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AQUATIC CHEMISTRY Problem Set# 5 Due Wednesday, April 8, 2020 by 11:59PM (1.) (14 points) For the following Cu(II) systems described below, derive the total dissolved metal equation, CuT, in terms of overall formation reaction constants (βi). Determine the concentration of free copper and other species in italics for each set of stated conditions. Ignore precipitation-dissolution reactions and activity corrections. Use the complexation constants from MH Table 6.3. (a) Determine the Cu speciation assuming that the ligands NH3 and Cl- are present and CuT = 10-6 M, NT = 0.5 M and ClT = 0.2 M at pH = 6. Include only the following complexes: L = OH- (CuL, CuL2, CuL4); L = NH3 (CuL, CuL2, CuL3, CuL4); L = Cl- (CuL). (b) Determine the Cu speciation at pH = 10 assuming that the system is open to the atmosphere and that PCO2 = 10-3.5 atm, PNH3 = 10-4.5 atm, CuT = 10-5 M, and ClT = 0.2 M. Include only the following complexes: L = OH- (CuL, CuL2, CuL4); L = NH3 (CuL, CuL2, CuL3, CuL4); L = Cl- (CuL); L = CO32- (CuL, CuL2). (2.) (10 points) Perhaps the most important metal ion complexing agents that occur naturally in aquatic and soil systems are humic and fulvic acids (HAs and FAs). These organic acids are residual organic matter formed during microbial decomposition of plant materials. Copper is quite often used as a phytotoxin to suppress the growth of algae in lakes and ponds. You are asked to give advice on the use of copper to control algae in a lake which has a pH = 6 and a FAT concentration of 5.0 mg/L (Molecular Weight = 5000 g/mol for the FA). You have toxicology data that indicates 10-8 M "free" copper (Cu2+) is needed to control algae. The equilibrium reaction and constant for copper complexation with FA at pH = 6 is as follows: Cu2+ + FA2- = CuFA LogK= 8.4 Ignoring the precipitation of copper but including hydroxide complexation (OH-: CuL, CuL2, CuL4), how much total copper (M) must be added to effectively control the algae? (You may also ignore protonation/deprotonation of FA.) (3.) (10 points) A lake with the simple composition: NaT = CaT = MgT = KT = ClT = [SO4]T = CT = 10-3 M contains an organic complexing agent, Y, characterized by HY = H+ + Y- logKa = -6 CuY+ = Cu2+ + Y- logK = -7 CaY+ = Ca2+ + Y- logK = -2 MgY+ = Mg2+ + Y- logK = -2 Calculate the copper speciation at pH 7 with CuT = 10-7 M and YT = 10-6 M. Consider the following complexes: L = OH- (CuL, CuL2), L = CO32- (CuL, CuL2), L = SO42- (CuL), L = Cl- (CuL), in addition to the organic ligand complexes listed above (but no solid phases). (4.) (15 points) (a) Draw a logC-pH solubility diagram for aqueous chromium and its hydroxide complexes in equilibrium with Cr(OH)3(s). Over what pH range does Cr(OH)2+ control Cr(OH)3(s) solubility? (b) At pH = 8.0, calculate the total soluble Cr that would be in equilibrium with Cr(OH)3(s) in the presence of a total EDTA of 10-5 M (Note: for Cr, use only OH- complexes and the CrL EDTA complex; for EDTA, consider HL, H2L, H3L, CrL only).