Question

A solution might have any, all, or none of the following cations: Al3+(aq), Ba2+ (aq), PB2+(aq) or Na+(aq). You follow the following procedure:
1. Addition of HBr(aq) caused a precipitate to form.
2. You filtered off the precipitate, added H2SO4(aq) and formed another precipitate.
3. Again, you filter it, and add KOH(aq), but no precipitate formed.

The questions are:
What cation(s) was/were in precipitate?
Which cation(s) must be absent from the original solution?