Phosphoric acid is neutralized by potassium hydroxide according to the following reaction: KOH (aq) + H3PO4 (aq) → K3PO4 (aq) + H2O (1)
What is the concentration of a phosphoric acid solution if 25.0 mL are exactly neutralized by 20.0 mL of 2.000 M KOH solution?

Balance the reaction first:

3KOH + H3PO4 —> K3PO4 + 3H2O

So for every mol of H3PO4, you need 3 mol of OH- to fully neutralize the acid, since H3PO4 is polyprotic.

0.0200 L KOH • (2.000 mol KOH / L KOH) • (1 mol H3PO4 / 3 mol KOH) = 0.0133 mol H3PO4

Divide this by the volume of H3PO4 to get the concentration.

0.0133 mol H3PO4 / 0.0250 L = 0.532 M H3PO4

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