Question

The equivalent weight of phosphoric acid (\[{{H}_{3}}P{{O}_{4}}\]) in the reaction is:
\[NaOH\text{ }+\text{ }{{H}_{3}}P{{O}_{4}}\text{ }\to \text{ }Na{{H}_{2}}P{{O}_{4}}\text{ }+\text{ }{{H}_{2}}O\]
A. 25
B. 49
C. 59
D. 98

Answer 1

Hint: For calculation of equivalent weight, we should know the molar mass and “n” equivalent factor. For determination of equivalent weight of phosphoric acid we have to find “n”. And in phosphoric acid it is about replaceable hydrogen.

Step by step answer:
\[NaOH\text{ }+\text{ }{{H}_{3}}P{{O}_{4}}\text{ }\to \text{ }Na{{H}_{2}}P{{O}_{4}}\text{ }+\text{ }{{H}_{2}}O\]
\[Equivalent\,weight=\dfrac{molar\,mass}{''n''\,number\,of\,equivalents}\]
We can easily find the molar mass of \[{{H}_{3}}P{{O}_{4}}\]
Molecular weight calculation$\to $
Number of H= 3 $\to $3 × 1 = 3
Number of P= 1$\to $1 × 31 = 31
Number of O= 4$\to $4 × 16 = 64
Molecular weight = 3 + 31 + 64= 98
Now we will find a number of equivalents that is “n”.\[{{H}_{3}}P{{O}_{4}}\] Is acid so “n” it will have basicity.
\[{{H}_{3}}P{{O}_{4}}\text{ }\to \text{ }HPO_{4}^{2-}\text{ }+\text{ }2{{H}^{+}}\]
2 hydrogen ions are left. Therefore, the valence factor = 2
\[Equivalent\,weight\,of\,{{H}_{3}}P{{O}_{4}}=\dfrac{molar\,mass\,of\,phosphoric\,acid}{number\,of\,equivalents(valency\,factor)}=\dfrac{98}{2}\]= 49
Equivalent weight of \[{{H}_{3}}P{{O}_{4}}\] is 49.

Additional information:
Equivalent weight of an acid: we should know that the valence factor of an acid is equal to its basicity. The basicity of an acid is equal to a replaceable hydrogen ion (proton) in its aqueous solution. Importantly, basicity is not the same as the number of hydrogen atoms in acid molecules. Consider acetic acid (\[C{{H}_{3}}COOH\]). It contains 4 hydrogen atoms in it, but only 1 replaceable hydrogen ion. As such, the basicity of acetic acid is 1.

Note: We should understand the concept of finding the valence factor for acid and base. Phosphorus based acids like phosphoric acid (\[{{H}_{3}}P{{O}_{4}}\]), phosphorous acid (\[{{H}_{3}}P{{O}_{3}}\]) and hypo-phosphorous acid (\[{{H}_{3}}P{{O}_{2}}\]) need special attention of us to understand their basicity. By looking at formula, it appears that these compounds may furnish OH ions, but bond strengths between phosphorus and oxygen (P-O) and phosphorus and hydrogen (P-H) are stronger than between oxygen and hydrogen (O-H) in –OH group. As such, these molecules release hydrogen ions from –OH group and behave as acid. Clearly, basicity of phosphoric acid (\[{{H}_{3}}P{{O}_{4}}\]), phosphorous acid (\[{{H}_{3}}P{{O}_{3}}\]) and hypo-phosphorous acid (\[{{H}_{3}}P{{O}_{2}}\]) are 3, 2 and 1 respectively.