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

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.