A chemist has flask containing a mixture of two gases at STP. The average velocity of a Gas 1 molecule is twice that of a Gas 2 molecule. Among the following, Gases 1 and 2 are most likely:
He = 4 g/mol
Ne = 20 g/mol
Ar = 40 g/mol
Kr = 80 g/mol
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Correct Answer: D. Ne and Kr
Two gases having the same temperature have the same average molecular kinetic energy. Therefore, (1/2)m1v12 = (1/2)m2v22, which gives m2/m1 = (v1/v2)2. Since v1/v2 = 2, we have m2/m1 = 22 = 4. Among the available choices, the only gases with a mass ratio close to 4 to 1 are krypton (83.8 g/mol) and neon (20.2 g/mol).
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