3.2.1Additional lessons
Representing Formulas of Organic Compounds — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 59 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Types of Formula
Empirical
- Empirical The simplest whole-number ratio of atoms of each element, such as CH₂ for ethene and for hex-1-ene.
Molecular
- Molecular The actual number of atoms of each element in one molecule, such as C₆H₁₂ for hex-1-ene.
Full structural
- Full structural Shows every atom and every bond, so the arrangement of atoms in a molecule is clear.
Condensed structural
- Condensed structural Writes groups along the chain, such as CH₃CH₂CH₂OH, leaving out bond lines.
Skeletal
- Skeletal Uses a zigzag line for the carbon chain with atoms assumed at each vertex and end, and only the functional groups shown.
| Compound | Molecular | Condensed |
|---|---|---|
| Ethanol | C₂H₆O | CH₃CH₂OH |
| Propanoic acid | C₃H₆O₂ | CH₃CH₂COOH |
| Butane | C₄H₁₀ | CH₃CH₂CH₂CH₃ |
Section 2 of 5Writing and Reading
1
Count the carbon atoms along the longest chain, then add branches, which are alkyl groups such as –CH₃.
2
Each carbon in a saturated structure forms four bonds, so hydrogen atoms complete the valence of each carbon.
3
Functional groups such as –OH, –COOH and C=O are written at the position where they are attached.
4
In a skeletal formula, each line end and each bend is a carbon atom, and hydrogen atoms attached to carbon are not shown.
5
The molecular formula is found by counting all atoms, and the empirical formula by dividing by the highest common factor.
Section 3 of 5Choosing the Right Representation
Choosing the Right Representation
Isomers
- Isomers Different structural formulas with the same molecular formula show that isomers exist, such as ethanol and methoxymethane.
3D features
- 3D features Wedge and dash bonds show the three-dimensional shape around a carbon atom in stereochemistry.
Analysis
- Analysis Combustion analysis gives mass percentages, which lead to empirical formulas before mass spectrometry gives the molecular formula.
Clarity
- Clarity A skeletal formula is quickest for large molecules, but a full structural formula is clearest for showing bond types.
Section 4 of 5Worked Example
Question
- A compound contains 54.5% C, 9.1% H and 36.4% O by mass and has a relative molecular mass of 88.
- Find its empirical and molecular formulas.
- Divide by atomic masses
- C 54.5 ÷ 12.01 = 4.54;
- H 9.1 ÷ 1.008 = 9.03;
- O 36.4 ÷ 16.00 = 2.28.
- C 54.5 ÷ 12.01 = 4.54;
- Divide by the smallest value (2.28): C 1.99, H 3.97, O 1.00, giving the ratio 2 : 4 : 1.
- Empirical formula is C₂H₄O, with Mᵣ = 44.
- 88 ÷ 44 = 2, so the molecular formula is C₄H₈O₂.
Answer Empirical C₂H₄O; molecular C₄H₈O₂
Section 5 of 5Exam Checklist & Tips
3.2.1 – Key Points for the Exam
- Empirical formula The simplest whole-number ratio of atoms of each element in a compound.
- Molecular formula The actual number of each type of atom in one molecule.
- Structural formula A formula showing how atoms are connected.
- Skeletal formula A simplified drawing in which carbon and hydrogen atoms are implied.
- Functional group An atom or group of atoms that determines characteristic reactions.
Common Mistakes
- Counting a bend or end of a skeletal structure as a missing atom rather than a carbon atom.
- Giving the empirical formula when the question asks for the molecular formula.
- Writing CH₃COH instead of CH₃CHO for ethanal.
- Forgetting hydrogen atoms attached to carbon when counting atoms from a skeletal drawing.
Last updated:
\n