A straight glass tube has two inlets X and Y at the two ends. The length of the tube is 200 cm. HCl gas through inlet X and `NH_3` gas through inlet Y are allowed to enter the tube at the same time. White fumes first appear at a point Pinside the tube. Find the distance of P from X.

Explanation

Molar mass of `HCl (g) =1+35.5=36.5 g`
Molar mass of `NH_(3) (g)=14+3xx1=17.0 g`
According to Graham's law of diffusion

Let the distance between points `X` and `P` be `d cm`.
Distance between point `Y` and `P =(200-d) cm`
The distance travelled by the vapour of gas directly proportional to rate of diffusion. Therefore,
`d=0.6824xx200-0.6824d`
or `d+0.6824d=0.6824xx200=136.48`
or `1.6824 d=136.48`
or `d=136.48/1.6824=81.12 cm`
Thus, the distance between point `P` and `X=81.12 cm`, i.e., `81.12 cm` from the `HCl` end.