Question

Okay, so I learned about Graham's Law and ideal gases in Chemistry class - for example, that hydrogen gas effuses 4 times faster than oxygen gas. But this is relative - how is absolute effusion determined in STP?

Answer 1

effusion can be measured fairly simply with a device like this. an evacuated flask is fitted with a syringe containing a sample of gas. Because the flask is a vacuum, the gas in the syringe will escape through the opening into the flask. The time it takes for the syringe to empty can easily be recorded, and you'll get a rate of effusion in terms of cm^3/s. Different gases will escape at different rates, so you can compare an unknown gas to a known gas (usually helium), to determine the molar mass.