Hydrocarbons: Alkanes and Alkenes: Question 1

Worked solution

Step 1: Recall the general formula for alkenes

Alkenes form a homologous series with general formula: $C_nH_{2n}$

where $n$ is the number of carbon atoms in one molecule.

Step 2: Substitute the given number of carbon atoms

Here $n = 9$, so the number of hydrogen atoms is: $2n = 2 \times 9 = 18$

Step 3: Check why the other options are wrong

• A (9): this wrongly assumes the hydrogen count equals the carbon count.
• B (16): this comes from an incorrect calculation of $2n$, such as $2n - 2$.
• D (20): this is what you would get from the alkane general formula, $2n + 2 = 2(9)+2=20$, not the alkene general formula.

Final answers

• One molecule of this alkene contains $\boxed{18}$ hydrogen atoms, option C.