Derive the molecular formula of a hydrocarbon, the mass fraction of carbon in which is 75%.

Derive the molecular formula of a hydrocarbon, the mass fraction of carbon in which is 75%. Its relative density for oxygen is 0.5.

Given:

w% (C) = 75%

D (O2) = 0.5

To find:

Formula -?

Decision:

1) Find the molecular weight of a hydrocarbon by its oxygen density:

D (O2) = Mr (UV): Mr (O2)

Mr (HC) = D (O2) * Mr (O2) = 0.5 * 32 = 16

2) Knowing the mass fraction of carbon in the compound, we find its molecular weight and the number of atoms:

16 – 100%

x – 75%,

then x = 75% * 16: 100% = 12

N (C) = 12: Mr (C) = 12: 12 = 1 atom.

Then the molecular weight of hydrogen accounts for 16 – 12 = 4,

N (H) = 4: 1 = 4 atoms.

We got the formula CH4 – methane.

Answer: CH4 is methane.