Chemistry help PLEASE!?

An unknown, which is a pure compound, forms a solution when water is added. A precipitate forms when Ba(NO3)2(aq) is added to this solution, but now when Cu(NO3)2(aq) is added. When HC2H3O2(aq) is added to the unknown solid, no gas is produced. When NaOH(aq) is added to the solid, a gas with a pungent odor is produced. What is the identity of the unknown?