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You want to design a new ionic compound, where the total positive charge is 6+ and the total negative charge is 6−. Which of the following combinations gives the correct number of ions? (i) 2 Al³⁺ and 3 Cl⁻ (ii) 3 Mg²⁺ and 1 PO₄³⁻ (iii) 2 Fe³⁺ and 3 O²⁻ (iv) 3 Ca²⁺ and 2 SO₄²⁻
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- (i) 2 Al³⁺ = 6+ but 3 Cl⁻ = 3− — not balanced.
- (ii) 3 Mg²⁺ = 6+ and 1 PO₄³⁻ = 3− — not balanced.
- (iii) 2 Fe³⁺ = 6+ and 3 O²⁻ = 6− — correct (Fe₂O₃).
- (iv) 3 Ca²⁺ = 6+ and 2 SO₄²⁻ = 4− — not balanced.
Answer: (iii)
Combination
Total charge → verdict
(i)
2 Al³⁺ and 3 Cl⁻
+6 and −3 —
Wrong
. Charges do not balance.
(ii)
3 Mg²⁺ and 1 PO₄³⁻
+6 and −3 —
Wrong
. Negative charge is only 3.
(iii)
2 Fe³⁺ and 3 O²⁻
+6 and −6 —
Correct
. This is the answer (Fe₂O₃).
(iv)
3 Ca²⁺ and 2 SO₄²⁻
+6 and −4 —
Wrong
. Charges do not balance.
