Question

Among the following, the correct order of acidity is
A. $HCl{O_3} < HCl{O_4} < HCl{O_2} < HClO$
B. $HCl{O_{}} < HCl{O_2} < HCl{O_3} < HCl{O_4}$
C. $HCl{O_2} < HCl{O_{}} < HCl{O_3} < HCl{O_4}$
D. $HCl{O_4} < HCl{O_2} < HCl{O_{}} < HCl{O_3}$

Answer 1

Acidity can be found out by oxidation state of the element. Higher is the oxidation state of the halogen means higher is the acidity of that compound. It also says that it has the tendency to accept lone pairs of electrons.

Step by step answer: Acidity is the amount of acid in a substance. It is measured on a scale called $pH$ scale. On this scale $pH$ value 7 is neutral and $pH$ value less than 7 shows acidity. Moving from 7 to 0 increases acidity.
$HCl{O_4}$ is also called perchloric acid which is a mineral acid. It is usually found as aqueous solution and is a colourless compound which is stronger than sulphuric acid and nitric acid. It is a powerful oxidiser when hot.
$HCl{O_3}$ is called chloric acid which is a chlorine oxoacid. It is a conjugate acid of chlorate. It is a strong acid and oxidizing agent. Chloric acid is prepared by the reaction of sulfuric acid with barium chlorate, the insoluble barium sulphate being removed by precipitation.
$HCl{O_2}$ is commonly called Chlorous acid is an inorganic compound and a weak acid. In this acid chlorine has +3 oxidation state. The pure substance is unstable, disproportionating to hypochlorous acid and chloric acid. Although the acid is difficult to obtain in pure substance, the conjugate base chlorite derived from this acid is stable. It is also a powerful oxidizing agent.
$HClO$ is called hypochlorous acid which is a weak acid that forms when chlorine dissolves in water. It is an oxidizer and a primary disinfection agent of chlorine solutions.
Now we can see that all four are oxidizers. Also, the acidity of the compound is based on the oxidation states of the halogen. So, higher the oxidation state means it is more acidic in nature. Calculating the oxidation states of the four compounds we get,
-$HClO$
Here, 1 is the oxidation state of the hydrogen atom, $x$ is the chlorine atom and -2 is the oxygen atom.
$1 + x - 2 = 0$
$\Rightarrow x = 1$
-$HCl{O_2}$
$1 + x - 2(2) = 0$
$\Rightarrow x = 3$
-$HCl{O_3}$
$1 + x - 3(2) = 0$
$\Rightarrow x = 5$

-$HCl{O_4}$
$1 + x - 4(2) = 0$
$\Rightarrow x = 7$
Therefore the order of oxidation state is $HCl{O_{}} < HCl{O_2} < HCl{O_3} < HCl{O_4}$ and also the order of acidity would be the same.
Therefore, the correct answer is option B.

Note: The oxidation states can be found out by multiplying the oxidation number of each atom with the number of that atom and then adding the oxidation number of all atoms with each other.