If it takes 24.58 mL of a 0.022 M sodium hydroxide solution to titrate 145 mL of an unknown solution of phosphoric acid, what is the concentration of the acid solution?

Answer 1

You should first write the correctly balanced equation for this reaction:

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

Next, find the moles of NaOH used:

24.58 ml x 1 L / 1000 ml x 0.022 mol / L = 0.00054076 moles

Next, use this value and stoichiometry of the balanced equation to find moles of Na₃PO₄:

0.00054076 mols NaOH x 1 mol Na₃PO₄ / 3 mols NaOH = 0.00018025 moles

Finally, use the volume of Na₃PO₄ and these moles to calculate concentration of Na₃PO₄:

0.00018025 mols / 145 ml x 1000 ml / L = 0.0012 moles / L = 0.0012 M (to 2 sig. figs.)