Question

Which one of the following oxyacids of chlorine is the least oxidising in nature?
A) $HOCl$
B) $HCl{{O}_{2}}$
C) $HCl{{O}_{4}}$
D) $HCl{{O}_{3}}$

Answer 1

The answer here is dependent on the calculation of the oxidation state of chlorine in each compound and one which has the highest oxidation number then that compound will be the least oxidising compound in nature.

Complete answer:
In the classes of inorganic chemistry, we have dealt with the topics of calculation of oxidation states of an element in the given compound and change in its oxidation states when they combine with other reactants.
Now, we shall see which among the given compounds have low oxidising power.
- Oxidation state of any atom is defined as the total number of electrons that an atom gains or loses in order to form a chemical bond.
- Oxidation state is also called as oxidation number is calculated and the values can either be in a positive integer or a negative integer.
In option A) $HOCl$, also called hypochlorous acid has the oxidation number as shown below where the oxidation state of chlorine is considered as ‘x’.
$x+(+1)+(-2)=0$
$\Rightarrow x=+1$
Similarly in option B) $HCl{{O}_{2}}$ also called as chlorous acid the oxidation state of chlorine is calculated as,
$x+(+1)+(-2\times 2)=0$
$\Rightarrow x=+3$
In option C) $HCl{{O}_{4}}$ that is perchloric acid, the oxidation state is as calculated below,
$x+(+1)+(-2\times 4)=0$
$\Rightarrow x=+7$
Similarly, in option D) $HCl{{O}_{3}}$ also known as chloric acid, the oxidation state of chlorine in this case is,
$x+(+1)+(-2\times 3)=0$
$\Rightarrow x=+5$
Now, if we see all the oxidation states of the given compounds the highest oxidation state is of $HCl{{O}_{4}}$ that is +7 and therefore, this compound has the least oxidising power as it cannot lose more of electrons.

Thus, the answer is option C) $HCl{{O}_{4}}$.

Note:
Be thorough with the common names given to the compounds as the questions based on this can also be asked instead of molecular formula and also note that there are only four oxyacids of chlorine.