<span>This problem is solved by the law of effusion, which states that the rate of effusion is ~ 1/sqrt(molecular weight). Calculate the molecular weights of the different gases:
HBr = 80.91 g/mol
NO2 = 46.0055 g/mol
C2H6 = 30.07 g/mol
Thus, the approximate relative rates are HBr ~ 0.112, NO2 = 0.1474 and C2H6 = 0.1824.
Answer: C2H6 effuses fastest.</span>
