Acetylene [C2H2] burns in air forming carbon dioxide and water vapour. Calculate the volume of air required to completely burn 50 cm3 of acetylene. [Assume air contains 20% oxygen].

[By Lussac’s law]

2C₂H₂    +     5O₂      →     4CO2    +    2H₂O

2 vol        :     5 vol     →    4 vol       :    2 vol

To calculate the volume of air required,

C₂H₂      :      O₂

2             :       5

50          :        x

Therefore, volume of oxygen (x),

⇒ x = 50 × 5\over 2 = 125 cm³

When oxygen is 20% then air is 100%

Therefore when, oxygen is 125 cm³ then air is

⇒ x = 125 × 100\over 20 = 625 cm³

Therefore, volume of air required is 625 cm³.