How much 1.5 m sodium hydroxide is necessary to exactly neutralize 20.0 ml of 2.5 m phosphoric acid?

Answer is: 100 mL of sodium hydroxide.

Chemical reaction: 3NaOH + H ₃PO₄ → Na₃PO₄ + 3H₂O.

V (H₃PO₄) = 20.0 mL : 1000 mL/L = 0.02 L.

n (H₃PO₄) = V (H₃PO₄) · c (H₃PO₄).

n (H₃PO₄) = 0.02 L · 2.5 mol/L.

n (H₃PO₄) = 0.05 mol.

From chemical reaction: n (H₃PO₄) : n (NaOH) = 1 : 3.

n (NaOH) = 0.15 mol.

V (NaOH) = n (NaOH) : c (NaOH).

V (NaOH) = 0.15 mol : 1.5 mol/L.

V (NaOH) = 0.1 L · 1000 mL/L = 100 mL.