Determine the mass fraction of carbon in an alkane, the relative density of which in air is 3.448.

Given: CxNu – alkane class hydrocarbon;
D (air) = 3.448;
M (air) = 29 g / mol;
Find: molecular formula CxHy;
W (C) =?
Let’s implement the solution:
1. Let’s calculate the molecular weight of organic matter:
M (CxHy) = D (air) * M (air) = 3.448 * 29 = 99.99 g / mol.
2. Determine the molecular formula:
CnH2n + 2 is the general formula for alkanes;
CnH2n + 2 = 99.99;
12n = 99.99-4;
12n = 95.99;
n = 7.9;
C7H16 – heptane.
3. Let’s make calculations using the formula, determine the mass fraction of carbon:
Y (C) = W / M * 100;
W (C) = 7/12 * 100 = 84%.
Answer: in heptane, the mass fraction of carbon is 84%.