In hydrocarbon, the mass fraction of Carbon is 0.857. The relative density of hydrocarbon vapors in air is 1.45.

In hydrocarbon, the mass fraction of Carbon is 0.857. The relative density of hydrocarbon vapors in air is 1.45. Determine the true formula of the hydrocarbon.

Let’s implement the solution:
1. Let’s define the molar mass of a substance by the formula:
M (CxHy) = D (air) * M (air) = 1.45 * 29 = 42 g / mol;
2. Determine the number of moles of carbon:
Y (C) = m / M = 85.7 / 12 = 7 mol;
3. Let’s calculate the mass of carbon:
m (C) = 0.857 * 42 = 35.99 = 36 g;
m (H) = 42 – 36 = 6 g;
Y (H) = m / M = 6/1 = 6;
4. The sum of the masses is equal to: m (C) + m (H) = 36 + 6 = 42 g, thus corresponding to the value of the molecular weight. Suppose that the general formula of the hydrocarbon is Сn H2n
5. Determine the value – n according to the general formula:
12n + 2 = 42 g / mol;
12n = 40;
n = 3, it follows that the molecular formula of the substance: C3H6 – propene;
M (C3H6) = 12 * 3 + 1 * 6 = 42 g / mol;
Answer: the true formula of the substance: C3H6.