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Chemistry Chapter 7 : Topic 5
Ionic Formula Derivation
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A student is given the ions Al³⁺ and O²⁻ and is asked to write a neutral ionic compound. The student applies the criss-cross method but forgets to reduce the ratio to the simplest form. Which final formula is correct and chemically acceptable?
Reasoning: Charges are criss-crossed (Al₂O₃), and the ratio is already in simplest whole numbers, giving a neutral compound.
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During a lab activity, students combine Mg²⁺ ions with Cl⁻ ions. One student writes MgCl₂ while another writes Mg₂Cl.
Reasoning: Two chloride ions (each - 1) are needed to balance Mg²⁺ for neutrality.
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A compound contains Fe³⁺ and SO₄²⁻ ions. Which formula correctly represents the compound using charge balance logic?
Reasoning: Criss-cross charges give Fe₂(SO₄)₃ to balance + 6 and - 6 total charge.
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A student writes Ca₂O₂ for calcium oxide after applying the criss-cross method. Why is this incorrect?
Reasoning: Ca₂O₂ simplifies to CaO in lowest ratio form.
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In an ionic compound, Na⁺ combines with PO₄²⁻. What is the correct formula?
Reasoning: Three Na⁺ ions are required to balance - 3 charge of phosphate.
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A compound is formed between Al³⁺ and SO₃²⁻. A student writes Al₂SO₃ instead of the correct formula. What is the correct representation?
Reasoning: Cross multiplication gives Al₂(SO₃)₃ for charge neutrality.
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Which principle is MOST important in deriving ionic formulas?
Reasoning: Ionic compounds must be electrically neutral.
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A compound is formed using Cu²⁺ and OH⁻ ions. What is the correct formula?
Reasoning: Two hydroxide ions balance Cu²⁺ charge.
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A student uses oxidation numbers to form Fe²⁺ and Cl⁻ compound. What is the correct formula?
Reasoning: One Fe²⁺ needs two Cl⁻ ions for neutrality.
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Why are parentheses used in formulas like Ca(OH)₂ but not in NaCl ?
Reasoning: Parentheses are used when multiple polyatomic ions exist.
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A compound is formed from Al³⁺ and NO₃⁻ ions. What is the correct formula?
Reasoning: One Al³⁺ balances three nitrate ions.
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A student writes Mg₂Cl₂ for magnesium chloride. Why is this formula considered incorrect in standard representation?
Reasoning: MgCl₂ is the simplest correct formula.
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Which of the following correctly describes oxidation number in ionic formula formation?
Reasoning: Oxidation number represents assumed charge in compounds.
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A compound is formed between Ca²⁺ and PO₄³⁻ ions. What is correct formula?
Reasoning: Charge balance gives Ca₃(PO₄)₂.
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Which step is essential after applying criss-cross method?
Reasoning: Final formula must be in simplest whole-number ratio.
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A compound contains NH₄⁺ and CO₃²⁻ ions. What is correct formula?
Reasoning: Two ammonium ions balance carbonate ion.
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Why is FeCl₃ different from FeCl₂?
Reasoning: Fe can be + 2 or + 3 depending on compound.
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A student incorrectly writes AlO for aluminium oxide. What is correct formula logic?
Reasoning: Al³⁺ and O²⁻ require ratio 2 :3.
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Which compound requires parentheses in its correct formula?
Reasoning: Multiple nitrate ions require grouping.
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A compound formed from Zn²⁺ and NO₃ ⁻ ions will have formula:
Reasoning: One Zn²⁺ balances two nitrate ions.
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Which statement BEST explains ionic formula formation?
Reasoning: Neutrality is the basis of ionic compounds.
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A compound is formed from Na⁺ and S²⁻. What is correct formula?
Reasoning: Two sodium ions balance sulfide ion.
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Why is oxidation number important in formula formation?
Reasoning: It ensures correct charge balance in compounds.
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A compound contains Ba²⁺ and Cl⁻ ions. What is correct formula?
Reasoning: One Ba²⁺ requires two chloride ions.
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A student writes Al₃O₂ instead of Al₂O₃. What concept is violated?
Reasoning: Incorrect ratio does not balance total charge.
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