Determine the formula for a hydrocarbon containing 80% carbon. The relative density for hydrogen is 15.

Let’s find the mass content of hydrogen in an unknown hydrocarbon:

w (H) = 100 – w (C) = 100 – 80 = 20 (%).

Mass ratio of elements in an unknown hydrocarbon:

w (C): w (H) = 80: 20

Taking into account the atomic weights of the elements, let’s move on to the quantitative ratio:

n (C): n (H) = w (C) / M (C): w (H) / M (H) = 80/12: 20/1 = 6.67: 20 = 1: 3

This means that in an unknown hydrocarbon there are 3 H atoms per 1 C atom – thus, its formula can be written as CnH3n.

Let’s find the molecular weight of the unknown hydrocarbon:

M (CnH3n) = M (H2) * DH2 (CnH3n) = 2 * 15 = 30 (amu).

Let’s compose and solve the equation:

12 * n + 1 * 3n = 30, whence n = 2 and the hydrocarbon formula: C2H6 (ethane).