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Combining Equilibrium Equations

Please provide solution for (a) and (b) with explanation of how it was derived.

Early groups used Pb plumbing in their homes more than 2000 years ago. This Pb pipe has since corroded in the soil to form cerrusite (PbCO3) and hydrocerrusite ( Pb3(CO3)2(OH)2 ).

Essington et al., 2004. The soil mineralogy of lead at Horaceʼs
Villa. Soil Sci. Soc. Am. J., 68, 979)

(a) Assuming a CO2 partial pressure in the soil of about 0.02 atm , calculate the free lead ion solubility, (Pb2+), in soil solution at pH 5, 6, 7 and 8.

(b) Plot the calculated log (Pb2+) versus pH for these minerals on the same graph, with pH on the x-axis.

Reaction Equilibrium Constants :

PbCO3(s) + 2H+ = Pb2+ + CO2(g) + H2O log Ko = 4.89

Pb3(CO3)2(OH)2(s) + 6H+ = 3 Pb2+ + 2 CO2(g) + 4 H2O log Ko = 18.49

Solution Summary

This solution demonstrates how to combine two equilibrium equations and solve for various concentrations under different conditions.