Question

For $20{\text{ ml}}$ of hydrocarbon requires $100{\text{ ml}}$ of oxygen for complete combustion. In this reaction ${\text{60 ml}}$of carbon dioxide is produced. The formula of hydrocarbon is?
A.${{\text{C}}_2}{H_4}$
B.${{\text{C}}_3}{H_8}$
C.${{\text{C}}_3}{H_6}$
D.${{\text{C}}_6}{H_6}$

Answer 1

Combustion can be defined as a chemical exothermic reaction at a high temperature in which a substance (generally hydrocarbons) reacts with oxygen to produce gaseous substances or mixture of fumes.

Complete answer:
To solve this question, we have to make use of stoichiometry. We have to begin by balancing the different atoms present on both sides using their stoichiometric coefficients.
So let’s begin,
Since we all know very well that,
At STP, $1$ mole of any gas acquires $22.4l$.
So, accordingly, $20{\text{ ml}}$ of hydrocarbon will correspond to $0.89{\text{ moles}}$
And $100{\text{ ml}}$ of oxygen and ${\text{60 ml}}$ of carbon dioxide will correspond to ${\text{4}}{\text{.46}}$ and ${\text{2}}{\text{.67}}$ moles respectively.
So, the combustion of $1$ mol of hydrocarbon needs ${\text{5}}$moles of oxygen and ${\text{3}}$moles of carbon dioxide will be produced.
So, it is clear that the molecular formula of hydrocarbons must be having ${\text{3}}$ carbon atoms in it.
And also out of ${\text{5}}$ moles of oxygen, ${\text{3}}$ moles will be used by carbon dioxide.
The remaining ${\text{2}}$ moles of oxygen will combine with ${\text{4}}$ moles of water, containing ${\text{8}}$ moles of hydrogen atom.
Thus, the hydrocarbon contains the ${\text{8}}$ hydrogen atom.
Hence, hydrocarbons contain ${\text{3}}$ carbon atoms and ${\text{8}}$ hydrogen atoms.
So option B is the correct answer i.e. ${{\text{C}}_3}{H_8}$.

Note:
Let’s have a look at the complete combustion reaction of this hydrocarbon with oxygen.
And the reaction is:
${{\text{C}}_3}{H_8} + 5{O_2} \to 3C{O_2} + 4{H_2}O$