Question

$20$ ml of a hydrocarbon requires $100$ ml of oxygen for complete combustion. In this reaction, $60$ ml of carbon dioxide is produced. The formula of hydrocarbon is:

• A. $C_2{H}_4$
• B. $C_3{H}_6$
• C. $C_3{H}_8$
• D. $C_6{H}_6$

Answer 1

The correct option is C $C_3{H}_8$

At STP, $1$ mole of any gas occupies $22.4$ L.
$20$ ml of hydrocarbon, $100$ ml of oxygen and $60$ ml of carbon dioxide corresponds to $0.89$, $4.46$ and $2.67$ moles respectively.
Hence, the combustion of $1$ mole of hydrocarbon will require $5$ moles of oxygen and will produce $3$ moles of carbon dioxide.
Thus, the molecular formula contains $3$ carbon atoms.
Out of $5$ moles of oxygen, $3$ moles will combine with carbon to form $3$ moles of carbon dioxide.
The remaining two moles of oxygen will combine with $8$ moles of hydrogen atoms (from the hydrocarbon) to form $4$ moles of water.
Thus, the molecular formula contains $8$ hydrogen atoms.
The hydrocarbon is $C_3{H}_8$ and the combustion reaction is $C_3{H}_8+5O_2\to 3C{O}_2+4H_{2}O$.