A student forgot to add the reaction mixture to the round bottomed flask at 27 ^(@)C but instead he/she placed the flask on the flame. After a lapse of time, he realized his mistake, and using a pyrometer he found the temperature of the flask was 477 ^(@)C. What fraction of air would have been expelled out?

A student forgot to add the reaction mixture to the round bottomed fla

1.	A student forgot to add the reaction mixture to the round bottomed flask at 27 ^(@)C but instead he/she placed the flask on the flame. After a lapse of time, he realized his mistake, and using a pyrometer he found the temperature of the flask was 477 ^(@)C. What fraction of air would have been expelled out?
Answer» Solution :When an open flask is heated, its volume and pressure may be regarded as CONSTANT. Suppose the NUMBER of moles of air in the flask before and after heating are `n_1 and n_2` RESPECTIVELY. According to the IDEAL gas equation, `PV =n_1 RT_1 = n_1 xx R xx (273+27)` (before heating ) ...(i) and `PV =n_2 RT_2 = n_2 xx R xx (273+477)` (after heating) ...(ii) Dividing eq. (i) by (ii), we have `1 = (n_1 xx 300)/(n_2 xx 750)` `:."" n_2 = 300/750 xx n_1 = 2/5 n_1` Hence, number of moles of air expelled on heating `=n_1 - n_2 =n_1 - 2/5 n_1 = 3/5 n_1` THEREFORE, the fraction of air expelled `=(3/5 n_1)/n_1=3/5`