Derive the molecular formula of the hydrocarbon according to the data: mass fraction of carbon

| February 2, 2021 | education

Derive the molecular formula of the hydrocarbon according to the data: mass fraction of carbon = 85.7%, relative density in air = 1.45.

Given:

w% (C) = 85.7%

D (air) = 1.45

To find:

Formula -?

Decision:

1) Find the molecular weight of the hydrocarbon based on its air density:

D (HC) = Mr (HC): Mr (air), – since air does not have a definite formula, its molecular weight is conventionally taken as 29.

Mr (HC) = D (HC) * Mr (air) = 1.45 * 29 = 42.

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

Mr (C) = 42 * 0.857 = 36

N (C) = 36: Mr (C) = 36: 12 = 3 atoms.

3) Find the number of hydrogen atoms:

Mr (H) = 42 – 36 = 6

N (H) = 6: Mr (H) = 6: 1 = 6 atoms.

4) The resulting formula of the substance: C3H6 – cyclopentane or propylene, depending on which class of hydrocarbons was considered.

Answer: C3H6 is cyclopentane or propylene.



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