Q.On hydrolysis, PCl3 gives

Step 1. Write the hydrolysis reaction.

PCl3 + 3H2O -> H3PO3 + 3HCl.

Step 2. Understand the mechanism.

A water molecule first coordinates to phosphorus via its available vacant 3d orbital, forming an unstable PCl3.H2O adduct; this then eliminates HCl in three successive steps (PCl3.H2O -> POHCl2 + HCl; POHCl2 + H2O -> H2PO2Cl + HCl; H2PO2Cl + H2O -> H3PO3 + HCl), each step replacing one Cl by OH, exactly as SiCl4 hydrolyses.

Step 3. Identify the product's basicity/structure.

Orthophosphorous acid, H3PO3, has one P-H (non-acidic), one P=O, and two P-OH groups, making it dibasic despite the formula suggesting three acidic hydrogens.

Step 4. Eliminate the distractors. …

Track the phosphorus oxidation state (unchanged at +3 through hydrolysis) and recall that PCl3 hydrolyses fully to the +3 oxoacid …

Confusing PCl3's hydrolysis product (H3PO3) with PCl5's (H3PO4), since both rea …