In an effusion experiment it required 40 s for a certain number of moles of a gas of unknown molar mass to pass through a small orifice into a vacuum. Under the same conditions 16 s were required for the same number of moles of O2 to effuse. What is the molar mass of the unknown gas?
A. 5.1 g/mol B. 12.8 g/mol C. 80 g/mol D. 100 g/mol E. 200 g/mol