Background info:

Many waste water treatment facilities have strict guidelines on the amounts of transition metals that can be discharged into the sewer systems. This creates a waste disposal problem since student experiments in general chemistry courses can generate large volumes of aqueous wastes containing high concentrations of some of these transition metals. In the first year chemistry lab, we have generated a large amount of aqueous copper containing

waste. At this point, the waste solution contains the aqueous copper species, Cu(H2O)62+, recall that in aqueous solution the first-row transition metals exist as the aquated ions, for simplicity we often abbreviate these as Cu2+(aq). A cost-effective method for removing copper from aqueous solution is to precipitate the ions using a source of sulphide ion, S2-. By adding sodium sulphide (Na2S) to the waste solution, we should be able to precipitate copper (KspCuS = 6.7 x 10-42) as copper sulphide and subsequently remove it by filtration: Cu2+(aq)+ S2-(aq)→CuS(aq) Your task is to analyze the copper waste solution and answer the following questions:

Concentration of Cu2+ is 0.1398 M

Question:

1. What assumptions are we making in this analysis regarding the other components present in the waste solution?
2. What mass of Na2S must be added / L of solution, to precipitate the Cu2+ ions ?