Question

a gaseous compound of carbon and hydrogen contains 80% carbon by mass, one dm3 of the compound at s.t.p weighing 1.35g. calculate

Answer 1

calculate what?

assume you have 100 grams.

then 80 grams is carbon, or 80/12 moles
and 20 grams is H, or 20/1 moles H

C=80/12=6.66 moles
H= 20

divide each by the lowest
C=1
H=3 empirical formula: CH3

Now, 22.4 liters has a mass of 1.35*22.4 = 30.24grams
divide that by the empirical mass
30.24/15=2
Molecular formula: twice the empirical formula, or C2H6

Your instructor is just too easy.