Class 9

9.4.3 Empirical Formula

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

MCQs Based Quiz

9.4.3 Empirical Formula

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What does an empirical formula represent?

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Which type of compounds are always represented by their empirical formulas?

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What is the empirical formula of calcium fluoride?

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Why do H₂O₂ (molecular formula) and HO (empirical formula) differ?

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Which pair shares the same empirical formula?

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For which compound is the molecular formula identical to its empirical formula?

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Why can't empirical formulas distinguish between benzene and acetylene?

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What fundamental limitation does an empirical formula have?

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If a compound's empirical formula is CH₂O, its molecular formula could be:

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For what type of substances empirical formula may be different from molecular formulas.

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Short Questions

Q1. What is meant by empirical formula of a compound.

A chemical formula that shows the simplest whole number ratio of atoms present in a compound is called its empirical formula.

For example, empirical formula of benzene (C6H6) is CH.

Q2. What is the empirical formula of hydrogen peroxide (H2O2)?

The empirical formula of hydrogen peroxide (H2O2) is HO.

Q3. What is the empirical formula of benzene (C6H6)?

The empirical formula of benzene (C6H6) is CH.

Q4. What is the empirical formula of acetylene (C2H2)?

The empirical formula of acetylene (C2H2) is CH.

Q6. What is the empirical formula of water (H2O)?

Water already contains the simplest whole number ratio of atoms in its molecules. So, its empirical formula is same H2O.

Q7. Is it possible for a compound to have same empirical formula and molecular formula?

Yes, it is possible. For example, water has similar empirical formula (H2O) and molecular formula (H2O).

Q8. Is it possible for two compounds to have similar empirical formulas?

Yes, it is possible for two different compounds to have similar empirical formulas. For example, both benzene (C6H6) and acetylene (C2H2) have the same empirical formula CH.

Q9. Give two examples of the compounds which have same empirical and molecular formula.

  1. Carbon dioxide has similar molecular formula CO2 and empirical formula CO2.
  2. Water also has similar molecular formula H2O and empirical formula H2

Descriptive Question

Q1. Write an explanatory note on empirical formula.

A formula that shows the simplest whole number ratio of atoms present in a compound is called empirical formula.

Empirical Formula of Ionic Compounds:

All the ionic compounds are represented by their empirical formulas because they show the simplest ratio of ions in them. For example, the empirical formula of calcium fluoride is CaF2 which shows the ratio of calcium and fluoride ions in its crystal.

Empirical Formula of Covalent Compounds:

Covalent Compounds With Different Empirical and Molecular Formulas: There exist many covalent compounds whose empirical formulas are different from their molecular formulas. For example, the molecular formula of hydrogen peroxide is H2O2 but its empirical formula HO is different. Similarly, benzene is another covalent compound with different molecular (C6H6) and empirical (CH) formulas.

Covalent Compounds With Same Empirical and Molecular Formulas: There are also some covalent compounds whose molecular and empirical formulas are same. For example, water’s molecular formula is H2O and its empirical formula is also H2O. This is because atoms are already in their simplest whole number ratio.

Different Covalent Compounds with Similar Empirical Formulas: It is possible for two different covalent compounds to have similar empirical formulas if they have similar whole number ratio of atoms. For example, both benzene (C6H6) and acetylene (C2H2) have same empirical formula CH.