Find the molecular formula of the hydrocarbon, the carbon content of which is 80%

Find the molecular formula of the hydrocarbon, the carbon content of which is 80%, and the hydrogen content is 20%, the relative density for hydrogen is 15.

Let’s designate the unknown substance as CxHy.
Let’s compose the ratio of the percentage to molar mass.
CxHy = 80/12: 20/1 = 6.7: 20.
Let’s round to the nearest whole number by dividing by the smallest value.
CxNy = 1: 3.
The molar mass of the obtained substance is avgmr (CH2) = 15.
Let us find the true molar mass of a substance, knowing the vapor density of hydrogen.
Mr (source) = D (H2) • Mr (H2) = 15.4 • 2 = 30.8.
Let’s set the coefficients according to the formula.
CxHy = Mr (source) / Mr (CH3) = 30.8 / 15 = 2.
So the formula of the substance C2H6 is ethane.