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Last updated date: 06th Dec 2023
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MVSAT Dec 2023

Assertion: $ HCl $ is collected by upward displacement of air.
Reason: $ HCl $ gas is lighter than air.
A) If both assertion and reason are correct and reason is the correct explanation of the assertion.
B) If both assertion and reason are correct but reason is not the correct explanation of the assertion.
C) If the assertion is correct, but the reason is incorrect.
D) If the assertion is incorrect, but the reason is correct.

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Hint: In the above question, we will calculate the molecular mass of the gas to predict if it weighs lighter than the air. The upward displacement of the air depends upon the weight and the density of the molecule with respect to the air.

Complete answer:
We know that all the gases that are lighter than air has a molecular weight which is less than ${l^{ - 1}} $
Molecular weight of $ HCl $ :
= Molecular weight of $ {H^ + } $ Molecular weight of $ Cl $
 $ = 1.01 + 35.45 $
 $ ={l^{ - 1}} $
Which is greater than ${l^{ - 1}} $
Therefore, $ HCl $ gas is heavier than air which contradicts the reason mentioned in the question. Hence, the reason is incorrect.
Upward displacement of air is the process of heating solid reactants to produce and absorb a gas that is less dense (lighter) than air. The less dense gas reaches into the denser air and displaces it downwards. The term "upward delivery" or "displacement" refers to this type of gas preparation. Since hydrogen chloride gas is $ 1.28 $ times heavier than air, it is obtained by downward delivery (upward displacement of air). Since it is strongly soluble in water, it is not collected over water.
Hence, the assertion is correct.
Therefore, the correct option is (C) If the assertion is correct, but the reason is incorrect.

When $ HCl $ is dissolved in water, it breaks down into the ions $ {H^ + } $ and $ C{l^ - } $ . These ions are what keep water stable, and they're usually quite dissimilar to one another. $ HCl $ dissolves in water despite its covalent nature because it is able to break up into ions.