Question

An open flask containing air is heated from ${27}^{\circ }C$ to ${127}^{\circ }C$ K. What percentage of air will escape if the pressure is kept constant?

Answer 1

To keep the pressure constant, air has to be expelled, i.e. number of moles decreases. Note that the volume of the flask is also constant!
$n_1=n$, $T_1=300\text{K}$, $T_2=400\text{K}$, $n_2=?$
At constant pressure and volume $n_1{T}_1=n_2{T}_2$
$\therefore n_2=\frac{n_1{T}_1}{T_2}=\frac{n\times 300}{400}=\frac{3n}{4}$
$\therefore$ amount of air escaped = initial moles - final moles =$\frac{n}{4}$
$\therefore \%$ air escaped $=\frac{\frac{n}{4}}{n}\times 100=25\%$