8a. Cerussite (P6CO3) is a secondary mineral often associated with Pb ores (i.e., galena). It is...

 

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8a. Cerussite (P6CO3) is a secondary mineral often associated with Pb ores (i.e., galena). It is also sometimes found in old lead pipes. When lead ores or pipes dissolve in the presence of dissolved inorganic carbon, Pb carbonates can precipitate. Calculate the concentration of Pb2* in water at 25°C in equilibrium with cerussite (PbCO3) and atmospheric CO2 (Pco2=10-3.5) for pH 5.0 and 8.5. Assume activity coefficients of 1. PbCO, O Pb2+ + CO?- K= 10-12.8 3(s) (aq) 3(aq) * Kco2 = 10-1.46 CO2(g) + H20 → H2CO3"(aq) H2CO3 (aq) HCO3'(aq) + H* (aq) HCO3 (aq) → CO3² (aq) K1 = 10-6.35 + H*(aq) K2 = 10-10.33 %3D 8b. Health Canada considers water above 10 ppb (4.8 mmol/kg) unsafe to drink. Since freshwater usually has a pH somewhere around 6 or lower, are old Pb pipes (which likely have cerussite deposits) a problem? Explain your answer using your answers above. 8a. Cerussite (P6CO3) is a secondary mineral often associated with Pb ores (i.e., galena). It is also sometimes found in old lead pipes. When lead ores or pipes dissolve in the presence of dissolved inorganic carbon, Pb carbonates can precipitate. Calculate the concentration of Pb2* in water at 25°C in equilibrium with cerussite (PbCO3) and atmospheric CO2 (Pco2=10-3.5) for pH 5.0 and 8.5. Assume activity coefficients of 1. PbCO, O Pb2+ + CO?- K= 10-12.8 3(s) (aq) 3(aq) * Kco2 = 10-1.46 CO2(g) + H20 → H2CO3"(aq) H2CO3 (aq) HCO3'(aq) + H* (aq) HCO3 (aq) → CO3² (aq) K1 = 10-6.35 + H*(aq) K2 = 10-10.33 %3D 8b. Health Canada considers water above 10 ppb (4.8 mmol/kg) unsafe to drink. Since freshwater usually has a pH somewhere around 6 or lower, are old Pb pipes (which likely have cerussite deposits) a problem? Explain your answer using your answers above.