Find the molecular formula for a hydrocarbon with a carbon mass fraction of 85.7%.

Find the molecular formula for a hydrocarbon with a carbon mass fraction of 85.7%. The relative nitrogen vapor density of this substance is 2.

To solve the problem, let’s designate hydrocarbon: CxNu.
Let’s calculate the molar mass:
M (CxHy) = D (N2) * M (N2) = 2 * 14 = 28 g / mol;
Determine the amount of moles of carbon, hydrogen:
Y (C) = 85.7 / 12 = 7.14 mol;
m (C) = 28 * 0.857 = 23.99 g;
m (H) = 28-23.99 = 4.01 g;
Y (H) = m / M = 4.01 / 2 = 2 mol;
The ratio X: Y = C: H = 7.14: 2 = 3.57: 1. The simplest formula is C3H;
Let’s define the formula: СnH2n = 28; 12n + 2 = 28; n = 2. Molecular formula: C2H4 (ethylene)
M (C2H4) = 12 * 2 + 1 * 4 = 28 g / mol.
Answer: the molecular formula of the hydrocarbon is C2H4.