#6

Titration of 25.0 mL H3PO4-solution (with unknown concentration) with 0.110 mol/L NaOH(aq). The first equivalence point is reached after adding 26.4 mL NaOH. 1. Calculate the original H3PO4-concentration. 2. Calculate the pH at the start of the titration (0 mL added). 3. Calculate the pH at the first equivalence point. 4. Which volume of NaOH(aq) must be added to reach the second equivalence point? 5. Calculate the pH at the second equivalence point.

1a	How many moles of NaOH are present in 26.4 mL of 0.110 mol/L NaOH?
1b	How many moles of H3PO4 were present at the start?
1c	Calculate the original H3PO4-concentration.
2a	What is H3PO4?
2b	Calculate the H+-concentration at the start. Only the first ionization step of H3PO4 must be taken into account.
2c	Calculate pH at the start.
3a	At the first equivalence point all H3PO4 will be converted into NaH2PO4. What is NaH2PO4?
3b	Calculate the pH.
4a	Which reaction will occur when we add more NaOH after the first equivalence point?
4b	How many mL of NaOH-solution must be added to complete this reaction?
4c	Which volume of NaOH(aq) must be added in total to reach the second equivalence point?
5a	At the 2nd equivalence point all H3PO4 will be converted into Na2HPO4. What is Na2HPO4?
5b	Calculate the pH.