Explanation:
When 2 moles of sodium hydroxide (NaOH) reacts with 1 mole of phosphoric acid (H3PO4) it gives 1 moles disodium phosphate Na2HPO4 and 2 moles of water.
0.5 molar solution indicates half moles of molecules are present in per ltr solution.
to prepare disodium phosphate you need to dilute phosphoric acid solution to 0.25 molar and add equal quantity of 0.5 molar solution.
[tex]2 NaOH + H_3PO_4 \: \rightarrow \: Na_2HPO_4 + 2 H_2O[/tex]
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