Question

If $5mL$ of a gas containing only carbon and hydrogen is mixed with an excess of oxygen $\left(30mL\right)$ and the mixture is exploded by means of an electric spark, the volume of the mixed gases become $25mL$. On adding concentrated solution of potassium hydroxide, the volume further decreases to $15mL$ and the residual gas being pure oxygen. All volumes have been reduced to $STP$. Calculate the molecular formula of the hydrocarbon gas.

• A. $C_3{H}_6$
• B. $C_2{H}_2$
• C. $C_2{H}_6$
• D. $C_2{H}_4$

Answer 1

Answer: (D)

$C_2{H}_4$

$C_a{H}_b+\left(\frac{4a+b}{4}\right)O_2\to aC{O}_2+\frac{b}{2}{H}_{2}O$
$1$ $\left(\frac{4a+b}{4}\right)$ $a$ $\frac{b}{2}$ Initial
$5$ $5\left(\frac{4a+b}{4}\right)$ $5a$ $\frac{5b}{2}$ Final
$C{O}_2$ produced $=25-15=10mL$
$5a=10$
or, $a=2$
$O_2$ used $=30-15=15mL$
$\therefore 5\left(\frac{4a+b}{4}\right)=15$
or, $5\left(\frac{(4\times 2)+b}{4}\right)=15$
$b=4$
Hence, the formula is $C_2{H}_4$.