Precipitates and pH?

These two questions are really bugging me. Anyone have any ideas? A 0.05 M solution of hydrogen sulfide (H2S) contains 0.15 M nickel chloride NiCl2, and 0.35 M mercury nitrate Hg(NO3)2. What pH is required to precipitate the maximum amount of HgS but none of the NiS? Even before the industrial age, rainwater... show more These two questions are really bugging me. Anyone have any ideas?

A 0.05 M solution of hydrogen sulfide (H2S) contains 0.15 M nickel chloride NiCl2, and 0.35 M mercury nitrate Hg(NO3)2. What pH is required to precipitate the maximum amount of HgS but none of the NiS?

Even before the industrial age, rainwater was discovered to be slightly acidic from dissolved CO2. What is the pH of a bottle of rainwater collected in 1863 if the solubility of CO2 in pure water is 88 mL CO2 gas in 100 mL H2O. The Ka1 of H2CO3 is equal to 4.5 x 10^-7. the volume percent of CO2 in air is 0.033%.
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