Chemistry 9th Chap 7 : Topic 5 - Maryam Publications

Chemistry Chapter 7 : Topic 5

Ionic Formula Derivation

1 / 25

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?

A) Al₂O₆

B) AlO

C) Al₂O₃

D) Al₃O₂

2 / 25

During a lab activity, students combine Mg²⁺ ions with Cl⁻ ions. One student writes MgCl₂ while another writes Mg₂Cl.

A) Chlorine changes its charge to + 2 in compounds.

B) Mg²⁺ needs two Cl⁻ ions to balance charge neutrality.

C) Mg and Cl form covalent bonds first, then ionic bonds.

D) Magnesium always forms MgCl because charges cancel automatically.

3 / 25

A compound contains Fe³⁺ and SO₄²⁻ ions. Which formula correctly represents the compound using charge balance logic?

A) Fe₂(SO₄)₃

B) Fe₂SO₄

C) FeSO₄

D) Fe₃(SO₄)₂

4 / 25

A student writes Ca₂O₂ for calcium oxide after applying the criss-cross method. Why is this incorrect?

A) Calcium has variable valency.

B) Calcium cannot form ionic compounds.

C) The ratio must be reduced to simplest whole numbers.

D) Oxygen changes its oxidation number.

5 / 25

In an ionic compound, Na⁺ combines with PO₄²⁻. What is the correct formula?

A) Na₃PO₄

B) Na₃(PO₄)₂

C) Na₂PO₄

D) NaPO₄

6 / 25

A compound is formed between Al³⁺ and SO₃²⁻. A student writes Al₂SO₃ instead of the correct formula. What is the correct representation?

A) Al₂(SO₃)₃

B) Al₃(SO₃)₂

C) Al₂SO₃ is already correct

D) AlSO₃

7 / 25

Which principle is MOST important in deriving ionic formulas?

A) Conservation of mass only

B) Atomic number equality

C) Equal number of atoms of each element

D) Total positive charge equals total negative charge

8 / 25

A compound is formed using Cu²⁺ and OH⁻ ions. What is the correct formula?

A) Cu₂OH

B) Cu₂(OH)

C) CuOH

D) Cu(OH)₂

9 / 25

A student uses oxidation numbers to form Fe²⁺ and Cl⁻ compound. What is the correct formula?

A) Fe₂Cl₂

B) FeCl

C) FeCl₂

D) Fe₂Cl

10 / 25

Why are parentheses used in formulas like Ca(OH)₂ but not in NaCl ?

A) To indicate covalent character

B) To group polyatomic ions when more than one is present

C) To show ionic bonding

D) To separate metals from non-metals

11 / 25

A compound is formed from Al³⁺ and NO₃⁻ ions. What is the correct formula?

A) Al(NO₃)₃

B) Al₃(NO₃)₂

C) Al₂(NO₃)₃

D) AlNO₃

12 / 25

A student writes Mg₂Cl₂ for magnesium chloride. Why is this formula considered incorrect in standard representation?

A) It is not simplified to lowest ratio.

B) Chlorine changes oxidation state.

C) It is not electrically neutral.

D) Magnesium cannot bond with chlorine.

13 / 25

Which of the following correctly describes oxidation number in ionic formula formation?

A) Charge of atom in free state.

B) Mass number of ion.

C) Number of neutrons in atom.

D) Imaginary charge assigned based on electron transfer.

14 / 25

A compound is formed between Ca²⁺ and PO₄³⁻ ions. What is correct formula?

A) Ca₂(PO₄)₃

B) Ca₃PO₄

C) CaPO₄

D) Ca₃(PO₄)₂

15 / 25

Which step is essential after applying criss-cross method?

A) Convert to molecular form

B) Multiply atomic numbers

C) Change oxidation numbers

D) Reduce subscripts to simplest ratio

16 / 25

A compound contains NH₄⁺ and CO₃²⁻ ions. What is correct formula?

A) NH₄(CO₃)₂

B) NH₄CO₃

C) (NH₄)₂CO₃

D) NH₄₂CO₃

17 / 25

Why is FeCl₃ different from FeCl₂?

A) Chlorine forms covalent bonds.

B) Chlorine changes atomic number.

C) Iron cannot form ionic compounds.

D) Iron has variable oxidation states.

18 / 25

A student incorrectly writes AlO for aluminium oxide. What is correct formula logic?

A) Aluminium and oxygen form covalent bond.

B) Charges are not balanced in 1 :1 ratio.

C) Oxygen changes oxidation state.

D) Charges are equal so no criss-cross needed.

19 / 25

Which compound requires parentheses in its correct formula?

A) Ca(NO₃)₂

B) NaCl

C) KBr

D) MgO

20 / 25

A compound formed from Zn²⁺ and NO₃ ⁻ ions will have formula:

A) Zn₂(NO₃)₂

B) Zn₃(NO₃)₂

C) Zn(NO₃)₂

D) ZnNO₃

21 / 25

Which statement BEST explains ionic formula formation?

A) Molecular mass must be equal

B) Elements combine randomly

C) Total positive and negative charges are balanced

D) Atoms share electrons equally

22 / 25

A compound is formed from Na⁺ and S²⁻. What is correct formula?

A) Na₂S₂

B) NaS

C) NaS₂

D) Na₂S

23 / 25

Why is oxidation number important in formula formation?

A) It defines physical state

B) It determines color of compound

C) It helps balance ionic charges

D) It increases atomic size

24 / 25

A compound contains Ba²⁺ and Cl⁻ ions. What is correct formula?

A) BaCl

B) BaCl₂

C) Ba₂Cl

D) Ba₂Cl₂

25 / 25

A student writes Al₃O₂ instead of Al₂O₃. What concept is violated?

A) Charge neutrality

B) Atomic theory

C) Molecular symmetry

D) Law of conservation of energy

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