Question

# Choose the weak monobasic acid among the followinga.) ${H_3}B{O_3}$b.) ${H_3}P{O_3}$c.) ${H_3}P{O_4}$d.) $HN{O_3}$e.) ${H_4}{P_2}{O_7}$

Hint: The answer to this question is the molecule in which the central atom has vacant p orbital and it is small in size. So, it can easily accept electrons in its vacant p orbitals. It is often used as antiseptic and insecticide in our houses.

We know that an acid is a chemical species that can donate protons. It is based on the Bronsted lowry concept while according to lewis concept, an acid is a chemical species which has empty orbital and can accept lone pairs of electrons.
Now, moving to the question. In the question, it is monobasic acid. The monobasic acid can be defined as the acid that has only one hydrogen which can be donated to base in the acid-base reaction.

Let us now see the options one by one.

The option a.) is ${H_3}B{O_3}$ which is called Boric acid or hydrogen borate. It can be written as $B{\left( {OH} \right)_3}$. It has three OH groups but none of the OH groups give protons. But Boron has one p orbital which is vacant. This vacant p orbital accepts a lone pair from $^ - OH$ of water and as a result of this, a proton is liberated from the water molecule.
Thus, it is a weak monobasic acid.
Thus, option a.) can be the correct answer.

Moving to option b.) which is ${H_3}P{O_3}$. This is called phosphorus acid and has two OH groups which can easily donate two protons. Thus, it is dibasic in nature.

The option c.) is ${H_3}P{O_4}$ which is called Phosphoric acid. It has three OH groups and is thus tribasic in nature.

The option d.) is $HN{O_3}$. It is called nitric acid. It has one OH group. Thus, one proton can be donated. So, even this is a monobasic acid. But it is a strong monobasic acid. So, this option can also not be the correct answer.

The option e.) is ${H_4}{P_2}{O_7}$ which is called diphosphoric acid. This molecule has four OH groups and thus is tetrabasic in nature.

So. The correct answer is the option a.).

Note:

We can say ${H_3}B{O_3}$ is a weak monobasic acid on the basis of lewis concept. If we talk about the Bronsted lowry concept, then even ${H_3}B{O_3}$ has three OH groups and is thus tribasic in nature. But these three OH can not donate protons easily.
In the case of $HN{O_3}$, after the release of the proton, the negative charge is stabilised on two oxygen atoms by resonance. Thus, the product formed after the release of the proton is highly stable and so the donation is easy and hence it is strong in nature.