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A bottle of dry ammonia and a bottle of dry hydrogen chloride connected through a long tube are opened simultaneously at both ends. The white ammonium chloride ring first formed will be:
(A) At the center of the tube
(B) Near the hydrogen chloride bottle
(C) Near the ammonia bottle
(D) Throughout the length of the tube

Answer
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Hint: Ammonia is basic in character while hydrogen chloride is an acid. They both reacted in a test tube to form white rings of ammonium chloride ($N{H_4}Cl$). The formation of the ring depends upon the rate of diffusion of these two gases.

 Complete step by step answer:
 The chemical formula of ammonia is $N{H_3}$. It is formed by covalent bonding between Hydrogen and Nitrogen with a lone pair on nitrogen while the chemical formula of hydrogen chloride is $HCl$ and it is also formed by covalent bonding between Hydrogen and chlorine. When these compounds are passed in test tubes in their gaseous states they diffuse together to yield white rings of ammonium chloride ($N{H_4}Cl$). The reaction for this is;
$N{H_3}(g)\, + \,HCl(g) \to \,N{H_4}Cl(s)$
According to graham’s law of diffusion of gases, the rate of diffusion is inversely proportional to the molecular mass of the gases i.e.
$rate\,\alpha \dfrac{1}{{\sqrt M }}$
It means the gas that will have the higher molecular mass will diffuse slowly and the gas with lower molecular mass will diffuse faster. Now, we will compare the rate of diffusion of ammonia and hydrogen chloride.
The molecular mass of $N{H_3}$ is $17$ and molecular mass of $HCl$ is $36.5$. Hence, among ammonia and hydrogen chloride $N{H_3}$ has lower molecular mass, so it will have a higher rate of diffusion than $HCl$. So the white ammonium chloride ring first formed will be near the Hydrogen chloride bottle.
Therefore, option (B) is correct.

Note: Diffusion is the net movement of atoms or molecules from a region of high concentration to a region of low concentration. The rate of diffusion of gases depends upon the molecular mass of the gases.