The following data comes from Lake 6 of the Lubbock City Lakes System. In all ca

Question

The following data comes from Lake 6 of the Lubbock City Lakes System. In all calculations using the data given above, use at least four significant figures. Do not make any simplifications/assumptions when answering the following questions. On the basis of the compositional data provided for Lake 6, calculate the pH and pOH of the water. Consider the constraint that applies to all solutions. Make the necessary calculations to determine whether Lake 6 is saturated, under-saturated, or supersaturate with respect to gypsum, calcite, and fluorite.

Explanation / Answer

Answer for ii.

Gypsum = CaSO4 = Mol. wt = 136.1406 g/mol; SO4 content = 70.5%

solubility in water = 0.24 g/100ml at 20 °C (dihydrate) = 2.4 g/L =  2400 mg/L = SO4 content = 1692 mg/L

Calcite = CaCO3 = Mol. wt = 100.0869 g/mol; CO3 content = 60%

solubility in water = 0.013 g/L (25 °C) = 13 mg/L CO3 content =7.8 mg/L

Fluorite = CaF2 = Mol. wt = 78.07 g/mol; Fluoride content = 48.76%

in water = 0.0016 g/100 mL (20 °C) = 0.16 mg/ 100ml = 1.6 mg/L = 0.78 mg/L

In the above lake cocentration of the ions from sulfate, carbonate and fluoride ions is as follows:

Sulfate content (348 mg/L) in the lake is less than percentage of sulfate (1692 mg/L) from CaSO4 solubility in water. So w.r.t gypsum the lake is under saturated.

Carbonate content (0.0002715 M, 0.01629 g/L 16.29 mg/L) in the lake is more than percentage of carbonate (7.8 mg/L) from CaCO3 solubilityin water. So w.r.t calcite the lake is super saturated.

Fluoride content (4.3 mg/L) in the lake is more than percentage of fluoride  (0.78 mg/L) from CaF2 solubilityin water. So w.r.t fluorite the lake is super saturated.

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