The equivalent mass of NaOH is calculated based on the amount of hydroxide ions (OH⁻) it provides in a given chemical reaction. In the reaction:

2 NaOH + H3PO4 → Na2HPO4 + 2 H2O

Each molecule of NaOH contributes one hydroxide ion (OH⁻) to the reaction. The reaction involves two moles of NaOH reacting with one mole of phosphoric acid (H₃PO₄) to form one mole of disodium hydrogen phosphate (Na₂HPO₄) and two moles of water.

To determine the equivalent mass of NaOH in this specific reaction, we use the formula:

Equivalent mass = Molar mass of NaOH ÷ Number of OH⁻ ions donated per molecule

Since one molecule of NaOH releases one hydroxide ion (OH⁻) in the reaction, the equivalent mass of NaOH equals its molar mass. Thus:

Equivalent mass of NaOH = Molar mass of NaOH (M)