When an open vessel is heated from 300K to 500K, What percentage of the gas will escape out?

For open vessel 

$n_1{T}_1=n_2{T}_2\Rightarrow 1\times 300=n_2\times 500\Rightarrow n_2=\frac{3}{5}$

Number of  moles escaped out = $1-\frac{3}{5}=\frac{2}{5}=0.4$

% of gas escaped out = 40%