1. The problem statement, all variables and given/known data Calculate the analytical concentrations C2[KOH] and C2[H3PO4] in a solution where H3PO4 (originally of concentration 0.2 moldm-3 and volume 0.250 dm3) is titrated with KOH (originally of concentration 0.8 moldm-3, unknown volume used) until the pH reaches 7.8. 2. Relevant equations Mass balance: C2[H3PO4]=[H3PO4]+[H2PO4-]+[HPO42-]+[PO43-] C2[KOH]=[K+] Charge balance: [H+]+[K+]=[H2PO4-]+2*[HPO42-]+3*[PO43-]+[OH-] Equilibria: Kw=[H+]*[OH-] Ka1=[H2PO4-]*[H+]/[H3PO4] Ka2=[HPO42-]*[H+]/[H2PO4-] Ka3=[PO43-]*[H+]/[HPO42-] 3. The attempt at a solution The unknowns are: [OH-], [PO43-], [HPO42-], [H2PO4-], [H3PO4] and [K+]. These number 6; we have 6 equations. So a solution exists. [OH-]=Kw/[H+], but what about the rest? My problem is that C2[H3PO4] and C2[KOH] are unknown of themselves. They represent the analytical concentrations of H3PO4 and KOH in the final solution (of pH=7.90) after they have been added together. These are unfortunately the two values we need to work out in the end.