WAEC 2017 Chemistry · Question 18
The oxidation number of X in is
Options
- A.+1
- B.+3
- C.+4
- D.+5
Answer: D. +5
Explanation written with AI and checked against the answer key
Analyze the question: We need the oxidation number of X in the ion XO₄³⁻. The sum of the oxidation numbers of all atoms in an ion equals the charge on the ion. Concept: Oxygen usually has an oxidation number of -2 (except in peroxides and a few other cases). For a polyatomic ion: (oxidation number of X) + (sum of oxidation numbers of the other atoms) = overall charge Solution: Let the oxidation number of X = x. x + 4(-2) = -3 x - 8 = -3 x = -3 + 8 x = +5 Check: +5 + (4 × -2) = +5 - 8 = -3 ✓ X could be phosphorus (PO₄³⁻, phosphate) or arsenic (AsO₄³⁻), both of which are +5 in this ion. Answer: The oxidation number of X is +5, which is option D. Tip: For any ion, set up the equation "sum of oxidation numbers = charge." With oxygen at -2, just multiply by the number of oxygen atoms. For XO₄ⁿ⁻, the oxidation number is 8 - n. Here, 8 - 3 = 5. You can use this shortcut for phosphate, sulfate (SO₄²⁻ gives +6) and similar ions.AI can make mistakes. Report anything that looks wrong.
Past questions © the examination body (JAMB/WAEC/NECO). DeeepSolve is not affiliated with them. Rights holders: see /copyright.