Answer is: the formula for phosphorous acid is H₃PO₃.
Phosphorous acidis diprotic (lost two protons).
Dissociation of phosphorous acid in water:
H₃PO₃(aq) → 2H⁺(aq) + PHO₃²⁻(aq).
Solubility in water of this acid is 310 g/100 mL.
Other oxyacids of phosphorus are phosphoric acid (H₃PO₄) and hypophosphorous acid (H₃PO₂).