1. The problem statement, all variables and given/known data 8.9985 g of Na2HPO4 and 4.0211 g of NaH2PO4 * 7 H2O are added to 1 L of 0.20 M KCl in water. KCl does not participate in any way but to maintain ionic strength, at 0.20 M. H3PO4 (aq) + H2O → H2PO4- (aq) + H3O+, Ka1 = 7.11 x 10-3 Ka2 = 6.32 x 10-8 Ka3 = 7.1 x 10-13 (a) Give the mass balance for phosphate, and overall charge balance, ignoring KCl. (b) Calculate the pH. (c) After adding 4 g NaOH, what is the new pH? 3. The attempt at a solution So for problems like these, the setup is pretty much everything, so I wanted to make sure that I do at least the setup correct, as the rest of the problem falls out rather easily. I started out by calculating the molarity of Na2HPO4 and NaH2PO4: Na2HPO4: 141.96 g/mol, so 0.0634 mol. In 1 L solution, means 0.0634 M. NaH2PO4 * 7 H2O: 246.12 g/mol, so 0.0163 mol. In 1 L solution, means 0.0163 M. Therefore, mass balance: 0.0634 M + 0.0163 M = [H2PO4-] + [HPO42-] + [PO43-] Charge balance: [Na+] + [H3O+] = [H2PO4-] + 2 [HPO42-] + 3 [PO43-] But [Na+] = 0.0163 M, so charge balance: 0.0163 M + [H3O+] = [H2PO4-] + 2 [HPO42-] + 3 [PO43-] To find pH, I simply have to find a numerical expression for H3O+, and then do pH = –log[H3O+]. For part c, adding 4 g of NaOH changes the concentration of [Na+] ions in solution, which would change the expression of charge balance that I have, so I would just have to rework it with a different number for that value. I haven't gone through with the calculations yet, but I just want to know if I have the gist and setup of the problem correct. Thanks in advance! EDIT: I should add that we are given a table with hydrated radii of the ions, hence the statement about ionic strength, but this doesn't change too much other than the fact that I need to use the Debye–Hückel equation to find the activity coefficients.