Question

Anhydride of sulphuric acid is:
A) $\text{ S}{{\text{O}}_{\text{2}}}\text{ }$
B) $\text{ S}{{\text{O}}_{3}}\text{ }$
C) $\text{ }{{\text{H}}_{\text{2}}}{{\text{S}}_{\text{2}}}{{\text{O}}_{3}}\text{ }$
D) $\text{ }{{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{3}}\text{ }$

Answer 1

Anhydride is the compound that is obtained by the elimination of water. Anhydrides of inorganic acids are generally the oxides of non-metallic elements. For example, carbon dioxide $\text{ C}{{\text{O}}_{\text{2}}}\text{ }$ is an anhydride of carbonic acid $\text{ }{{\text{H}}_{\text{2}}}\text{C}{{\text{O}}_{3}}\text{ }$.

Complete Solution :
- The anhydride is the compound that is obtained by the elimination of water. The acid anhydrides are defined as the chemical compounds which are derived by the elimination of water molecules from an acid molecule. In inorganic chemistry, the acid anhydrides are referred to as acidic oxide.
- These acidic oxides absorb or interact with water molecules to form their corresponding oxyacid. The oxyacid is a compound that contains oxygen or carbonic acid. Anhydrides are highly reactive.
- We are interested in determining the anhydride of sulphuric acid. The anhydride can be determined by simply eliminating the water molecule $\text{ }{{\text{H}}_{\text{2}}}\text{O }$ from the molecular formula. The molecular formula of sulphuric acid is $\text{ }{{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}\text{ }$ . Lets eliminate a water molecule. We have,
$\text{ }{{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}\text{ }-{{\text{H}}_{\text{2}}}\text{O = S}{{\text{O}}_{\text{3}}}\text{ }$
- The elimination of water molecules results in the generation of sulphur trioxide. In other words, when sulphur trioxide gas is passed through the water produces sulphuric acid. The reaction is as shown below:
$\text{ S}{{\text{O}}_{\text{3}}}\text{(}g\text{) + }{{\text{H}}_{\text{2}}}\text{O(}l\text{) }\to \text{ }{{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}\text{(}aq\text{) }$
Therefore, sulphur trioxide $\text{ S}{{\text{O}}_{\text{3 }}}$ is an anhydride of sulphuric acid.

So, the correct answer is “Option B”.

Note: Note that, there is a simple trick to determine the anhydride. Consider an acid and then subtract a water molecule $\text{ }{{\text{H}}_{\text{2}}}\text{O }$ formula from its molecular formula. The obtained result is an anhydride of a compound. There are basic anhydride too. For example, calcium oxide $\text{ CaO }$ is an anhydride of calcium hydroxide $\text{ Ca(OH}{{\text{)}}_{\text{2}}}\text{ }$ . That is, $\text{ Ca(OH}{{\text{)}}_{\text{2}}}\text{ }-\text{ }{{\text{H}}_{\text{2}}}\text{O = CaO }$ .