A straight glass tube has two inlets `x` and `y` at 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 flames first appear at a point `P` inside the tube. Find the distance of `P` from `x`.

Explanation

According to Graham's law of diffusion
`(Rate of diffusion of HCl)/(Rate of diffusion of NH_(3))=sqrt((M_(NH_(3)))/(M_(HCl)))` (`1`)

Let the distance between `P` and `x` be `d cm`.
Then the distance between`P` and `y` will be `(200-d)cm`.
These two gases meet at point `P` in time `t`.
Rate of diffusion`=(Distance)/(Time)`
Rate of diffusion of `HCl=(d)/(t)`
Rate of diffusion of `NH_(3)=(200-d)/(t)`
Putting in equation (`i`), we get
`(d//t)/((200-d)//t)=sqrt((17)/(36.5))`
or `(d)/(200-d)=0.6824`
`:. d=81.12 cm`