Asolution containing a mixture of metal cations was treated with dilute hcl and a precipitate formed. the solution was filtered and h2s was bubbled through the acidic solution. a precipitate again formed and was filtered off. then, the ph was raised to about 8 and h2s was again bubbled through the solution. this time, no precipitate formed. finally, the solution was treated with a sodium carbonate solution, which resulted in formation of a precipitate. which metal ions were definitely present, which were definitely absent, and which may or may not have been present in the original mixture?