In a glass tube of 1 m length and uniform cross section, (g) from one end and (g) from the other end — States of Matter and Gaseous State Chemistry Question

Question

In a glass tube of 1 m length and uniform cross section, $\text{NH}_3$(g) from one end and $\text{BF}_3$(g) from the other end are sent at a time. The distance from the ammonia end where the addition compound, $\text{BF}_3 \cdot \text{NH}_3$ will first form is (B = 11; F = 19)