Question

Consider the titration of 100.0 mL of 0.0200 M H3PO4 with 0.121 M NaOH. Calculate the milliliters of base that must be added to reach the first, second, and third equivalence points.

Answer 1

First equivalence point : H₃PO₄ (aq) + NaOH (aq) NaH₂PO₄ (aq) + H₂O (l)

moles H₃PO₄ present = (molarity H₃PO₄) * (volume H₃PO₄)

moles H₃PO₄ present = (0.0200 M) * (100.0 mL)

moles H₃PO₄ present = 2.00 mmol

moles NaOH required = moles H₃PO₄ present

moles NaOH required = 2.00 mmol

volume NaOH required = (moles NaOH required) / (molarity NaOH)

volume NaOH required = (2.00 mol) / (0.121 M)

volume NaOH required = 16.53 mL

Second equivalence point : NaH₂PO₄ (aq) + NaOH (aq) Na₂HPO₄ (aq) + H₂O (l)

moles NaH₂PO₄ present = (molarity H₃PO₄) * (volume H₃PO₄)

moles NaH₂PO₄ present = (0.0200 M) * (100.0 mL)

moles NaH₂PO₄ present = 2.00 mmol

moles NaOH required = moles NaH₂PO₄ present

moles NaOH required = 2.00 mmol

volume NaOH required = (moles NaOH required) / (molarity NaOH)

volume NaOH required = (2.00 mol) / (0.121 M)

volume NaOH required = 16.53 mL

Total volume of NaOH required = 16.53 mL (for first equivalence point) + 16.53 mL (for second equivalence point)

Total volume of NaOH required = 33.06 mL

Third equivalence point : Na₂HPO₄ (aq) + NaOH (aq) Na₃PO₄ (aq) + H₂O (l)

moles Na₂HPO₄ present = (molarity H₃PO₄) * (volume H₃PO₄)

moles Na₂HPO₄ present = (0.0200 M) * (100.0 mL)

moles Na₂HPO₄ present = 2.00 mmol

moles NaOH required = moles Na₂HPO₄ present

moles NaOH required = 2.00 mmol

volume NaOH required = (moles NaOH required) / (molarity NaOH)

volume NaOH required = (2.00 mol) / (0.121 M)

volume NaOH required = 16.53 mL

Total volume of NaOH required = 16.53 mL (for first equivalence point) + 16.53 mL (for second equivalence point) + 16.53 mL (for third equivalence point)

Total volume of NaOH required = 49.59 mL