Which of the following gases will have the highest velocity at a given temperature?

a.) ne
b.) o2
c.) he
d.) cl2

The velocity of the gas is inversely proportional to the square root of the molar mass of the gass. Heavier molecules tend to travel slow compared to light molecules. Among these, He has the lowest molar mass of 4 g/mol hence this element has the greatest velocity among all molecules given.

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