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4.2

Empirical formula: the simplest ratio

Your guide: Miss SanaCounts atoms the way you count rupees: carefully.

Divide the subscripts down

The empirical formula gives the simplest whole-number ratio of the atoms in a compound. It tells you the ratio, not the real number of atoms in a molecule.

To find it, divide every subscript by their highest common factor. H2O2 divides by 2 to give HO. C6H6 divides by 6 to give CH. Glucose C6H12O6 (p. 56) gives CH2O.

Sometimes nothing divides: H2O, CH4 and NH3 already show the simplest ratio, so their empirical and molecular formulas are the same.

Ionic compounds have no molecules, so their formula unit is already the empirical formula: NaCl, CaF2, MgO, CaCO3.

Two different compounds can share one empirical formula: benzene C6H6 and acetylene C2H2 are both CH.

Step 1 / 7

Notes, short questions and MCQs

Read the full notes: key terms, model answers and MCQs with answers

Divide the subscripts down

The empirical formula gives the simplest whole-number ratio of the atoms in a compound. It tells you the ratio, not the real number of atoms in a molecule.

To find it, divide every subscript by their highest common factor. H2O2 divides by 2 to give HO. C6H6 divides by 6 to give CH. Glucose C6H12O6 (p. 56) gives CH2O.

Sometimes nothing divides: H2O, CH4 and NH3 already show the simplest ratio, so their empirical and molecular formulas are the same.

Ionic compounds have no molecules, so their formula unit is already the empirical formula: NaCl, CaF2, MgO, CaCO3.

Two different compounds can share one empirical formula: benzene C6H6 and acetylene C2H2 are both CH.

Key terms

Empirical formula
Comparing compositions.
Molecular formula
Describing one real molecule.
Highest common factor
Reducing a formula to its ratio.
Formula unit
Is already empirical for ionic compounds.

Short questions with model answers

  1. Q1. Find the empirical formula of benzene, C6H6. (p. 54)

    • C : H = 6 : 6
    • (6 : 6) ÷ 6 = 1 : 1

    Empirical formula CH.

  2. Q2. Find the empirical formula of NH3.

    • N : H = 1 : 3
    • No common factor

    Empirical formula NH3, the same as its molecular formula.

Common mistakes

  • ✗ HO is the formula of a real hydrogen peroxide molecule.

    ✓ HO is only the ratio; the molecule is H2O2.

  • ✗ Empirical and molecular formulas are always different.

    ✓ For H2O, CH4 and NH3 they are the same.

  • ✗ C6H12O6 simplifies to C3H6O3.

    ✓ Divide by the highest common factor, 6: CH2O.

MCQs

  1. 1. The empirical formula of H2O2 is:

    1. (a) H2O
    2. (b) HO
    3. (c) H2O2
    4. (d) HO2
    Show answer

    (b) Divide both subscripts by 2 (p. 54).

  2. 2. Which compound has the same empirical and molecular formula?

    1. (a) C6H6
    2. (b) C2H2
    3. (c) CH4
    4. (d) C6H12O6
    Show answer

    (c) 1 : 4 cannot be reduced.

  3. 3. Glucose C6H12O6 has the empirical formula:

    1. (a) C3H6O3
    2. (b) C2H4O2
    3. (c) CHO
    4. (d) CH2O
    Show answer

    (d) Divide by 6 (glucose, p. 56).

  4. 4. Benzene and acetylene share the empirical formula:

    1. (a) CH
    2. (b) C2H
    3. (c) CH2
    4. (d) C6H6
    Show answer

    (a) C6H6 and C2H2 both reduce to 1 : 1.

  5. 5. For calcium fluoride, the empirical formula is:

    1. (a) Ca2F
    2. (b) CaF
    3. (c) CaF2
    4. (d) Ca2F4
    Show answer

    (c) For ionic compounds the formula unit is the empirical formula (p. 53).

Quick revision

  • Empirical formula = simplest whole-number ratio of atoms.
  • Divide all subscripts by their highest common factor: C6H6 → CH.
  • H2O, CH4, NH3 and ionic formula units are already empirical.

Before you start

What is Class 9 Chemistry Empirical formula?
Empirical formula is the open Class 9 Chemistry lesson for Matric Part 1, Punjab board. It is a step-by-step lesson, not a PDF of the book.
Which boards is this Class 9 Chemistry lesson for?
The same Punjab textbook: Lahore, Rawalpindi, Gujranwala, Faisalabad, Multan, Sargodha, Bahawalpur, Dera Ghazi Khan, Sahiwal.
Can I study this in Urdu?
Yes. English, Urdu or Roman Urdu. Science terms stay as the exam writes them.
Is this a notes PDF?
No. The book stays with you. This page is the lesson and the check.

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