Determine the chemical formula of alkene, which has a relative vapor density of a substance for hydrogen of 49.

The hydrogen vapor density of a substance is the ratio of the molecular weight of that substance to the molecular weight of hydrogen. Let’s write the formula:
D (H2) = M (substance) / M (H2)
Whence the molecular weight of our substance is: D (H2) * M (H2) = 49 * 2 = 98 g / mol
The condition says that our substance is alkene, that is, we can use its general formula to find the number of carbon and hydrogen atoms in it.
General formula of alkenes: CnH2n
Let’s write down the formula for the total mass of such an alkene:
12 * n + 2 * n = 14 * n
Where from:
14 * n = 98
n = 98/14 = 7
The number of hydrogen atoms is: 7 * 2 = 14
Alkene formula: С7Н14