Empirical Formula and Molecular Formula Introduction

Introduction to Molecular and Empirical Formulas

• Understanding molecular formulas and empirical formulas.

• Differences between molecular and empirical formulas.

• Learning to write empirical formulas from given molecular formulas.

Molecular Formula

• Definition: Represents the actual number of atoms of each element in a compound.

• Example: Ethene (C4H8)

  • Contains carbon and hydrogen:

    • Carbon: 4 atoms → C4

    • Hydrogen: 8 atoms → H8

• The molecular formula tells us the exact composition of the compound.

Empirical Formula

• Definition: Represents the simplest or most reduced ratio of different atoms in a compound.

• Example with Ethene:

  • Ratio of atoms: 4 Carbons (C) : 8 Hydrogens (H)

  • To simplify:

    • Divide both by their greatest common divisor (4):

      • C: 4/4 = 1 → C

      • H: 8/4 = 2 → H2

  • Resulting empirical formula: CH2

Example with Another Compound: Cyanotriazide

• Molecular Formula Calculation:

  • Atoms: 3 Carbons (C) and 12 Nitrogens (N) → C3N12

• Empirical Formula Calculation:

  • Ratio: 3 Carbons : 12 Nitrogens → C3:N12

  • Simplification:

    • Divide both by 3:

      • C: 3/3 = 1 → C

      • N: 12/3 = 4 → N4

  • Resulting empirical formula: CN4

Empirical Formulas with Multiple Elements

• Example: Molecular formula with carbon, hydrogen, and oxygen: C5H10O5

• To find the empirical formula, observe the subscripts:

  • GCD of 5:

    • C: 5/5 = 1 → C

    • H: 10/5 = 2 → H2

    • O: 5/5 = 1 → O

  • Resulting empirical formula: CH2O

Cases Where Empirical Formula Equals Molecular Formula

• Example: P3N5 and C5H12

• No further simplification possible:

  • The empirical formula is the same as the molecular formula when no common factor for simplification exists.

• For P3N5: Empirical formula = P3N5.

• For C5H12: Empirical formula = C5H12.

Multiple Compounds, Same Empirical Formula

• Various compounds can share the same empirical formula:

  • Molecular formula C4H8 has empirical formula CH2.

  • Other compounds with same empirical formula include:

    • C2H4, C3H6, C5H10, C6H12.

• Notable ratio: In hydrocarbons, whenever there are twice as many hydrogens as carbons, the empirical formula will be CH2.

Conclusion

• Distinction:

  • Molecular formula gives total atoms of each element in a compound.

  • Empirical formula gives simplest ratio of those atoms.

• Important concepts:

  • Some molecular formulas cannot be simplified, leading to identical empirical formulas.

  • Several different molecular formulas can share the same empirical formula (ex: C4H8 and CH2).

Next Steps

• Practice writing empirical formulas to solidify understanding.

• Molecular Formula

  • Represents actual number of atoms in a compound.

  • Example: Ethene (C4H8)→ Contains carbon and hydrogen: C: 4, H: 8

• Empirical Formula

  • Simplest ratio of atoms in a compound.

  • Example: Ethene→ C: 4 : H: 8 → CH2

• Simplifying Ratios

  • Divide by the greatest common divisor.

  • Example: Cyanotriazide (C3N12)→ C: 3 : N: 12 → CN4

• Empirical Formulas with Multiple Elements

  • Example: C5H10O5→ Simplified to CH2O

• No Simplification Possible

  • Example: P3N5, C5H12→ Empirical formulas: P3N5 and C5H12

• Multiple Compounds, Same Empirical Formula

  • Different molecular formulas can share one empirical formula.

  • Example: C4H8 → CH2 also C2H4, C3H6, ...

• Conclusion

  • Molecular formula shows total atoms; empirical formula shows simplest ratio.

• Next Steps

  • Practice writing empirical formulas for understanding.