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Question

A volume of 15.6 mL of a 0.45 M solution of hydrogen phosphate (or phosphoric acid) reacts completely with 54.7 milliliters of sodium hydroxide. What is the molarity of the sodium hydroxide solution?

Answer 1

Not sure why you're still confused. You might want to tell us a little about what it is that you don't quite get. I answered this earlier with slightly different numbers, and here it is again.

(Tri)hydrogen phosphate is phosphoric acid = H₃PO₄

H₃PO₄ + 3NaOH ==> Na₃PO₄ + 3H₂O ... balanced equation

moles H₃PO₄ used = 15.6 ml x 1 L/1000 ml x 0.45 mol/L = 0.00702 moles H3PO4

moles NaOH required = 0.00702 mol H₃PO₄ x 3 mol NaOH / mol H₃PO₄ = 0.02106 moles NaOH

Molarity of NaOH = moles/liter = 0.02106 moles / 0.0547 liters = 0.385 M = 0.39 M (2 sig. figs.)

If you're still confused, leave a comment about WHY you're confused, and maybe we can straighten you out.