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Barium phosphite can have the molecular formula of BaHPO3 or Ba(H2PO3)2 with molecular weights of 216.3021 and 296.2739 g/mol, respectively. It is the latter that appears when a solution of phosphorous acid is reacted with the carbonate:
BaCO3 +H3PO3 ? Ba(H2PO3)2 +H2O + CO2

Barium phosphite is soluble in water so that a vacuum evaporation is needed to form the hemihydrate, Ba(H2- PO3)2·0.5H2O. It can also be prepared by the action of ammonium phosphite on barium chloride:
BaCl2 (aq) + 2NH4HPO3 (aq) ? Ba(H2PO3)2·0.5H2O + 2NH4Cl (aq).