A gaseous alkane is exploded with oxygen. The moles of $O_{2}$ for complete combustion and $C O_{2}$ formed is in the ratio $7 : 4$ . Deduce the molecular formula of alkane.

C_{2} H_{6}

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C H_{3}

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C_{3} H_{6}

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C_{2} H_{4}

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Solution

The correct option is B $C_{2} H_{6}$
Let the formula of the gaseous alkane be $C_{x} H_{y}$ . The balanced chemical equation for the combustion reaction is
$C_{x} H_{y} + \frac{2 x + \frac{y}{2}}{2} O_{2} \to x C O_{2} + \frac{y}{2} H_{2} O$ .
For complete combustion, the moles of $O_{2}$ required is $\frac{2 x + \frac{y}{2}}{2}$ .
The moles of $C O_{2}$ formed is $x$ .
The ratio of the moles of $O_{2}$ required to the moles of $C O_{2}$ formed is $\frac{\frac{2 x + \frac{y}{2}}{2}}{x} = \frac{7}{4}$ .
i.e., $7 x = 4 x + y$
or, $3 x = y$
$x : y = 1 : 3$
Hence, the molecular formula of the alkane is $C_{2} H_{6}$ .

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A gaseous alkane is exploded with oxygen. The moles of O2 for complete combustion and CO2 formed is in the ratio 7:4. Deduce the molecular formula of alkane.

A gaseous alkane is exploded with oxygen. The moles of $O_{2}$ for complete combustion and $C O_{2}$ formed is in the ratio $7 : 4$ . Deduce the molecular formula of alkane.