Question

When a hydrocarbon A undergoes complete combustion it requires 11 equivalents of oxygen and produces 4 equivalents of water. What is the molecular formula of $A$?

• A. $C _{11} H _8$
• B. $C _{11} H _4$
• C. $C _5 H _8$
• D. $C _9 H _8$

Hint:

When solving combustion problems, remember that the number of moles of oxygen is related to the number of moles of carbon and hydrogen in the molecule.

Answer 1

The Correct Option is D

$C_x{H}_y+\left(x+\frac{y}{4}\right)O_2\to xC{O}_2+\frac{y}{2}{H}_{2}O$
$\frac{y}{2}=4\therefore y=8$
$x+\frac{8}{4}=11$
$\therefore x=9$
$\therefore$ Hydrocarbon will be$=C_9{H}_8$
So, the correct option is (D): \(C_9H_8\)

Answer 2

Let the molecular formula of the hydrocarbon be \( C_xH_y \). The balanced combustion reaction is:

\[ C_xH_y + \left( \frac{x + \frac{y}{4}}{2} \right) O_2 \rightarrow x \, CO_2 + \frac{y}{2} \, H_2O \]

We are given that 11 equivalents of oxygen are required, and 4 equivalents of water are produced. Using stoichiometry:

\[ \frac{y}{2} = 4 \quad \Rightarrow \quad y = 8 \]

Substituting in the oxygen requirement:

\[ x + \frac{8}{4} = 11 \quad \Rightarrow \quad x + 2 = 22 \quad \Rightarrow \quad x = 9 \]

Thus, the molecular formula is \( C_9H_8 \).