The relative molecular weight of the unknown substance is 28. A sample of this substance contains carbon with

The relative molecular weight of the unknown substance is 28. A sample of this substance contains carbon with a chemical amount of 0.30 mol and hydrogen with a chemical amount of 0.60 mol. Establish the molecular formula of this substance.

Given:
Mr (C (x) H (y)) = 28 g / mol
n (C) = 0.3 mol
n (H) = 0.6 mol

To find:
C (x) H (y) -?

Decision:
1) The ratio between the number of atoms in the molecule C (x) H (y) is determined by the ratio of the chemical amount of each of the elements in the molecule:
x: y = n (C): n (H) = 0.3: 0.6 = 1: 2;
Thus, the simplest formula for a substance is CH2.

2) The simplest formula of a substance does not always coincide with its true formula. The formula CH2 does not correspond to the valence of H, as well as the relative molecular weight of the substance according to the condition (28 g / mol). therefore
Mr (C (x) H (y)) / Mr (CH2) = 28/14 = 2;
That is, the number of atoms in a molecule should be 2 times more than in CH2:
x: y = 2: 4.
Therefore, the true formula of the unknown substance is C2H4 (ethylene).

Answer: The formula for the unknown substance is C2H4 (ethylene).