Two elemental gases ($A$ and $B$) are reacting to form a liquid ($C$) in a steady state process as per the reaction $A+B{\rightarrow}C$. The single-pass conversion of the reaction is only $20\%$ and hence recycle is used. The product is separated completely in pure form. The fresh feed has $49$ mol$\%$ of $A$ and $B$ each along with $2$ mol$\%$ impurities. The maximum allowable impurities in the recycle stream is $20$ mol$\%$. The amount of purge stream (in moles) per $100$ moles of the fresh feed is ___________