Question

a mixture of cyclo propane contains 85.7%C and 14.3%HS by mass. at 50.0°C and 0.984 atm pressure, 1.56 g cyclopropane has a volume of 1.00L. what is the molecular formula of cyclopropane?

Answer 1

Let us 1st find the molecular mass
T = 50.0 oC = (50+273) K = 323 K
P = 0.984 atm
V = 1 L

use:
P*V = n*R*T
P*V = (mass/molar mass) * R*T
0.984 * 1 = (1.56/MW)*0.0821*323
MW = 42 g/mol

Now lets find the empirical formula:
Consider 100 g of substance
C : 85.7 g
H : 14.3 g

divide each by their molar mass
C : 85.7/12 = 7.14 mol
H : 14.3/1 = 14.3 mol

divide each by smallest number:
C : 7.14/7.14 = 1
H : 14.3/7.14 = 2

empirical formula = CH2
empirical formula mass = 12 + 2*1 = 14

MW = 42 g/mol
multiplying factor =42/14 = 3

molecular formula is C3H6