IB Chemistry Topic S3.2.1 Representation of Organic Compounds Revision Notes (SL/HL)

IB Chemistry Topic S3.2.1 Representation of Organic Compounds Revision Notes (SL/HL) | RevisionDojo

Written by official IB examiners

Types of Formulas and Interconversion of Representations

Consider that you're in a chemistry lab, analyzing a small vial of an unknown organic compound.
You’re tasked with figuring out its molecular structure.
Where do you begin?
You might start by examining its formula, but which one?
A single compound can be represented in multiple ways: as an empirical formula, molecular formula, structural formula, or skeletal formula.

Empirical Formula: The Simplest Ratio

Definition

Empirical formula

The empirical formula represents the simplest whole-number ratio of atoms of each element in a compound.

It tells you the relative proportions of atoms but not the actual number of atoms in a molecule.

Example

Glucose has the molecular formula C₆H₁₂O₆, but its empirical formula is CH₂O.
This means that, for every carbon atom, there are two hydrogens and one oxygen in the simplest ratio.

Key Features:

Provides no structural information.
Useful for understanding the relative composition of a compound.

Example

If a compound has 40% carbon, 6.7% hydrogen, and 53.3% oxygen by mass, its empirical formula can be determined.
Divide each percentage by the atomic mass of the element:
- Carbon: $\frac{40}{12.01} = 3.33$
- Hydrogen: $ \frac{6.7}{1.01} = 6.63 $
- Oxygen: $ \frac{53.3}{16.00} = 3.33 $
Divide by the smallest value (3.33) to get the ratio: C₁H₂O₁, or simply CH₂O.

Tip

When calculating empirical formulas, ensure all percentages add up to 100%.
If not, account for rounding errors or missing data.

Molecular Formula: The Actual Atom Count

Definition

Molecular formula

The molecular formula specifies the exact number of atoms of each element in a molecule.

Example

For glucose, the molecular formula C₆H₁₂O₆ shows that it contains six carbons, twelve hydrogens, and six oxygens.

Key Features:

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Types of Formulas and Interconversion of Representations

Consider that you're in a chemistry lab, analyzing a small vial of an unknown organic compound.
You’re tasked with figuring out its molecular structure.
Where do you begin?
You might start by examining its formula, but which one?
A single compound can be represented in multiple ways: as an empirical formula, molecular formula, structural formula, or skeletal formula.

Empirical Formula: The Simplest Ratio

Definition

Empirical formula

The empirical formula represents the simplest whole-number ratio of atoms of each element in a compound.

It tells you the relative proportions of atoms but not the actual number of atoms in a molecule.

Example

Glucose has the molecular formula C₆H₁₂O₆, but its empirical formula is CH₂O.
This means that, for every carbon atom, there are two hydrogens and one oxygen in the simplest ratio.

Key Features:

Provides no structural information.
Useful for understanding the relative composition of a compound.

Example

If a compound has 40% carbon, 6.7% hydrogen, and 53.3% oxygen by mass, its empirical formula can be determined.
Divide each percentage by the atomic mass of the element:
- Carbon: $\frac{40}{12.01} = 3.33$
- Hydrogen: $ \frac{6.7}{1.01} = 6.63 $
- Oxygen: $ \frac{53.3}{16.00} = 3.33 $
Divide by the smallest value (3.33) to get the ratio: C₁H₂O₁, or simply CH₂O.

Tip

When calculating empirical formulas, ensure all percentages add up to 100%.
If not, account for rounding errors or missing data.

Molecular Formula: The Actual Atom Count

Definition

Molecular formula

The molecular formula specifies the exact number of atoms of each element in a molecule.

Example

For glucose, the molecular formula C₆H₁₂O₆ shows that it contains six carbons, twelve hydrogens, and six oxygens.

Key Features:

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Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

IB Chemistry S3.2.1 Representation of Organic Compounds Notes

Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

Types of Formulas and Interconversion of Representations

Empirical Formula: The Simplest Ratio

Key Features:

Molecular Formula: The Actual Atom Count

Key Features:

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Types of Formulas and Interconversion of Representations

Empirical Formula: The Simplest Ratio

Key Features:

Molecular Formula: The Actual Atom Count

Key Features:

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Duis aute irure dolor in reprehenderit

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