In a glass tube of 1 m length and uniform cross section, (g) from one end and BF3(g) from the other — States of Matter and Gaseous State Chemistry Question

Question

In a glass tube of 1 m length and uniform cross section, $NH_3$(g) from one end and BF3(g) from the other end are sent at a time. The distance from the ammonia end where the addition compound, BF3·$NH_3$ will first form is (B = 11; F = 19)