Chemistry Solutions Stoichiometry

Question

A volume of 11.7 mL of a 0.22 M solution of hydrogen phosphate (or phosphoric acid) reacts completely with 59.7 milliliters of sodium hydroxide. What is the molarity of the sodium hydroxide solution?

Answer 1

(Tri)hydrogen phosphate is phosphoric acid = H3PO4

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

moles H3PO4 used = 11.7 ml x 1 L/1000 ml x 0.22 mol/L = 0.00257 moles H3PO4

moles NaOH required = 0.00257 mol H3PO4 x 3 mol NaOH / mol H3PO4 = 0.00772 moles NaOH

Molarity of NaOH = moles/liter = 0.00722 moles / 0.0597 liters = 0.129 M = 0.13 M (2 sig. figs.)