Question

In the Dewar's method of separation of noble gases, the mixture of noble gases is kept in contact with coconut charcoal at $ 173\, K $ . Which one of the following gaseous mixtures is not absorbed in the charcoal?
(A) Ar, Kr
(B) He, Ne
(C) Xe, Ar
(D) Xe, Kr

Answer 1

Hint : Dewar’s charcoal adsorption method is a process which is used to separate a mixture of noble gases in a flask with the help of coconut charcoal which adsorb different gases at a different value of temperatures. The process of adsorption depends on the temperature. Lower the atomic mass of Noble gas, the lower will be the temperature which is required to absorb the gas.

Complete step by step answer
The dissociation of the gases is done in Dewar’s flask. This flask is double-walled filled with activated coconut charcoal at the center. A tube is placed above the flask, to introduce the gaseous mixture and to separate the unadsorbed gases.
In the first step, the mixture of noble gases is placed in contact along with charcoal kept in the Dewar's flask at a temperature of $ 173K$. It remains at that temperature for about one hour where krypton, argon, and xenon get adsorbed while helium and neon stay unadsorbed and are taken out of the flask.
In the second step mixture of He and Ne gases is placed into another bulb having coconut charcoal $ - 2 $ and is kept at a temperature of $ 93\,K $ . Here, Ne gas is adsorbed and helium gas is pumped out. The charcoal is warmed to retrieve Ne.
In the third step Argon, Krypton, and Xenon get absorbed in coming in contact with another charcoal $ - 3 $ . Argon gas diffused at $ 77\,K $ , and is recovered later.
In the fourth step, Krypton and Xenon which are in contact with Charcoal $ - 1 $ are dissociated by increasing the temperature of the charcoal to $ 183\,K $ .
From the given information we can deduce that He and Ne are the two gases that are not absorbed on the charcoal at $ 173\,K $ .
Therefore, Option (B) is correct.

Note
Adsorption capacity power of noble gases on the charcoal depends on the following factors:
At low temperatures, the atomic mass of gases increases and so does the adsorption capacity.
Adsorption capacity also varies inversely with temperature i.e. as the temperature increases the adsorption capacity decreases.