Derive the molecular formula of a substance in which the mass

Derive the molecular formula of a substance in which the mass fraction of carbon is 85.72%, for Hydrogen – 14.28%. The relative density of matter in water is 28.

Let us write down the unknown formula of the hydrocarbon in the form of CxHy.
Let us express the percentage and molar mass as a ratio.
CxHy = 85.72 / 12: 14.28 / 1 = 7.14: 14.28 = 1: 2.
Molar mass for the found substance Mr (CH2) = 14.
The true molar mass is Mr (CxHy) = D (H2) • Mr (H2) = 28 • 2 = 56.
Let’s set the true indices.
CxHy = Mr (source) / Mr (CH2) = 56/14 = 4.
So the formula of the substance C4H8 is butene.