Question

$H_{3}P{O}_2$ and $H_4{P}_2{O}_7$ are respectively:
A. Tribasic and tetrabasic acids
B. Dibasic and tetrabasic acids
C. Monobasic and tetrabasic acids
D. Tribasic and dibasic acids

Answer 1

Hint: We know that basicity of an acid is given by the number of ionizable $H^{+}$ that it can give up.

Complete step by step answer:
We have different theories that can be used to define acids including Arrhenius theory and Brönsted-Lowry theory. According to both of these theories, acids can be defined as $H^{+}$ donors. For example, hydrochloric acid $\left(HCl\right)$ and sulfuric acid$\left(H_{2}S{O}_4\right)$ as we can see from their ionization:
$\begin{array}{c}HCl\overrightarrow{\leftarrow \phantom{{}_{\text{vbox}to.5ex\text{vss}}}}{H}^{+}+C{l}^{-}\\ H_{2}S{O}_4\overrightarrow{\leftarrow \phantom{{}_{\text{vbox}to.5ex\text{vss}}}}2H^{+}+S{O}_4^{2-}\end{array}$
As we can see that hydrochloric acid has one ionizable $H^{+}$ whereas sulfuric acid has two ionizable $H^{+}$. This can form a basis to categorize acids as well and giving a new term basicity of acids which is equal to the number of ionizable $H^{+}$ present in the acid. Depending on the basicity, we can have monobasic, dibasic, tribasic, tetrabsic acids having $1,2,3and4$ ionizable $H^{+}$ respectively and so on.
So, in the above examples, $HCl$ is a monobasic acid whereas $H_{2}S{O}_4$ is a dibasic acid.
Now, let’s have a look at the given acids: $H_{3}P{O}_2$ and $H_4{P}_2{O}_7$
The structure of these acids can be shown as follows:

As we can see that in the hypophosphorous acid with the formula $H_{3}P{O}_2$, we have only one $P-OH$ bond whereas other two hydrogens are directly attached to $P$ and are non-ionizable. So, $H_{3}P{O}_2$ is a monobasic acid.
Now, in case of pyrophosphoric acid with the formula $H_4{P}_2{O}_7$, we have four $P-OH$ bonds which gives us four ionizable protons. So, $H_4{P}_2{O}_7$ is a tetrabasic acid.

Therefore, from the above explanation the correct option is (C).

Note:
We have to look carefully at the structure of as basicity cannot be simply deduced from looking at the chemical formula only.