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# The tribasic acid is: A. ${{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{\text{4}}}$B. ${{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{2}}$C. ${{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{2}}$D. $\text{HP}{{\text{O}}_{3}}$

Answer Verified
Hint: A tribasic acid in a reaction donates three hydrogen atoms (ions) in an acid-base reaction. So every tribasic acid has three replaceable hydrogen atoms. Dibasic acid refers to an acid having two replaceable hydrogen atoms and monobasic has only one replaceable hydrogen atom.

Complete answer:
The basicity of an acid is equivalent to the number of ${{H}^{+}}$produced by an acid, when it gets ionized in the aqueous medium. The ${{H}^{+}}$ can only be given then it is attached to an oxygen atom in case of phosphorus oxides. So, the number of hydrogens that are attached to the oxygen atom contribute to the basicity of the acid. ${{H}_{3}}P{{O}_{4}}$ is a tribasic acid. Here, all the three hydrogen atoms are replaceable. In the case of ${{H}_{3}}P{{O}_{4}}$one of the hydrogen atoms is bound to phosphate atom therefore acting as a dibasic acid. has two hydrogen atoms attached to the phosphate ion and therefore is a monobasic acid. $HP{{O}_{3}}$ is also a monobasic acid as it has only one hydrogen which is replaceable. ${{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{2}}$
also has only two hydrogens attached to oxygen atoms so it possesses only two replaceable hydrogens and it is thus dibasic.
The correct option is therefore option (A) which is phosphoric acid ${{H}_{3}}P{{O}_{4}}$ .
So, the correct answer is “Option A”.

Note: Only those hydrogen atoms are counted to determine basicity of an acid which are replaceable and not all the hydrogens present in the acid. The hydrogens directly attached to carbon or any other less electronegative element are not acidic and are not released as ${{H}^{+}}$ in the aqueous solutions.