$\mathrm{Ca}(\mathrm{OH}{)}_2+{\mathrm{H}}_3{\mathrm{PO}}_4\to {\mathrm{CaHPO}}_4+2{\mathrm{H}}_2\mathrm{O}$. The equivalent weight of H₃PO₄ in the above reaction is

H₃​PO₄​ is an acid and the equivalent mass of an acid is calculated by using the formula:

Equivalent wt.= $\frac{\mathrm{molecular} \mathrm{mass} \mathrm{of} \mathrm{acid}}{\mathrm{no}. \mathrm{of} \mathrm{replacable} \mathrm{atoms}}$​

Molar mass of H₃​PO₄ ​= 3 × 1 + 31 + 4 × 16 = 98

From the given reaction:

H₃​PO₃ ​+ Ca(OH)₂ ​→ CaHPO₄ ​+ H₂​O

As, the replaceable hydrogen ions in acid is 2, by putting the values in above equation, we get:

Equivalent mass of H₃​PO₄ ​= $\frac{98}{2}$​= 49