Solved Problems on Hydrogen and its Compounds

Question 1:  Why is hard water softened before its use in boilers? 

Sol. Hard water cannot be used in boilers because on heating it gives precipitates of CaCO₃ and MgCO₃ alongwith CaSO₄ which forms scales in boilers. To avoid the scales formation, hard water is softened before its use in boiler.

Question 2: What volume of oxygen at NTP will be obtained by the complete decomposition of 100 ml of H₂O₂ marked 30 volume?

Sol. 1 ml H₂O₂ of 30 volume liberates 30 ml O₂ at NTP.

100 ml H₂O₂ of 30 volumes liberate 30 ´ 100 ml O₂ at NTP or 3 litre O_2­ at NTP.

Question 3: Cu does not liberate H₂ from dil H₂SO₄. Explain.

Sol. Cu is placed below H in electrochemical series having oxidation potential -0.34 V hence Cu can not transfer electron to H⁺ ions of H₂SO₄ to liberate H₂.

Question 4. A small amount of phosphoric acid or glycerol or acetanilide is added in H₂O_2­ during its storage. Explain.

Sol. All the three additives act as negative catalyst for the decomposition of H₂O₂ and thus decomposition of H₂O is checked off.

Question 5. Which isotope of hydrogen is used as a trace in organic reactions?

Sol. Hydrogen has three isotopes, Viz, H, D and T. Due to difference in masses, the rate constants of three isotopes with the same substance are different. In other words, both D and T show isotope effect. But since T is not only radioactive but is also least abundant hydrogen isotope. Therefore D is used as a trace to study the mechanism of organic reactions.

Question 6. H₂O₂ is a very good solvent for ionic compounds. Explain.

Sol. The dielectric constant of pure H₂O₂ is 93.7 (which also increases on dilution (97 for 90% pure; 120 for 65% pure) which is appreciably high to weaken the electrostatic forces of attraction among ions and to pass them in solution state.

Question 7. Anhydrous  is not used for preparing  why?

Sol. Anhydrous  is not used because the  formed during the reaction forms a protective layer around the unreacted  and the reaction stops after some time.

Question 8. Statues coated with white lead on long exposure to atmosphere turns black and the original colour can be restored on treatment with. Why?

Sol. On long exposure to atmosphere, white lead is converted into black PbS due to the action of  present in the atmosphere. As a result, statues turn black

On treatment of these blackened statues with, the black PbS get oxidized to white  and the colour is restored

Question 9. Show with suitable example hydrogen acts as reducing agent as well as oxidizing agent?

Sol. In the reaction of  with metals to form metal hydrides, it acts as an oxidizing agent.

Here Na has been oxidized to Na⁺ while  is reduced to hydride  ion.

In the reaction of heated cupric oxide with dihydrogen to form  and copper metal, dihydrogen acts as a reducing agent

Here, CuO is reduced to Cu while dihydrogen is oxidized to.

Question 10. Ozone is used for purifying air in crowded place such as cinema halls, tube railways, tunnels etc explain?

Sol.is easily decomposes to produce  thereby purifying air at crowed places.

Question 11. Why is sodium chloride less soluble in heavy water than in ordinary water? 

Sol. The dielectric constant of  is less than that of, therefore ionic compounds like NaCl are less soluble in  than in.

Question 12. Why electrolysis of ordinary water occurs faster than heavy water?

Sol. This is due to lower bond dissociation energy of protium bonds in H – O – H, which is less than deuterium bonds in D – O – D, so electrolysis of  occurs much faster than that of.

Question 13. Name the class of hydrides to which H₂O, B₂H₆, NaH and LaH₃ belong. What is understood by ‘Hydride Gap’?

Sol. H₂O - covalent hydride, B₂H₆ - covalent hydride, NaH - ionic hydride, LaH₃ - metallic hydride.Hydride Gap: The metals of 7, 8, 9 groups do not form hydride. This is called the hydride gap.

Question 14. Concentrated sulphuric acid can not be used for drying. Why?

Sol. This is because conc. on absorbing water from moisture generates so much heat that hydrogen catches fire.

Question 15. A mixture of hydrazine and  with Cu(II)  catalyst is used as rocket propellant. Why?

Sol. The reaction between hydrazine and  is highly exothermic and is accompanied by a large increase in the volumes of the product and hence this mixture is used as a rocket propelled

• Objective:

Question 1. HCl is added to the following oxides. Which one would give?

(A) 

(B) 

(C) 

(D) none of the above

Sol. Since none of the oxides is a peroxide. Hence none of them would give.

Questionl 2.  When the same amount of zinc is treated separately with excess of sulphuric acid and excess of sodium hydroxide, the ratio of volumes of hydrogen evolved is

(A) 1 : 1                                                         (B)  1 : 2

(C) 2 : 1                                                         (D)  9 : 4

Sol.         

The ratio of volumes of  evolved in both the cases is 1 : 1.

Question 3.   Which of the following is the true structure of H₂O₂?

Sol. Oxygen atom is  hybridized having two large pair of electrons. Therefore the             structure of hydrogen peroxide is  

Question 4. When electric current is passed through an ionic hydride in the molten state

(A) hydrogen is liberated at the anode              

(B) hydrogen is liberated at the anode

(C) no reaction takes place

(D) hydride ion migrates towards cathode

Sol. Ionic hydride contains  ion which liberates  at the anode.

Question 5. Which of the following can not be oxidized by  ?

(A )KI + HCl
(B) 
(C) PbS
(D) 

Sol. is more powerful oxidizing agent than. So  reduces  to.

Question 6. The reaction                                                  

Ag₂O + H₂O₂ → 2Ag + H₂O + O₂ takes place in

(A) basic medium
(B) bleaching agent
(C) neutral medium
(D) both in acidic and basic medium

Sol.     on oxidation gives  only in basic medium

Question 7. Which of the following compounds turns white on treatment with.

(A) HgS                                                          (B)  PbS

(C) NiS                                                           (D)  CuS

Sol. 

Question 8. The strongest base is

(A)                                                           (B)  

(C)                                                         (D)  

Sol. Basic character decreases as the size of central atom increases in hydrides of group 15 members

Question 9.   Semi water gas is a mixture of

(A)                                                     (B)  

(C)                                              (D)  none

Sol. Mixture of CO +  +  is called semi water gas.

Question 10. A hydride of nitrogen which is acidic is

(A)                                                          (B)  

(C)                                                         (D)   

Sol.  (hydrazoic acid) is the acidic hydride of nitrogen

•  True/False

Question 11. Tritium can be obtained from natural sources.

Sol. False

Question 12. H₂O₂ act as antichlor.

Sol. True

Question 13. Sodium carbonate can remove only temporary hardness of water.

Sol. False

Question 14. Ortho hydrogen has lower energy and is more stable than para hydrogen.

Sol. True

Question 15. Bleaching action of H₂O is due to its reducing nature.

Sol. False

You can also refer to following links

• JEE Organic Chemistry Syllabus 

•  Reference books of Organic Chemistry

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