What is the molar mass of a compound if a gaseous sample has a density of 2.807 g/L at 18 °C and 728 torr?
P*molar mass = density*RT
Solve for molar mass.
Nope. P must be in atmospheres. 728/760 = ? atm.
Also it appears to me you have set up the reciprocal.
M = dRT/P