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A-Level Trends in the elements across a period and down a group: worked solution
3 marks. Full working, one step per line.
Question
Nitrogen sits directly above phosphorus in the Periodic Table. Outline how an oxide of phosphorus and a chloride of phosphorus each behave when added to water, giving equations where relevant and predicting the pH of the resulting solutions.
Worked answer
Oxide: phosphorus(V) oxide reacts with water to give phosphoric(V) acid. P4O10 + 6H2O -> 4H3PO4 H3PO4 is a fairly strong acid, so the solution is acidic, pH about 1-2. Chloride: phosphorus pentachloride is hydrolysed by water. PCl5 + 4H2O -> H3PO4 + 5HCl (or PCl3 + 3H2O -> H3PO3 + 3HCl) The HCl produced is a strong acid, so the solution is strongly acidic, pH about 1-2. Both the oxide and the chloride of phosphorus give acidic solutions of low pH.
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This question is part of A-Level Trends in the elements across a period and down a group, in A-Level H2 Chemistry.
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