The mass fraction of carbon in the hydrocarbon is 84.21%, the mass fraction of hydrogen is 15.79%

| September 21, 2021 | education

The mass fraction of carbon in the hydrocarbon is 84.21%, the mass fraction of hydrogen is 15.79%. The relative density of the hydrocarbon in air is 3.93. Derive its molecular formula.

1. Let’s find the masses and quantities of elements in 100 g of hydrocarbon:

m (C) = (m (in-va) * ω (C)) / 100% = (100 g * 84.21%) / 100% = 84.21 g.

n (C) = m (C) / M (C) = 84.21 g / 12 g / mol = 7 mol.

m (H) = (m (in-va) * ω (H)) / 100% = (100 g * 15.79%) / 100% = 15.79 g

n (H) = m (H) / M (H) = 15.79 g / 1 g / mol = 15.79 mol.

2. Find the molar mass of the hydrocarbon:

M (in-va) = Dair * M (air) = 3.93 * 29 g / mol = 114 g / mol.

3. Let’s find the formula of the substance:

n (C): n (H) = 7 mol: 15.79 mol = 1: 2.25 = 4: 9 ⇒ initial formula C4H9.

M (C4H9) = 57 g / mol, which means you need to double the formula, since the molecular formula of the substance should be 114 g / mol ⇒ C8H18.

Answer: C8H18.



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