JEE Chemistry Hydrogen MCQs Set 04

Multiple Choice Questions (MCQs) for JEE Chemistry: Hydrogen

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Multiple Choice Questions

Question 1: 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 cathode
(c) No reaction takes place
(d) The hydride ion migrates towards the cathode
Show Answer & Explanation

Answer: (a) Hydrogen is liberated at the anode

Explanation:
1. Ionic hydrides contain H- ions.
2. During electrolysis H- migrates to the anode and is oxidised: 2H- gives H2 + 2e-.

Teacher's Note:
1. Anions always move to the anode.

Question 2: The oxidation states shown by hydrogen are:
(a) -1 only
(b) 0 only
(c) +1, -1 and 0
(d) +1 only
Show Answer & Explanation

Answer: (c) +1, -1 and 0

Explanation:
1. Hydrogen is +1 in HCl and H2O, -1 in metal hydrides such as NaH and CaH2, and 0 in H2.

Teacher's Note:
1. Check hydrides, acids and the free element.

Question 3: Hydrogen has three isotopes. The number of possible diatomic molecules is:
(a) 2
(b) 6
(c) 9
(d) 12
Show Answer & Explanation

Answer: (b) 6

Explanation:
1. With isotopes H, D, T the molecules are H2, D2, T2 (3 identical pairs) and HD, HT, DT (3 mixed pairs).
2. Total = 3 + 3 = 6.

Teacher's Note:
1. Choose 2 from 3 with repetition: n(n+1)/2 = 6.

Question 4: In which of the following compounds does hydrogen have an oxidation state of -1?
(a) CH4
(b) NH3
(c) HCl
(d) CaH2
Show Answer & Explanation

Answer: (d) CaH2

Explanation:
1. In CH4, NH3 and HCl hydrogen is less electronegative than the other atom, so it is +1.
2. In CaH2 the metal is far more electropositive, so hydrogen is -1.

Teacher's Note:
1. Metal hydrides contain H-.

Question 5: In the Bosch process, which gas is utilised for the production of hydrogen?
(a) Producer gas
(b) Water gas
(c) Coal gas
(d) None of these
Show Answer & Explanation

Answer: (b) Water gas

Explanation:
1. Water gas (CO + H2) is mixed with steam over an iron oxide catalyst: CO + H2O gives CO2 + H2.
2. CO2 is removed by scrubbing, leaving hydrogen.

Teacher's Note:
1. Water gas shift reaction.

Question 6: Tritium undergoes radioactive decay giving:
(a) Alpha particles
(b) Neutrons
(c) Beta particles
(d) Gamma rays
Show Answer & Explanation

Answer: (c) Beta particles

Explanation:
1. 3H is unstable because of its two neutrons.
2. It decays by beta emission: 3H gives 3He + e- + antineutrino.

Teacher's Note:
1. Tritium is the only radioactive isotope of hydrogen.

Question 7: When SO3 is treated with heavy water, the product is:
(a) Deuterium and sulphuric acid
(b) Deuterium and sulphurous acid
(c) Only deuterium
(d) Dideuterosulphuric acid
Show Answer & Explanation

Answer: (d) Dideuterosulphuric acid

Explanation:
1. SO3 + H2O gives H2SO4.
2. By analogy SO3 + D2O gives D2SO4, dideuterosulphuric acid.

Teacher's Note:
1. Replace H by D in the product.

Question 8: Which statement concerning protium, deuterium and tritium is NOT true?
(a) They are isotopes of each other
(b) They have similar electronic configurations
(c) They exist in nature in the ratio 1 : 2 : 3
(d) Their mass numbers are in the ratio 1 : 2 : 3
Show Answer & Explanation

Answer: (c) They exist in nature in the ratio 1 : 2 : 3

Explanation:
1. All three have one proton and one electron, and mass numbers 1, 2 and 3.
2. In nature protium is 99.985%, deuterium about 0.015% and tritium only in traces, so the ratio 1 : 2 : 3 is wrong.

Teacher's Note:
1. Mass-number ratio is not the abundance ratio.

