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:
$\begin{array}{c}
HCl \mathbin{\lower.3ex\hbox{$\buildrel\textstyle\rightarrow\over
{\smash{\leftarrow}\vphantom{_{\vbox to.5ex{\vss}}}}$}} {H^ + } + C{l^ - }\\
{H_2}S{O_4} \mathbin{\lower.3ex\hbox{$\buildrel\textstyle\rightarrow\over
{\smash{\leftarrow}\vphantom{_{\vbox to.5ex{\vss}}}}$}} 2{H^ + } + SO_4^{2 - }
\end{array}$
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 $1,2,3\;and\;4$ 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.