Molecular Formula of the Compound!

A gaseous compound containing hydrogen and carbon is decomposed and found to contain 85.63% C and 14.37% H by mass. The mass of 258 mL of the gas, measured at STP, is found to be 0.646 g. What is the molecular formula of the compound?

Take a hypothetical sample of 100.000 grams:

(14.37 g H) / (1.007947 g H/mol) = 14.2567 mol H2
(85.63 g C) / (12.01078 g C/mol) = 7.1294 mol C

Divide by the smaller number of moles:
(14.2567 mol H2) / 7.1294 = 2.000
( 7.1294 mol C) / 7.1294 = 1.000
So the empirical formula is:
CH2

PV = nRT
n / V = P / RT = (1.000 atm) / ((0.08205746 L atm/ K mol) * (273.15 K)) =
0.044615 mol/L
(0.646 g) / (0.258 L) = 2.50387 g/L
(2.50387 g/L) / (0.044615 mol/L) = 56.1217 g/mol

CH2 = 14.02667 g/mol
(56.1217 g/mol) / (14.02667 g/mol) = 4.00
So multiply the empirical formula by 4 to find the molecular formula:
C4H8