Oxide hydration stoichiometry identifies phosphorus

Steps:

• Identify the key reaction: one oxide molecule + 6 H2O yields an acidic compound.
• Recall phosphorus(V) oxide formula: P4O10 + 6 H2O → 4 H3PO4 (phosphoric acid).
• Check stoichiometry: P4O10 counts as one oxide molecule, matching exactly six waters.
• Eliminate options by comparing their oxide reactions to this pattern.

Why B is correct:

• P4O10 reacts with 6 H2O to form H3PO4, a strong acid, per the balanced equation for phosphorus pentoxide hydration.

Why the others are wrong:

• A. Aluminium oxide (Al2O3) + 3 H2O → 2 Al(OH)3 (amphoteric, not acidic with 6 H2O).
• C. Silicon dioxide (SiO2) + 2 H2O → H4SiO4 (uses 2 H2O, weak acid).
• D. Sulfur trioxide (SO3) + H2O → H2SO4 (uses 1 H2O).

Final answer: B