Question 9: Synthetic detergents are more effective than soaps in hard water because:
(a) They are highly soluble in water
(b) Their Ca2+ and Mg2+ salts are water soluble
(c) Their Ca2+ and Mg2+ salts are insoluble in water
(d) None of these
Show Answer & Explanation

Answer: (b) Their Ca2+ and Mg2+ salts are water soluble

Explanation:
1. Soaps form insoluble Ca and Mg salts (scum) in hard water and lose their cleansing power.
2. Detergents (sulphonates, sulphates) give soluble Ca and Mg salts, so they keep lathering.

Teacher's Note:
1. Scum formation is the problem with soap.

Question 10: Temporary hardness of water may be removed by adding:
(a) CaCO3
(b) Ca(OH)2
(c) CaSO4
(d) HCl
Show Answer & Explanation

Answer: (b) Ca(OH)2

Explanation:
1. Clark's method adds the exact quantity of slaked lime: Ca(HCO3)2 + Ca(OH)2 gives 2CaCO3 + 2H2O.
2. The bicarbonates are precipitated as carbonates.

Teacher's Note:
1. CaSO4 would add hardness.

Question 11: Which pair of ions makes water hard?
(a) Na+, SO42-
(b) K+, HCO3-
(c) Ca2+, NO3-
(d) NH4+, Cl-
Show Answer & Explanation

Answer: (c) Ca2+, NO3-

Explanation:
1. Hardness is caused by dissolved Ca2+ and Mg2+ salts.
2. Only the pair containing Ca2+ makes water hard.

Teacher's Note:
1. Look for Ca2+ or Mg2+.

Question 12: When zeolite (hydrated sodium aluminium silicate) is treated with hard water, the sodium ions are exchanged with:
(a) OH- ions
(b) SO42- ions
(c) Ca2+ ions
(d) H+ ions
Show Answer & Explanation

Answer: (c) Ca2+ ions

Explanation:
1. In the permutit process Na2Z + Ca2+ gives CaZ + 2Na+.
2. The sodium ions of zeolite exchange with Ca2+ and Mg2+ ions.

Teacher's Note:
1. Cation exchange swaps Na+ for Ca2+.

Question 13: Which of the following is NOT true?
(a) Hardness of water depends on its behaviour towards soap
(b) Temporary hardness is due to the presence of Ca and Mg bicarbonates
(c) Permanent hardness is due to soluble Ca and Mg sulphates, chlorides and nitrates
(d) Permanent hardness can be removed by boiling the water
Show Answer & Explanation

Answer: (d) Permanent hardness can be removed by boiling the water

Explanation:
1. Permanent hardness is caused by soluble chlorides and sulphates, which are not decomposed on boiling.
2. Only temporary hardness is removed by boiling.

Teacher's Note:
1. Boiling removes bicarbonates only.

Question 14: The molarity of pure water at 4 degrees C is:
(a) 1 M
(b) 2.5 M
(c) 5 M
(d) 55.5 M
Show Answer & Explanation

Answer: (d) 55.5 M

Explanation:
1. Density at 4 degrees C is 1 g/mL, so 1000 mL has 1000 g of water.
2. Moles = 1000/18 = 55.5, so molarity is 55.5 M.

Teacher's Note:
1. Molarity of pure water is about 55.5.

Question 15: Heavy water freezes at:
(a) 0 degrees C
(b) 3.8 degrees C
(c) 38 degrees C
(d) -0.38 degrees C
Show Answer & Explanation

Answer: (b) 3.8 degrees C

Explanation:
1. D2O has stronger hydrogen bonds than H2O.
2. Its melting point is 3.8 degrees C against 0 degrees C for ordinary water.

Teacher's Note:
1. Heavy water has higher melting and boiling points.

Question 16: The pH of D2O and H2O at 298 K are respectively:
(a) 7.0, 7.0
(b) 7.35, 7.0
(c) 7.0, 6.85
(d) 6.85, 7.35
Show Answer & Explanation

Answer: (b) 7.35, 7.0

Explanation:
1. The ionic product of D2O is smaller than that of H2O (about 1.35 x 10-15 against 1 x 10-14), so [D+] is lower.
2. pD is about 7.35 and pH is 7.0.

Teacher's Note:
1. Heavy water is less ionised.

