Question

Nitrogen gas effuses through an opening 1.59 times faster than an unknown gas.

A. Calculate the molecular mass of the unknown gas.

B. Make a reasonable prediction as to what the unknown gas is.

Answer 1

A) Nitrogen has an ATOMIC mass number of 14, but nitrogen gas consists of N₂ molecules, so the mass to use in this problem is 28 g/mol. Rates of effusion ∝ 1/√(mass), so

√(mass unknown) /√28 = (rate N₂ effusion)/(rate unknown effusion) = 1.59
∴ mass unknown = (1.59)²(28) = 70.78 g/mol

B) One possible gas that comes close for this mass is NF₃.