Equivalence point

Question

In a titration of 36.0 mL of a 0.250 M solution of a triprotic acid H₃PO₄ (phosphoric acid) with 0.600 M Ca(OH)₂, how many mL of base are required to reach the third equivalence point?

Answer 1

First, write the correctly balanced equation for the neutralization reaction:

2H₃PO₄ + 3Ca(OH)₂ ==> 6H₂O + Ca₃(PO₄)₂ ... balance equation

Note that it takes 3 moles of Ca(OH)₂ to react with 2 moles H₃PO₄ to reach the 3rd equivalence point.

moles H₃PO₄ present = 36.0 ml x 1 L/1000 ml x 0.250 mol/L = 0.00900 moles H₃PO₄

moles Ca(OH)₂ needed = 0.00900 mol H₃PO₄ x 3 mol Ca(OH)₂ / 2 mol H₃PO₄ = 0.0135 mol Ca(OH)₂

Volume of Ca(OH)₂: (0.600 mol/L) (x L) = 0.0135 mol and x = 0.0225 L = 22.5 mls Ca(OH)₂ needed