平衡后: PbS CuS几乎沉淀完全!!
[H+] ==0.05 *2*2 == 0.2mol/L
[H+]^2[S2-]/[H2S] ==Ka1Ka2
[S2-] == Ka1KA2[H2S]/[H+]^2 ==3*10^-20mol/L
[Cu2+] ==6*10^-36/3*10^-20 == 2*10^-16 mol/L
[Pb2+] ==8*10^-28/3*10^-20 == 2.7*10^-8 mol/L
[Fe2+]不沉淀!