkable features in the condensed phase (liquid and solid phases). In comparison to H₂S and H₂Se, this results in a high freezing point, high boiling point, high heat of vaporisation, and high heat of fusion.

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Option (i) and (ii)

Presence of calcium and magnesium salts in the form of hydrogen carbonate, chloride and sulphate in water makes water 'hard'.

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Option (ii) and (iii)

Electron precise hydrides have the required number of electrons to write their conventional Lewis structures. All elements of group 14 form such compounds (e.g., CH₄) which are tetrahedral in geometry.

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Option (i) and (iv)

All elements in group 13 are Lewis acids because they form electron-deficient compounds. Excess electrons are found as lone pairs in electron-rich hydrides.

These compounds are made up of elements from groups 15-17. (NH₃ has one lone pair, H₂O has two, and HF has three.) As a result, they act as Lewis bases.

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Option (i), (ii) and (iii)

Ionic hydrides are crystalline, non-volatile and non-conducting in solid state

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(i) → (j); (ii) → (h); (iii) → (f) : (iv) → (a); (v) → (b); (vi) → (e)

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(i)→ (d); (ii)→ (b); (iii)→ (c); (iv)→ (a)

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(i)→ (e); (ii)→ (d); (iii)→ (b); (iv)→ (a); (v)→ (c)

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(i) → (b); (ii) → (c); (iii) → (a)

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(i) Statements A and R both are correct and R is the correct explanation of A.

MCl₂+Na₂CO₃→ MCO₃+2NaCl

MSO₄+Na₂CO₃→MCO₃+Na₂SO₄

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(i) Statements A and R both are correct and R is the correct explanation of A. Since, metals like Pd and Pt adsorbs a large volume of hydrogen, hence, these are used as a storage media for it. 

P? = X? (P? – P? ) + P?
ATQ
550 = 1/4 (P? – P? ) + P?
2200 = P? – P? + 4P?
560 = 1/5 (P? – P? ) + P?
2200 = P? – P? + 5P?
P? + 3P? = 2200
P? + 4P? = 2800
P? = 600
P? = 400 mmHg

first order reaction
K = (2.303/t) log (a? / (a? - x)
K = (2.303/90) log (a? / 0.25a? ) = 0.0154
t = 60% = (2.303/K) log (a? / a? ) . (2)
= (2.303 / 0.0154) x (1 - 0.602) = 59.51 mins ≈ 60

N? + 3H? → 2NH?
(2.8 x 10³) / 28 → 0.1 x 10³ mol LR
(1 x 10³) / 2 → 0.5 x 10³ mol

Mass of NH? produced = 0.2 × 10³ × 17 = 3.4 kg = 3400 g

Kindly consider the following Image

eq of H? O? = eq of KMnO?
(0.316 / 158) x 5 = 0.01
(0.01 x 17 / 0.2) x 100 = 85%

Reaction is

Mass of product, 1, 2, 2- tetrobromopropane obtained

= $\frac{1\times 360}{160\times 2}\times \frac{27}{100}$

= 3.30375

= 3.0375 × 10^-1