Question

When gas C is mixed with gas B, dense white fumes are seen and there is no other product. What is the name of gas C? [Note: Gas B turns moist red litmus paper blue].
A. $N{H_3}$
B. $C{l_2}$
C. $HCl$
D. $S{O_2}$

Answer 1

Hint: Litmus test is very useful to determine the acidity or basicity of a compound. For this the solution or gas of the compound is applied or absorbed on moist litmus paper. Acids turn blue litmus red and bases turn red litmus blue.

Complete step by step answer: This is an example of acid base reaction in which a salt is generated in gas phase. The given sets of options are all gases. Let us consider the gases one by one.
Ammonia gas: Ammonia gas is a moderately basic gas. It readily dissolves in water to produce ammonium hydroxide solution. The solution shows alkaline properties. Ammonia readily reacts with acids to produce salts. Ammonia gas accepts hydrogen ions and so is basic in nature.
Chlorine gas: Chlorine gas is monobasic acid. It reacts with water to produce hypochlorous acid. This lowers the pH of the solution and changes blue litmus red.
Hydrochloric acid: Hydrochloric acid is a very strong acid. It readily donates protons to bases and is very reactive towards acid base reaction. In general hydrochloric acid solution in water is used to remove bases like pyridine from reaction mixture.
Sulphur dioxide gas: Sulphur dioxide is an acidic gas. In sulphur dioxide the central atom sulfur is electron deficient and can accept electrons which make it a Lewis acid. It also converts blue litmus red.
Out of the four options available only ammonia and hydrochloric acid reacts to produce a white gas which is the vapour of ammonium chloride. The corresponding reaction is as follows:
$N{H_3}(g) + HCl(g) \to N{H_4}Cl(g)$
The gas B turns moist red litmus paper blue so it is ammonia.
The name of gas C is $HCl$ gas i.e. option C.
So, the correct answer is “Option C”.

Note: This reaction is very useful to check the presence of $HCl$ gas liberating as the product from a reaction. The gas is passed through a paper which is dipped in ammonia solution. The reverse is also performed to check the generation of ammonia gas from a reaction.