9.4.2 Chemical Formula

This is the second lecture from Chapter 4: “Stoichiometry” in the new Class 9 Chemistry book (Punjab Board – PCTB). It discusses the chemical formula of elements and compounds. The lecture also includes a multiple-choice quiz, short question and long question notes.

MCQs Based Quiz

9.4.2 Chemical Formula

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Elements like O₂ and N₂ exist as:

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The chemical formula O₃ represents:

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Ionic compounds like NaCl are represented by:

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Which of the following substances can be represented by only a symbol?

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Which of the following substances can be represented by a formula unit?

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Which of the following substances can be represented by a molecular formula?

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Covalent compounds like $\mathrm{H_2O}$ are represented by:

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Which of these is a covalent compound?

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The formula unit of table salt is NaCl. What does this formula indicate?

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Which of the following element is not represented by only its symbol?

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The average score is 100%

Short Questions

Which types of elements are represented by their symbols only?

Elements that exist in the form of group (aggregate) of atoms can be represented by their symbols alone.

For example: Sodium (Na), Calcium (Ca), Carbon (C) and Iron (Fe).

Give some examples of elements that exist in the form of molecules.

Oxygen (O2 and O3), Hydrogen (H2) and Nitrogen (N2).

What is meant by formula unit of an ionic compound?

Formula unit of an ionic compound is the simplest ratio of opposite ions bonded in the form of a crystal.

For example, the formula unit of NaCl indicates that its crystal contains a 1:1 ratio of both ions.

What is a molecular formula?

A covalent compound is represented by a chemical formula which indicates the types and number of atoms bonded together in that compound. This formula is called the molecular formula of that compound.

For example, molecular formula of water is (H2O).

What information can you get from the molecular formula of water (H2O)?

The chemical formula of water tells us that in every molecule of water, two hydrogen atoms are bonded with one oxygen atom.

How would you differentiate between the chemical formula of an element and that of a compound? Give examples. Write down the names of ionic and covalent compounds whose formulas have been given in this article.

A chemical formula of an element only contains one type of atoms. Whereas the chemical formula of a compound contains at least two different types of atoms. For example, chemical formula of oxygen is O2 and the chemical formula of water is H2O.

Names of Ionic Compounds:

NaCl: Sodium chloride

CaCl2: Calcium chloride

KBr: Potassium Bromide

BaCl2: Barium chloride

Names of Covalent Compounds:

H2O: Water

NH3: Ammonia

CH4: Methane

HCl: Hydrogen chloride

HF: Hydrogen fluoride

H2S: Hydrogen sulphide

PH3: Phosphine

H2O2: Hydrogen peroxide

H2SO4: Sulphuric Acid

CO2: Carbon dioxide

CO: Carbon monoxide

C6H6: Benzene

Descriptive Question

Q1. Write a detailed note on chemical formulas of substances.

Elements:

  1. Some elements exist in the form of group (aggregate) of atoms. Such elements are represented by their symbols alone. For example, Na, Ca, C, Fe, etc.
  2. Elements that exist as molecules are represented by a chemical formula that represents the number of atoms in their one molecule. For example, O2, O3, N2 and H2. Ozone is a form of oxygen element, and its formula (O3) tells us that one molecule of ozone contains three oxygen atoms bonded together.

Compounds:

Ionic Compounds: Ionic compounds are represented by a chemical formula called formula unit. Formula unit of an ionic compound is the simplest ratio of ions bonded in the form of a crystal.

For example, the ratio of ions in the crystal of sodium chloride is 1:1. So its formula unit is NaCl.

Some other examples of formula unit include, CaCl2, KBr and BaCl2.

Covalent Compounds: A covalent compound is represented by a chemical formula which the types and number of atoms bonded together in that compound is called the molecular formula of that compound.

For example, molecular formula of water is H2O. This formula represents that a water molecule contains two hydrogen atoms, and one oxygen atom covalently bonded together.

Some other examples, includes HCl, HF, H2S, PH3, H2O2, H2SO4, CO2, CO, C6H6, etc.