Cyclopropane mixed in the proper ratio with oxygen = anestetic. At 755 mm Hg and 25 C density = 1.71 g/L

a) What is the molar mass of cyclopropane?
b) Cyclopropane is made up of 85.7% C and 14.3% H. What is the molecular formula of cyclopropane?

Let us see, you need to use the ideal gas equation in a slightly different format, so let us go through those steps

PV = nRT, where n = m/M, where m is the wt of the compound and M the molar mass
Thus PV = (m/M)RT or P = (m/M)RT / V, since m/V is density
then P = dRT/M
or M = dRT/P, substituting the values you will get
M = 1.71*0.0821*298*760/755 = 42.11 (note you need to convert the 755 mm Hg to atm units, 1 atm = 760 mm Hg)

So the molar mass of cyclopropane is 42

b) 100 g cyclopropane contain 85.7 g C
then 42 g will contain 42*85.7/100 = 35.994 or 36 g C or 36/12 = 3 moles of C
so the molecular formula will be C3H6

There you have it
similarly 42*14.3/100 = 6.0 moles of H