Question

A gaseous alkane on complete combustion gives $CO_{2} \, and \, H_{2}O$ . If the ratio of moles of $O_{2}$ needed for combustion and moles of $CO_{2}$ formed is $5:3.$ find out the formula of alkane.

• A. $C_{4}H_{10}$
• B. $C_{5}H_{12}$
• C. $C_{3}H_{8}$
• D. $C_{2}H_{6}$

Answer 1

Answer: (C)

Let the alkane be $C_{n}H_{2 n + 2}$

$C_{n}H_{2 n + 2}+\left[n + \frac{n + 1 \, }{2}\right]O_{2} \rightarrow nCO_{2}+\left(\right.n+1\left.\right)H_{2}O$

$\frac{M o l e \, o f \, O_{2}}{M o l e \, o f C O_{2} \, }=\frac{n + \frac{n + 1 \, }{2}}{n}=\frac{5}{3}$

$n=3 \, $

Thus the alkane is $C_{3}H_{8}$