Hint: Sulphurous acid is also known as trioxo sulphuric acid. It is used as a reducing agent, and it doesn’t dissociate into the hydrogen ion completely. Now, the acidity of sulphurous acid could be known.
Complete step by step solution: First, let us know about the sulphurous acid. Sulphurous acid exists in liquid form, and is colourless. It has a pungent burning smell of sulphur. It has chemical formula H$_2$SO$_3$, known to be sulphur oxoacid, conjugate acid of hydrogen sulfite, and tautomer of sulfonic acid. Sulphurous acid is highly soluble in water. Thus, it dissociates, and leads to the formation of bisulfite, and sulphite ions. The dissociation reaction is:
Now, if we have aqueous sulphurous acid, as mentioned it dissociates completely into hydrogen ions, whereas the bisulfite ion formed is too strong in the presence of a base like water to maintain the proton.
Thus, it is considered to be a weak acid, whereas sulphuric acid is a strong acid. In the end we can conclude that sulphurous acid is a weak acid.
Additional information: Sulphurous acid is corrosive in nature, if it comes in contact with the eye, it causes damage, and inhalation can irritate the neck, and throat. It is a dibasic acid, and used as disinfectants.
Note: Don’t get confused between the sulphuric acid, and the sulphurous acid. We have already discussed the sulphurous acid. The chemical formula of sulphuric acid is H$_2$SO$_4$. It is a strong acid as it dissociates completely into hydrogen ions.
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