Question

A hydrocarbon has $4$ hydrogen atoms. Give its molecular formula if its alkyne.

• A. ${CH}_4$
• B. $C_3{H}_4$
• C. $C_2{H}_6$
• D. $C_2{H}_4$

Answer 1

The correct option is B

${\mathrm{C}}_3{\mathrm{H}}_4$

Explanation for correct option:

(B) ${\mathrm{C}}_3{\mathrm{H}}_4$

• An alkyne is an unsaturated hydrocarbon that contains at least one C-C triple bond.
• The general formula for alkynes is specified as ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}-2}$. Here, $\mathrm{n}$ represents the number of carbon atoms.
• The number of hydrogen atoms for the molecular formula is 4.
• The number of the carbon atoms can be calculated as follows:

$$\begin{gathered} 2\text{n}-2=4 \\ \text{n}=3 \end{gathered}$$

So, the number of carbon atoms in the molecular formula is 3. Hence, the molecular formula of alkyne will be ${\mathrm{C}}_3{\mathrm{H}}_4$.

Explanation for incorrect options:

(A) Methane $\left({\mathrm{CH}}_4\right)$ is a saturated hydrocarbon and it does not have any C-C triple bond thus, it is an alkane not alkyne.

(C) Ethane $\left({\mathrm{C}}_2{\mathrm{H}}_6\right)$ has C-C single bond thus, it is an alkane not alkyne.

(D) Ethene $\left({\mathrm{C}}_2{\mathrm{H}}_4\right)$ is an unsaturated hydrocarbon and it has one C-C double bond thus, it is an alkene not alkyne.

Hence, the correct option is (B) i.e. ${\mathrm{C}}_3{\mathrm{H}}_4$.