Question

Determine the molar mass of a gas with a density of 1.3786 g/L at 80.00 degress celsius and exactly 1 atm.

Answer 1

You can use the gas law of PV = nRT but you must work on that to do it directly.
P*molar mass = density*R*T.
I can show you how you get that equation if you haven't seen it before.