Vapour Density is an important concept in Class 11 Chemistry used to calculate the molecular mass (molar mass) of gases.
Formula: Molar Mass = 2 × Vapour Density.
V.D. has no units (it is a ratio).
Reference gas: Always compared to Hydrogen (H₂).
Vapour Density is defined as the ratio of the mass of a given volume of a gas to the mass of the same volume of hydrogen gas, under the same conditions of temperature and pressure.
Formula:
Vapour Density (V.D.) = Mass of 'n' molecules of gas / Mass of 'n' molecules of H₂
Relation with Molar Mass:
Molar Mass = 2 × Vapour Density
OR
Vapour Density = Molar Mass / 2
Q: The vapour density of CO₂ is 22. Find its molar mass.
Molar Mass = 2 × V.D. = 2 × 22 = 44 g/mol
(Actual molar mass of CO₂ = 12 + 16 + 16 = 44 g/mol ✓)
Vapour Density (V.D.) is the ratio of mass of a gas to the mass of equal volume of hydrogen. The relation with molar mass is: Molar Mass = 2 × Vapour Density.
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