Okay, I need a procedure for how to determine the molecular weight of the gas in a Bic Lighter by collecting?

Question

the Gas over water.

Answer 1

You'll need to weigh the lighter before and after collecting the gas because you'll need to know the mass of the gas that you collected.

Once you've collected the gas, hopefully, you've done that in a way that will allow you to know the volume, pressure and temperature of the gas.

Now, you'll use the ideal gas law with one slight variation. The term "n" in the ideal gas law is the number of moles of the gas. That term is equal to the mass of the gas divided by the molecular weight of the gas: n=m/MW

So, using the ideal gas law, you can solve for MW as the only term you don't know.