Mass fraction of carbon 53.3%, hydrogen – 15.6%, nitrogen – 31.1%, air density 1.551. Derive the molecular formula.

Let’s find the molecular weight of the unknown substance:
29 * 1.551 = 45 g / mol (29 g / mol is the molecular weight of air);

Find the molecular weight of carbon in the compound:
45 * 0.533 = 24 g / mol;

The relative atomic mass of carbon is 12. Let’s find the number of carbon atoms in the substance:
24/12 = 2 atoms;

Find the molecular weight of hydrogen in the compound:
45 * 0.156 = 7 (the number of hydrogen atoms is 7, since the relative atomic mass of hydrogen is 1);

Let’s find the molecular weight of nitrogen in the substance:
45 – 24 – 7 = 14 g / mol (the number of nitrogen atoms in the substance is 1, since the relative atomic mass of nitrogen is 14);

Answer: the unknown substance is ethylamine, its formula is C2H5NH2.