On igniting a mixture of acetylene [C2H2] and oxygen, 200 cm3 of CO2 is collected at s.t.p. Calculate the volume of acetylene & O2 at s.t.p. in the original mixture.

[By Lussac’s law]

2C₂H₂   +    5O₂  →   4CO₂ + 2 H₂O

2 vol       :    5 vol →   4 vol   : 2 vol

To calculate the volume of acetylene

CO₂   :  C₂H₂

4        :    2

200   :   x

⇒ x = 200 × 2\over 4 = 100 cm³

Hence, volume of acetylene in the original mixture is 100 cm³.

To calculate the volume of oxygen

CO₂   :  O₂

4        :   5

200   :   x

⇒ x = 200 × 5\over 4 = 250 cm³

Hence, volume of oxygen in the original mixture is 250 cm³.