You have performed a potentiometric titration of an acid solution.
Reactions that occurred during this titration are the following:
H+ + Cl- + Na+ + OH- <--> H2O + Na+ + Cl- (ionic)
H+ + OH- <--> H2O (net ionic)
H3PO4 + OH- <--> H2O + H2PO4-
H2PO4- + OH- <--> H2O + HPO42-
Calculate the concentration of [HCl] and [H3PO4] in your solution in units of mM by setting up a mass balance equation for each endpoint. The following information will be used to set up and carry out your calculations:
H3PO4 <--> H+ + H2PO4- (K1 = 7.11e-3)
H2PO4- <--> H+ + HPO42- (K2 = 6.32E-8)
HPO42- <--> H+ + PO43- (K3 = 4.5E-13)
The first equivalence point will be used for calculating the sum of HCl and the first proton of the phosphoric acid. The second equivalence point marks the neutralization of the second phosphoric acid proton. The third phosphoric acid proton is too weak to be titrated effectively. At each of the first two equivalence points, the pH goes through an abrupt change. When the equivalence points have been located, the molarity of each of the acids can be obtained.
Volume of NaOH added to reach 1st equivalence point: 25.05 mL
Volume of NaOH added to reach 2nd equivalence point: 37.43 mL
Other pertinent values you may or may not need to find the [HCl] and [H3PO4] concentrations via mass balance:
Concentration of [NaOH] = 1.01100 M
Volume of pipet used to obtain unknown sample = 25.02 mL
Total volume of unknown solution: 100 mL