Question 17: Which of the following is NOT true?
(a) Ordinary water is electrolysed more rapidly than D2O
(b) The reaction between H2 and Cl2 is much faster than between D2 and Cl2
(c) D2O freezes at a lower temperature than H2O
(d) The bond dissociation energy of D2 is greater than that of H2
Show Answer & Explanation

Answer: (c) D2O freezes at a lower temperature than H2O

Explanation:
1. (a) True: the lighter isotope reacts faster.
2. (b) True: kinetic isotope effect.
3. (c) False: D2O freezes at 3.8 degrees C, higher than H2O at 0 degrees C.
4. (d) True: 443 kJ/mol against 436 kJ/mol.

Teacher's Note:
1. Heavy isotopes give stronger bonds.

Question 18: In which of the following reactions is hydrogen peroxide acting as a reducing agent?
(a) 2FeCl2 + 2HCl + H2O2 gives 2FeCl3 + 2H2O
(b) Cl2 + H2O2 gives 2HCl + O2
(c) 2HI + H2O2 gives 2H2O + I2
(d) H2SO3 + H2O2 gives H2SO4 + H2O
Show Answer & Explanation

Answer: (b) Cl2 + H2O2 gives 2HCl + O2

Explanation:
1. In (a), (c) and (d) the oxygen of H2O2 goes from -1 to -2, so H2O2 is an oxidising agent.
2. In (b) oxygen goes from -1 to 0 (O2), so H2O2 is oxidised and acts as a reducing agent.

Teacher's Note:
1. Look for O2 evolved.

Question 19: The percentage strength (w/v) of a 10 volume hydrogen peroxide solution is:
(a) 3.00%
(b) 4.045%
(c) 2.509%
(d) 3.035%
Show Answer & Explanation

Answer: (d) 3.035%

Explanation:
1. 10 volume means 1 mL of the solution gives 10 mL of O2 at STP.
2. 2H2O2 gives 2H2O + O2: 68 g gives 22400 mL, so 10 mL O2 comes from 0.0304 g H2O2.
3. So 1 mL contains 0.03036 g, which is 3.036 g per 100 mL (3.035%).

Teacher's Note:
1. Percentage strength = volume strength x 0.3036.

Question 20: In the laboratory, H2O2 is obtained by:
(a) Cold H2SO4 and BaO2
(b) HCl and BaO2
(c) Conc. H2SO4 and Na2O2
(d) H2 + O2
Show Answer & Explanation

Answer: (a) Cold H2SO4 and BaO2

Explanation:
1. BaO2.8H2O + H2SO4 (dilute, ice cold) gives H2O2 + BaSO4.
2. BaSO4 is insoluble and is filtered off.

Teacher's Note:
1. Use cold dilute acid so H2O2 does not decompose.

Question 21: HCl is added to the following oxides. Which one would give H2O2?
(a) MnO2
(b) PbO2
(c) BaO2
(d) None of these
Show Answer & Explanation

Answer: (c) BaO2

Explanation:
1. BaO2 is a true peroxide (contains O22-).
2. BaO2 + 2HCl gives BaCl2 + H2O2, whereas MnO2 and PbO2 are dioxides that give chlorine.

Teacher's Note:
1. Only peroxides release H2O2 with acids.

Question 22: The reaction H2S + H2O2 gives S + 2H2O shows which nature of H2O2?
(a) Acidic
(b) Alkaline
(c) Oxidising
(d) Reducing
Show Answer & Explanation

Answer: (c) Oxidising

Explanation:
1. Sulphur changes from -2 in H2S to 0 in S, so H2S is oxidised.
2. H2O2 is the oxidising agent.

Teacher's Note:
1. The substance that causes oxidation is the oxidant.

Question 23: The volume strength of 1.5 N H2O2 solution is:
(a) 8.4 litres
(b) 4.2 litres
(c) 16.8 litres
(d) 5.2 litres
Show Answer & Explanation

Answer: (a) 8.4 litres

Explanation:
1. Volume strength = 5.6 x normality.
2. = 5.6 x 1.5 = 8.4.

Teacher's Note:
1. 5.6 is the conversion factor from normality.

Question 24: The volume of oxygen liberated from 15 mL of 20 volume H2O2 is:
(a) 250 mL
(b) 300 mL
(c) 150 mL
(d) 200 mL
Show Answer & Explanation

Answer: (b) 300 mL

Explanation:
1. 20 volume means 1 mL gives 20 mL of O2.
2. 15 mL gives 15 x 20 = 300 mL.

Teacher's Note:
1. Volume strength multiplied by the volume gives the O2.

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