Questions & explanations
1. Which of the following methods removes permanent hardness of water by forming a soluble complex with Ca2+ and Mg2+ ions?
- Boiling
- Clark's process
- Washing soda addition
- Calgon process
Answer: Calgon process
Permanent hardness is caused by Ca2+/Mg2+ chlorides and sulphates. Calgon (sodium hexametaphosphate) sequesters these ions into a soluble complex, preventing them from reacting with soap. Boiling and Clark's process remove only temporary hardness. Washing soda precipitates carbonates, not forms a soluble complex.
2. In the laboratory preparation of dihydrogen, why is granulated zinc preferred over pure zinc?
- Granulated zinc is cheaper than pure zinc.
- Granulated zinc has a larger surface area, but pure zinc reacts faster.
- Impurities in granulated zinc form electrochemical cells that speed up the reaction.
- Pure zinc reacts explosively with dilute acid.
Answer: Impurities in granulated zinc form electrochemical cells that speed up the reaction.
Granulated zinc contains impurities like copper or lead. These impurities, along with zinc, form tiny electrochemical cells in the presence of acid, which accelerate the reaction. Pure zinc reacts slowly because a layer of hydrogen gas forms on its surface, preventing further contact with acid.
3. What is the dihedral angle in H2O2 in the gas phase?
- 94.8°
- 111.5°
- 90.2°
- 180°
Answer: 94.8°
The dihedral angle in gas-phase H2O2 is determined by the balance between steric repulsion and lone-pair interactions. Experimental measurements (e.g., microwave spectroscopy) give a value of about 94.8°. This non-planar 'open book' structure minimizes repulsion between the O–H bonds.
4. Heavy water is used as a moderator in nuclear reactors because it:
- Absorbs neutrons to control the reaction
- Slows down neutrons without absorbing them
- Increases the speed of neutrons
- Acts as a fuel in the reactor
Answer: Slows down neutrons without absorbing them
Heavy water (D2O) slows down fast neutrons to thermal energies via collisions, and deuterium has a low neutron absorption cross-section, so it does not absorb neutrons. This makes it an effective moderator. Options a, c, and d describe functions of control rods, not moderators.
5. Which of the following statements about hydrogen is correct?
- Hydrogen easily loses an electron to form H⁺ in water.
- Hydride ion (H⁻) is as stable as chloride ion (Cl⁻).
- Hydrogen is placed in Group 1 because it resembles alkali metals in all properties.
- Hydrogen has a very high ionization enthalpy of 1312 kJ/mol.
Answer: Hydrogen has a very high ionization enthalpy of 1312 kJ/mol.
Hydrogen has an ionization enthalpy of 1312 kJ/mol, which is much higher than that of alkali metals (e.g., Na = 496 kJ/mol). Therefore, H⁺ is not stable in aqueous solution and exists only as H₃O⁺. This high ionization enthalpy is a key difference from alkali metals.
6. A 20-volume H2O2 solution is used. 25 mL of it is titrated against acidified KMnO4 requiring 35.7 mL. What is the molarity of KMnO4?
- 0.50 M
- 0.25 M
- 0.75 M
- 1.00 M
Answer: 0.25 M
20-volume H2O2 means 1 L yields 20 L O2 at STP. Moles O2 = 20/22.4 = 0.8929, so moles H2O2 = 2×0.8929 = 1.786 M. In acidic medium, n-factor of H2O2 is 2, of KMnO4 is 5. Equivalents H2O2 = 1.786×2×0.025 = 0.0893. Equivalents KMnO4 = M×5×0.0357 = 0.0893 → M = 0.25 M.
7. The H-O-H bond angle in water is 104.5° because of:
- sp hybridisation of oxygen with no lone pairs
- sp2 hybridisation of oxygen with one lone pair
- sp3 hybridisation of oxygen with two lone pairs causing greater repulsion
- pure p orbitals of oxygen forming a linear shape
Answer: sp3 hybridisation of oxygen with two lone pairs causing greater repulsion
In water, oxygen is sp3 hybridised with two bond pairs and two lone pairs. According to VSEPR theory, lone pair-lone pair repulsion is stronger than bond pair-bond pair repulsion, which compresses the H-O-H angle from the ideal tetrahedral 109.5° to 104.5°.
8. Which of the following physical properties of heavy water (D2O) is correct compared to ordinary water?
- Higher boiling point
- Lower density
- Lower boiling point
- Same boiling point
Answer: Higher boiling point
Heavy water has stronger hydrogen bonds due to deuterium, leading to a higher boiling point (101.4°C) and higher density (1.106 g/cm³) compared to ordinary water. The other options are incorrect because D2O has higher, not lower, boiling point and density.
9. Why is hydrogen placed separately at the top of the periodic table?
- Because its electronic configuration 1s¹ shows dual resemblance to both alkali metals and halogens
- Because it has the lowest atomic mass among all elements
- Because it is the only element that can form both H⁺ and H⁻ ions
- Because it is a gas at room temperature
Answer: Because its electronic configuration 1s¹ shows dual resemblance to both alkali metals and halogens
Hydrogen has electronic configuration 1s¹, which can be interpreted as one valence electron like alkali metals or one electron short of noble gas like halogens. This dual character makes its position anomalous, so it is placed separately at the top.
10. A water sample has 200 mg/L Ca(HCO3)2 and 100 mg/L MgSO4. What mass of Ca(OH)2 per litre is needed to remove only the temporary hardness?
- 45.7 mg
- 182.8 mg
- 91.4 mg
- 137.1 mg
Answer: 91.4 mg
Only Ca(HCO3)2 causes temporary hardness. Molar mass Ca(HCO3)2 = 162 g/mol, so 200 mg/L = 0.200/162 = 0.001235 mol/L. Clark's process: Ca(HCO3)2 + Ca(OH)2 → 2CaCO3 + 2H2O, 1:1 mole ratio. Mass Ca(OH)2 = 0.001235 × 74 = 0.0914 g/L = 91.4 mg/L.
11. In the reaction H2O2 + Cl2 → 2HCl + O2, what is the role of H2O2?
- Neither oxidising nor reducing agent
- Oxidising agent
- Both oxidising and reducing agent
- Reducing agent
Answer: Reducing agent
In H2O2, oxygen has oxidation state -1. It is oxidised to O2 (0), so H2O2 acts as a reducing agent. Chlorine is reduced to HCl (-1), confirming H2O2 is the reducing agent. There is no reduction of H2O2; thus it is only a reducing agent.
12. Which of the following pairs of compounds shows a resemblance between hydrogen and alkali metals?
- H₂O and Cl₂O
- HCl and NaCl
- H₂ and Cl₂
- H₂O and Na₂O
Answer: HCl and NaCl
Hydrogen resembles alkali metals in forming ionic hydrides and halides. Both form monovalent cations (H⁺ and Na⁺) that combine with halide ions to give compounds like HCl and NaCl, showing similar stoichiometry and ionic character.