Question

Which of the following is the strongest acid?
A. ${{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}}$
B. ${{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}}$
C. ${\rm{HN}}{{\rm{O}}_{\rm{2}}}$
D. ${\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{COOH}}$

Answer 1

Hint: We know that acid is a substance that can release hydrogen ion in aqueous solution and a base is a substance that can release hydroxide ion in aqueous solution. Some examples of acids are hydrochloric acid, sulphuric acid etc.

Complete step by step answer: Let’s understand when an acid is called a strong acid. Strong acid is the acid that undergoes complete dissociation in aqueous solution. Weak acid is the acid which does not undergo complete dissociation.

Now, come to the question. Option A is ${{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}}$
(phosphoric acid). Phosphoric acid $\left( {{{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}}} \right)$ shows less dissociation as comparison at sulphuric acid $\left( {{{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}}} \right)$ in solution. So, sulphuric acid is stronger than phosphoric acid.
 Option C is ${\rm{HN}}{{\rm{O}}_{\rm{2}}}$. It does not undergo complete dissociation in water. So, it is a weak acid.
Option D is ${\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{COOH}}$ (acetic acid). It also does not undergo complete dissociation in water. So, it is a weak acid.

So, we find that two acids in the options are weak acid and the remaining two are strong acids. Now, the strongest acid is the one that dissociates more. So, between phosphoric acid $\left( {{{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}}} \right)$ and sulphuric acid$\left( {{{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}}} \right)$ is sulphuric acid is strongest acid.
So, the correct answer is “Option B”.

Note: Always remember that, an acid loses one proton to form the conjugate base and the base accepts the proton and forms the conjugate acid. There is the difference of one proton in conjugate acid-base pairs. The strength of acid-base can be determined from the conjugate acid base pair. The more the base is strong, its conjugate acid is weak and stronger the acid, weaker its conjugate base.