Precipitation of Ions For BaSO4(s) Ba2+(aq) + SO42–(aq)... Ksp = [Ba2+] [SO42–] We could reach equilibrium from the left... i.e., start with BaSO4(s) or from the right... i.e., start w/Ba2+ (say, from Ba(NO3)2(aq)) and SO42– (say, from Na2SO4(aq)) At any given time, the ion product Q = [Ba2+] [SO42–] If Q > Ksp... more ppt will form. If Q < Ksp... more ions will dissolve. If Q = Ksp... ions and ppt are in eq.

Will a precipitate form from mixing 0.10 L of 8.0 x 10–3 M lead(II) nitrate and 0.40 L of 5.0 x 10–3 M sodium sulfate? Pb(NO3)2 and Na2SO4 (If any ppt forms, it will be of PbSO4. Look up the Ksp of PbSO4: 6.3 x 10–7.) 0.50 0.008(0.10) [Pb2+]init = = 0.0016 M 0.50 0.005(0.40) [SO42–]init = = 0.0040 M Q = [Pb2+] [SO42–] = 0.0016 (0.0040) = 6.4 x 10–6 Q > Ksp; ppt WILL form.

selective precipitation: using the different solubilities of ions to separate them Say we want to separate the Ag+ and Cu2+ in a soln that contains both. -- Add HCl. CuCl2 is… soluble, which means it won’t ppt out. AgCl is… insoluble (its Ksp is 1.8 x 10–10) so MOST of it will ppt out when we add HCl. Erlenmeyer flask AgCl(s) filter paper long-stem funnel Cu2+ ions still in soln (along w/a “wee-little” Ag+) -- Separate by... filtering out the AgCl(s). silver chloride