Question

A flask of ammonia is connected to a flask of an unknown acid HX by a 1.80 m glass tube. As the two gases diffuse down the tube, a white ring of NH4X forms 123 cm from the ammonia flask.

Calculate the molar mass of HX.

Answer 1

Solution-Using Grahams Law of diffusion
The velocity of diffusion of a gas is inversely proportional to the square root of its molar mass.
Therefore
V1/V2 = sqrt(M2/M1)
Since both of them traveled in the same time ratio of their velocities = ratio of distances they traveled.
So,
d1/d2 = sqrt(M2/M1)
Length of glass tube =1.80 m = 180cm
Distance travelled by Ammonia = 123cm
Distance travelled by HX = 180 - 123 = 57cm
123/57 = sqrt(M/17)

=>(2.15)^2= M/17

=>4.6225*17=M
M = 78.58 g/mol