Question

Which of the following is not an acid anhydride?
(A) $\text{C}{{\text{O}}_{\text{2}}}$
(B) CO
(C) $\text{S}{{\text{O}}_{\text{2}}}$
(D) $\text{S}{{\text{O}}_{\text{3}}}$

Answer 1

Hint: An acid anhydride is a compound formed after elimination of water molecules from an acid. Try adding water to each of the molecules and see if any acid is formed.

Complete answer:
An acid anhydride is a compound formed after elimination of water molecules from an acid. For example, acetic acid loses a molecule of water to form acetic anhydride.
Let’s consider the options,
(A) $\text{C}{{\text{O}}_{\text{2}}}$
When we just add water to carbon dioxide, we get carbonic acid.
\[{{H}_{2}}O+C{{O}_{2}}\rightleftharpoons {{H}_{2}}C{{O}_{3}}\]
Carbonic acid loses one molecule of water to form carbon dioxide. That indicates, carbon dioxide is an acid anhydride.
(B) CO
When we add water to carbon-monoxide, we don’t obtain any acid. So, CO is not an acid anhydride.
(C) $\text{S}{{\text{O}}_{\text{2}}}$
When we just add water to sulphur dioxide, we get sulphurous acid.
\[{{H}_{2}}O+S{{O}_{2}}\rightleftharpoons {{H}_{2}}S{{O}_{3}}\]
Sulphurous acid loses one molecule of water to form sulphur dioxide. That indicates, sulphur dioxide is an acid anhydride.
(D) $\text{S}{{\text{O}}_{\text{3}}}$
When we just add water to sulphur trioxide, we get sulphuric acid.
\[{{H}_{2}}O+S{{O}_{3}}\rightleftharpoons {{H}_{2}}S{{O}_{4}}\]
Sulphuric acid loses one molecule of water to form sulphur dioxide. That indicates, sulphur trioxide is an acid anhydride.

Therefore, we can conclude that the correct option is option (B) CO.

Note:
The question is a little confusing. Don’t think that anhydrides can only be organic. Keep in mind the definition of anhydride while solving the question. This question speaks about inorganic anhydrides.