Dissolution in chemistry



University of Houston

Question Description

1- What is the pH of a solution saturated with CaCO3 closed to the atmosphere?

2-What is the pH of a solution saturated with CaCO3 open to the atmosphere?

3- For a solution saturated with calcite (CaCO3), how does the solubility of Ca+2 change as a function of pH from 0 to 14 in open and closed systems?

4- Show how the solubility of Fe+2 changes as a function of pH from 0 to 14 given the following recipe [Fe-II] = 10-4 M and a total carbonate conc. (CT) = 10-3 M? Assume closed system.

5-Calculate the composition of a groundwater in equilibrium with PCO2 = 10-1.5 atm, calcite and kaolinite. Disregard all other potential solids.

6-Assuming an open system at equilibrium, only the carbonate system being important, an alkalinity of 1E-3 M and a pH of 9, would calcite precipitate (would solid calcite be present) if the calcium concentration were 1E-3 M?

7-A water contains 135 mg/L Ca+2 and 50 mg/L Mg+2. After the water is softened, the pH = 11 and the carbonate concentration [CO3-2] = 2.6E-5 M. Assume calcium precipitates as CaCO3 (pKs =8.40) Assume magnesium precipitates as Mg(OH)2 (pKs =10.6)

    • a) Calculate the total hardness (mg CaCO3 mg/L) in the water before softening.
    • b) Calculate the theoretical total hardness remaining in the water after softening based on the solubility of CaCO3 (s) and Mg(OH)2 (s).

    8- A metal plating factor produces a waste containing soluble zinc with a flow of 100 L/min for 8 hours per day. The waste is collected in a large tank. At the end of each day, the pH of the waste is raised to precipitate out the zinc. The sludge is removed and the pH of the supernatants is lower to pH 6 so that is can be discharged to the local sewer system.

    • a)Consider the removal of zinc from this waste by precipitation as zinc hydroxide (Zn(OH)2, pKs = 16.7). The initial concentration of zinc is 0.01 M. Calculate the pH that the wastewater should be raised in order to reduce the soluble zinc concentrations to
    • b)One day the operator forgets to remove the sludge before the pH is lowered to 6. He notices that all the sludge disappeared when the pH was lowered to 6. He proudly tells his supervisor that he discovered a method to eliminate the need to dispose of their metal sludge in an expensive landfill. Qualitatively, explain the operator’s amazing discovery. 0.05 mg/L (the EPA limit).

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