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Solution: Separate samples of a solution of an unknown salt are treate...

Question

Separate samples of a solution of an unknown salt are treated with dilute solutions of HBr, H2SO4, and NaOH. A precipitate forms in all three cases.Which of the following cations could the solution contain: K+, Pb2+, Ba2+

Video Transcript

Hey guys, so here what we have is we have three solutions, we have hydrobromic acid, we have sulphuric acid and we have sodium hydroxide. Now, here we're trying to figure out, does our solution contain a particular cation? So the cations are: potassium ion, the lead 2+ ion and barium ion.

They tell us that in every case this unknown cation is going to form a precipitate. So here, potassium ion's, automatically out, because potassium is always going to be soluble, so it's never going to make a precipitate. So our choices would either be lead 2+, or barium. Now here, barium would form a precipitate, if it were connected with sulphate ion. And become, barium sulphate, which would be a solid, but if barium connected with the bromide ion here, it would still be soluble according to the solubility rules, so barium is out. The answer is lead 2+, because lead 2+ no matter which one of these negative ions it connects with, in every case, it would make a precipitate. So lead 2+, with bromine, would give us this as a precipitate. Lead 2+ with sulphate would give us this as a precipitate. And lead 2+ with hydroxide would create this as a precipitate. So the cation would have to be Pb 2+.