Question

125 cm³ of ethane is burnt in just sufficient air (containing 20% oxygen by volume). Find the total volume of the mixture at constant temperature and pressure.

\[\ce{C2H6 + 7O2 -> 4CO2 + 6H2O}\]

Answer 1

Given, 125 cm³ C₂H₆

1. Oxygen required:

`125 xx 7/2`

= 437.5 cm³

2. Air required:

Air contains 20% O₂:

`(437.5 xx 100)/20`

= 2187.5 cm³

Nitrogen in air:

2187.5 − 437.5

= 1750 cm³

3. Gases formed:

CO₂ ​= `125 xx 4/2`

​= 250 cm³

H₂​O = `125 xx 6/2`

​= 375 cm³

Since the question only says constant temperature and pressure and does not say that the mixture is cooled, water remains as water vapour.

Therefore, Total volume = 1750 + 250 + 375 = 2375 cm³