Answer (Detailed Solution Below)

Option 4 : 20
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The correct answer is 20.

CONCEPT:

  • Heavy water: It is a form of water that contains only deuterium rather than the common hydrogen-1 isotope that makes up most of the hydrogen in normal water.
    • Formula: D2O
    • Boiling point:101.4 °C
    • Molar mass: 20 gm/mole
    • Melting point: 3.8 °C
    • Density: 1.11 gm/cm3
    • Refractive index:1.328

EXPLANATION:

  • Heavy water has deuterium(D) which is an isotope of hydrogen.
  • While hydrogen has 1 proton and 0 neutrons in its nucleus giving it an atomic weight of 1deuterium has 1 proton and 1 neutron in its nucleus.
  • So deuterium has atomic weight 2.
  • So heavy water, which has molecular formula D2O, has molecular mass = 16(for oxygen)+(2× 2)(for two deuteriums)= 20
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