4. The following problem will guide you through a series of simple problems to evaluate the final pH and the final concentration for a system removing A by oxidizing with oxygen to B (see eq. 3). This approach to finding a global solution to a complex problem by dissecting it in a series of easier problems is called problem analysis. The oxidation of metals followed by their precipitation is one of these complex problems because (i) both reactions change the pH, and (ii) the solubility of metals is affected by the pH. Thus, calculating the final conditions when designing a treatment system can be very complex. You will need to use the approach of problem analysis throughout your engineering career. The solubility of A and B hydroxides can be described by the following equations. Notice that pH affect their solubility, and that the pH would change during the precipitation. Thus, the complete solution to the problem would require solving for pH and solubility at the same time. a. What concentration of C in mg/L would you expect to find at equilibrium at a pH of D and a pH 7.00 if C is present in the A or B oxidation state in a solution at EC. b. During water treatment, aeration is employed to remove C by oxidizing A to B according to the chemical reaction H. What concentration of C(mg/L) would be present in a water that has had air bubbled through it at EC and pH7.00 (neglect pH changes)? c. What would be the pH after air bubbling of a water contaminated by G at a concentration of Fmg/L as C and an initial pH 7.00? Assume that the most oxidized form of C is at the concentration that you found Section (b). Hint: does the pH changes more due to C oxidation (eq. 4) or C precipitation (eq. 2)? What do you expect the combined result to be? Note that the dissolution of G is complete and is described by the eq. 3 . d. You want to use your calculations to design an C removing process. You realize that precipitation of C influences the pH and vice versa. Thus, you simplified the problem by solving the solubility problem at two different pHs. Based on you observations of the solubility of the most oxidized form of C in function of pH (step a), do you think that the calculated C concentration at pH7 (step b) would accurately represent the final concentration, or would overestimate/underestimate it (explain in a few sentences, no further calculation is necessary)? A:Mn2+; the solubility of Mn2+ is controlled by its precipitation as Mn(OH)2 according to the B:Mn3+; the solubility of Mn3+ is controlled by its precipitation as Mn2O3 according to the following equation: Mn2O3(s)+3H2O2Mn3+ (aq) +6OH(aq);Ksp25C2=2.8110Es eq. 2 C : manganese (Mn) G : manganous chloride ( MnCl2); dissolution according: MnCl2(s)Mn2+ (aq) +2Cl(ap) eq. 3 [Note that the single headed arrow mean a complete reaction.] D=3.75;E=13.5,F=5.83, Atomic masses are (g/mol);Fe:55.8,Cl:35.5,Cu:63.5,Mn:54.9,Cr:51.996
