You are given the cyclopropane composed of 85.7% C and 14.3% H by mass. You are also given the density of cyclopropane of 1.88 g/l at STP and you and you are asked what is the molecular formula of cyclopropane.
First, you have to find the empirical of cyclopropane and that is CH₂. Then, using PV = nRT, find the molecular weight of the compound.
PV = nRT
PV = (m/mm)RT
P(V/m) = RT/mm
mm = RTdensity/P
mm = (0.08206 L-atm/mol-K)(273K)(1.88g/L)/(1atm)
mm = 42.12 g/mol
The total number of moles of the empirical formula
CH₂ = 12 + 1(2) = 14 g/mol
(CH₂)n = 42.12g/mol/14g/mol
n = 3
then multiply this number to the empirical formula
The molecular formula is C₃H₆
