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Q.
The lightest gas is
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Solution:
Key Idea: The gas with lowest atomic mass is lightest gas. The atomic mass of given choices are as follows $ {{\text{N}}_{\text{2}}} $ atomic mass = 14.006 Ar atomic mass = 39.948 Rn atomic mass = 222.0 He atomic mass =4.003 $ \therefore $ He is the lightest gas.