Question

Air contains $20\%$ ${O_2}$ by volume. How many cc of air will be required for oxidation of 100 cc of acetylene.

Answer 1

To solve this question, we need to first understand the ratio of the volumes of the reactants required for this reaction. Then we can individually solve for first the volume of oxygen required, and then for the volume of air required.

Complete Step-by-Step Answer:
Before we move forward with the solution of the given question, let us first understand some important basic concepts.
Let us first understand the reaction that takes place when we oxidise acetylene. Acetylene is another name for ethylene and is a pure hydrocarbon. We know that burning pure hydrocarbons results in the formation of only carbon dioxide and water. Similarly, when we oxidise or burn ethylene, the products formed are just carbon dioxide and water. A balanced chemical reaction for the same can be given as:
$2{C_2}{H_2} + 5{O_2} \to 2C{O_2} + {H_2}O$
Hence, for every 2 moles of ethylene, we require 5 moles of oxygen. For gaseous reactants and products that are at the same temperature and pressure, the mole ratio is equal to the volume ratio. Hence, we can say that for oxidizing every 2 units volumes of acetylene, we require 5 units volumes of oxygen.
Let us now look at the data that has been given to us:
1. $\%$ composition of air $= 20\%$ of ${O_2}$
2.Volume of acetylene $= V = 100cc$
To find: volume of air required to oxidize the given sample of acetylene
The relation between the volume ratio of ethylene and oxygen is $2:5$. Hence, considering the required volume of oxygen to be ‘x’, we can say that:
$2V \to 5V$

$\Rightarrow$ $100cc \to x$
$\Rightarrow$ $\dfrac{{2V}}{{100}} = \dfrac{{5V}}{x}$
$\Rightarrow$ $x = 250cc$
Hence, 250 cc of oxygen would be required. Now we know that air contains $20\%$ oxygen by volume. Hence, we can say that:
$0.2({O_2}) \to 1(air)$
$250({O_2}) \to (n)(air)$
$\Rightarrow$ $n = \dfrac{{250}}{{(0.2)}}$
$\Rightarrow$ $n = 1250cc$

Hence, 1250 cc of air would be required for this reaction

Note: If there is too much carbon or too little oxygen present when hydrocarbons are burned, carbon monoxide (CO) may also be emitted. Sometimes unburned hydrocarbons are released into the air during incomplete combustion.