# Determine the alkane formula if the mass fraction of carbon is 81.82%.

May 28, 2021 | education

| **Determine the alkane formula if the mass fraction of carbon is 81.82%. The relative density of this alkane for hydrogen is 22. Make a structural formula and name this substance.**

1. Let us write down the general formula of alkanes: CnH2n + 2

2. Calculations:

M (CxHy) = M (H2) * D (H2) = 2 * 22 = 44 g / mol.

3. Determine the amount of carbon, hydrogen:

Y (C) = m / M = 81.82 / 12 = 6.81 mol;

m (C) = Y * M = 6.81 * 12 = 81.82 g;

m (H) = 100 – 81.82 = 18.18 g;

Y (H) = m / M = 18.18 / 1 = 18.18 mol.

4. Ratio:

X: Y = 6.81: 18.18 = 1: 2;

CH2 is the simplest formula.

5. Establish the formula for the hydrocarbon:

CnH2n + 2 = 44;

12n + 2 + 2 = 44;

12n = 40;

n = 3;

C3H8;

Checking: M (C3H8) = 12 * 3 + 8 = 44 g / mol.

Answer: C3H8 – propane.

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