the precipitate dissolving with addition of HNO3 confirms it is phosphate. Phosphate is unable to form precipitates in a low pH because the PO4 3- reacts with H3O+ (due to the acidic environment). This makes HPO42- (PO43-(aq) + H3O+(aq) < -- > HPO4 2-(aq) + H2O(l)) and also can make H2PO4 - as...