Question

: Air contains nearly $20\%$ oxygen by volume. The volume of air needed for complete combustion of 100 mL of acetylene will be:
A.500 mL
B.100 mL
C.250 mL
D.1250 mL

Answer 1

Oxidation basically refers to the addition of oxygen in a molecule. Addition of oxygen will occur both in carbon and hydrogen in case of acetylene. From the reaction of oxidation of hydrocarbons we can get to know how many moles of reactant will produce how much product and then we can calculate the volume using the same ratio. We can consider the total volume of air as 100 ml because the percentage is always calculated from 100.

Complete step by step solution:
Complete Oxidation of hydrocarbons always produces carbon dioxide and water. The molecular formula for acetylene is ${{\text{C}}_2}{{\text{H}}_2}$. We can write this in expanded form as ${\text{HC}} \equiv {\text{CH}}$. We will write the equation for oxidation of ${{\text{C}}_2}{{\text{H}}_2}$ as:
${\text{HC}} \equiv {\text{CH + }}\dfrac{5}{2}{{\text{O}}_2} \to 2{\text{C}}{{\text{O}}_2} + {{\text{H}}_2}{\text{O}}$
As we can see from that 1 mL of ${{\text{C}}_2}{{\text{H}}_2}$ require $\dfrac{5}{2}$ mL of oxygen for complete combustion. So for the combustion of 100 mL of ${{\text{C}}_2}{{\text{H}}_2}$ we will require $\dfrac{5}{2} \times 100{\text{ mL}}$of ${{\text{O}}_2}$, which is equal to $250{\text{ mL}}$.
We have been given that the volume of oxygen in air is $20\%$. Percentage is always calculated from $100$ so, from this we came to know that 20 ${\text{mL}}$ of oxygen is contained in 100 ml of air. But we want to know the volume of air required for $250{\text{ mL}}$, this will be:
$\dfrac{{100}}{{20}} \times 250{\text{ mL = 1250 mL}}$.

Hence the correct option is D..

Note:
Acetylene is an unsaturated hydrocarbon, containing one triple bond. It is the first member of alkynes series. It is unstable at room temperature and does not exist in liquid state. It is widely used as a fuel and in plastic derivatives.