Ammonia, ${\text{M}}_{\text{r}}=17$ and hydrogen chloride, ${\text{M}}_{\text{r}}=36.5$ diffuse in the tube shown .

When these gases meet, they produce a white cloud of ammonium chloride.

Where in the tube does the white cloud first appear?

According Graham’s law, rate of diffusion is inversely proportional to molecular weight. ${\text{rate}}_{\text{gas}}\propto \frac{1}{\sqrt{\text{M}}}$

${\text{For\hspace{0.17em}NH}}_{\text{3}},\text{\hspace{0.17em}molecular\hspace{0.17em}mass\hspace{0.17em}}=17$

$\text{For HC}l,\text{\hspace{0.17em}molecular\hspace{0.17em}mass =\hspace{0.17em}}36.5$

As HCl has greater molecular weight it can diffuse slowly.

${\text{NH}}_{\text{3}}+\text{HC}l\to {\text{NH}}_{\text{4}}\text{C}l$