Question

A student forgot to add the reaction mixture to the round bottomed open flask at 27°C and put it on the flame. After a lapse of time, he realized his mistake, using a pyrometer he found the temperature of the flask was 477°C. What fraction of air would have been expelled out?  

Answer 1

Initial mole of air in flask = n₁ 

Temperature = 21 + 273 = 300 K 

Final mole of air in flask = n₂ 

Temperature = 477 + 273 = 750 K 

The volume and pressure remains constant, thus, n₁ T₁ = n₂ T₂

n₂ = n₁ x 300/750 = 2/5 n₁

Thus, air expelled out on heating = n₁ - n₂ = n₁ - (2/5) n₁ = 3/5 n₁

or fraction of air expelled = 3/5.