Chemical Formulae of Common Compounds MCQs Quiz | Class 9
This Class IX Science (Code 086) quiz covers Unit I: Matter – Its Nature and Behaviour, specifically focusing on Writing and Identifying Chemical Formulae and Valency-based compound formation. Test your understanding of molecular compositions, valency crossing, and polyatomic ions. Attempt the 10 MCQs below, view your score instantly, and download the solution PDF for your exam preparation.
Overview: Writing Chemical Formulae
In Class 9 Science, Unit I, mastering the chemical formulae of common compounds is essential. A chemical formula represents the composition of a molecule of the substance in terms of the symbols of the elements present in it. It is the symbolic representation of a molecule’s composition.
Key Concepts
1. Symbols: Abbreviations used for the names of elements (e.g., H for Hydrogen, Na for Sodium).
2. Valency: The combining capacity of an element. It determines how atoms of an element will combine with atoms of another element.
3. Ions: Charged species. Cations are positively charged (e.g., Na+), and Anions are negatively charged (e.g., Cl-).
Rules for Writing Formulae
- The valencies or charges on the ion must balance.
- When a compound consists of a metal and a non-metal, the name or symbol of the metal is written first (e.g., Calcium oxide, CaO).
- In compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio (e.g., Ammonium sulphate, (NH4)2SO4).
Common Valencies to Remember
| Valency | Name (Symbol) | Polyatomic Ions |
|---|---|---|
| 1 | Sodium (Na+), Potassium (K+) | Ammonium (NH4+), Hydroxide (OH-) |
| 2 | Magnesium (Mg2+), Calcium (Ca2+) | Carbonate (CO3 2-), Sulphate (SO4 2-) |
| 3 | Aluminium (Al3+), Iron(III) (Fe3+) | Phosphate (PO4 3-) |
Steps to Form a Formula (Criss-Cross Method)
Example: Aluminium Oxide
- Write symbols: Al and O.
- Write valencies below: Al has valency 3, Oxygen has valency 2.
- Cross over the valencies: The subscript for Al becomes 2, and for O becomes 3.
- Result: Al2O3.
Extra Practice Questions
- Write the formula for Sodium Sulphide. (Answer: Na2S)
- What is the formula for Magnesium Hydroxide? (Answer: Mg(OH)2)
- Write the formula for Calcium Carbonate. (Answer: CaCO3)
- Identify the cation in Potassium Nitrate (KNO3). (Answer: Potassium, K+)
- What is the formula mass of H2O? (Answer: 18 u)
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