Question

One mole of an alkane on complete combustion required 8 mole of $O_2$, find out sum of carbon and hydrogen atoms in one molecule of the alkane.

Answer 1

Stoichiometry of hydrocarbon combustion. We use the balancing coefficients of oxygen to determine the number of carbon atoms in the alkane.

LOGIC:
1. The balanced equation for any alkane combustion is:
$C_n H_{2n+2} + \text{x} O_2 \rightarrow n CO_2 + (n+1) H_2O$
2. To balance Oxygen atoms on both sides:
$2\text{x} = 2n + (n + 1) = 3n + 1$
$\text{x} = \frac{3n + 1}{2}$
3. Given $\text{x} = 8$:
$8 = \frac{3n + 1}{2} \Rightarrow 16 = 3n + 1 \Rightarrow 3n = 15 \Rightarrow n = 5$
4. The formula of the alkane is $C_5H_{2(5)+2} = C_5H_{12}$.
5. Sum of atoms = $5 + 12 = 17$.