Find the molecular formula of a hydrocarbon, the mass fraction of hydrogen in which is 20%;

Find the molecular formula of a hydrocarbon, the mass fraction of hydrogen in which is 20%; the relative density of hydrocarbons in the air is 1.035

Let us determine the molecular weight of the hydrocarbon if its density by air is known:
M (CxHy) = M (air) * D (air) = 29 * 1.035 = 30 g / mol;
Let’s calculate the number of moles of hydrogen by the formula:
Y (H) = m / M = 20/1 = 20 mol;
From here, we find the mass of hydrogen:
m (H) = Y * M = 20 * 1 = 20 g;
Let’s calculate the mass of carbon:
X (C) = M (substances) – M (H2) = 30 – 20 = 10 g;
Ratio. C: H = 10: 20 = 1: 2;
The simplest formula: CH2;
Based on the simplest formula, we write down the general formula for the hydrocarbon;
СnH2n + 2, we find the value – n
12n + 2 + 2 = 30;
12n = 26;
n = 2, it follows that the molecular formula: C2H6, M (C2H6) = 2 * 12 + 1 * 6 = 30 g / mol.
Answer: The molecular formula C2H6 belongs to ethane.