Question

$20cc$ of a hydrocarbon, on complete combustion, gave 80 cc of $C{O}_2$ and 100 cc of $H_{2}O$ at STP. The empirical formula of that compound is:

• A. $C_2{H}_2$
• B. $C_2{H}_6$
• C. $C_3{H}_8$
• D. $C_4{H}_{10}$

Answer 1

Answer: (D)

$C_4{H}_{10}$

The balanced equation for the combustion is as follows:
$C_x{H}_y+[x+\frac{y}{4}]O_2\to xC{O}_2+\left[\frac{y}{2}\right]H_{2}O$
$20$ cc of $C_x{H}_y$ gives $=80$ cc of $C{O}_2$

moles correspond to volume at same conditions of pressure and temperature.
$1$ vol. of $C_x{H}_y$ $=4$ vol. of $C{O}_2$
$x=4$
$20$ cc of $C_x{H}_y$ gives $=100$ cc of $H_{2}O$
$1$ vol. of $C_x{H}_y$ $=5$ vol. of $H_{2}O$
$\therefore \frac{y}{2}=5;y=10$
$\therefore$ Formula of $C_x{H}_y$ $=C_4{H}_{10}$