Difference Between Molecular and Structural Formula

Main Difference – Molecular vs Structural Formula

Molecular formula and structural formula of compounds represent atoms of different chemical elements present in the compounds. The molecular formula gives the ratio between these elements. Structural formula gives the relative positions of these atoms. The main difference between molecular formula and structural formula is that molecular formula cannot indicate the relative positions of the atoms whereas structural formula indicates the relative positions of the atoms.

Key Areas Covered

1. What is Molecular Formula
     – Definition, Finding the Molecular Formula
2. What is Structural Formula
    – Definition, Uses
3. What is the Difference Between Molecular and Structural Formula
     – Comparison of Key Differences

Key Terms: Condensed Formula, Empirical Formula, Lewis Structure, Molecular Formula, Structural Formula

What is Molecular Formula

Molecular formula or chemical formula of a compound is the representation of the types of atoms and their ratios present in that compound. The molecular formula is given by symbols of chemical elements and the numbers representing the ratio between them. This ratio is the actual whole number of atoms (the simplest whole number is given by the empirical formula).

The molecular formula can be used in the nomenclature of simple molecules, but not for complex molecules. A molecular formula of a compound can be used to determine whether the compound is a binary compound, ternary compound, quaternary compound or has even more elements.

The molecular formula of a compound can be found when the required details are given. The following examples show how to find the molecular formula.

Question:

Find the molecular formula of a compound having C=40%, H=6.72% the rest is oxygen, and the molecular weight of the compound is found as 180 g/mol.

Answer:

Elements present are;

C = 40%
H = 6.72%
O = (100-{40+6.72}) % = 53.28%

The number of moles of each element present in 100g of the compound;

C = 40g/12gmol-1 = 3.33 mol
H = 6.72g/1gmol-1 = 6.72 mol
O = 53.28g/16gmol-1 = 3.33 mol

The simplest ratio between these elements are found by dividing each value by the largest value among them.

C = 3.33 mol / 6.72 mol = ½
H = 6.72 mol / 6.72 mol = 1
O = 3.33 mol / 6.72 mol = ½

Find the simplest whole number ratio between these atoms;

C = ½ x 2 = 1
H = 1 x 2 = 2
O = ½ x 2 = 1

Hence, the empirical formula of the compound is CH₂O.

To find the molecular formula using this empirical formula, we first need to find the number of empirical formula units present in the compound.

Molar mass of empirical formula unit = CH2O = (12 g/mol) + (1g/mol x 2) + (16g/mol)
                                                                     = 30 g/mol.

The number of empirical formula units present in the compound = 180 gmol-1 / 30 gmol-1 = 6
Then the molecular formula of the compound can be obtained by multiplying the empirical formula by 6.

Molecular formula = (CH2O) x 6 = C6H12O6

What is Structural Formula

Structural formula of a compound is the representation of the arrangement of atoms in the compound. This structural formula gives many details about the molecule and the properties of the compound also can be predicted using these details.

The structural formula can be used for the nomenclature of the molecule. This is because the structural formula shows all the functional groups present in the compound and their relative positions. The structural formula can also be used to predict the chemical properties of compound (such as polarity) and physical properties (such as boiling point).

There are few different ways that are used to give the structural formula. Lewis structures show the connectivity of atoms and the lone pairs or unpaired electron in the compound. The condensed formula is another type of structural formula, which gives the relative positions of atoms, but this is not much helpful (Ex: CH₃CH₂OH is the condensed formula of ethanol).

Figure 1: Lewis Structure of Water Molecule

An important type of structural formula in organic chemistry are skeletal formulas. Most complex organic molecules can be given via skeletal formulas. This type of formulas show the functional groups and their positions. But the hydrogen atoms attached to the carbon atoms are not indicated there.

Figure 2: Ethylene Glycol Skeletal Formula

In stereochemistry, there are many forms of structural formulas. For example, the structural formula indicates the cis-trans geometry of the compound, Newman projection and sawhorse projection of organic compounds, Fischer projection of sugar molecules, Haworth projection, etc.

Difference Between Molecular and Structural Formula

Definition

Molecular Formula: Molecular formula or chemical formula of a compound is the representation of the types of atoms and their ratios present in that compound.

Structural Formula: Structural formula of a compound is the representation of the arrangement of atoms in the compound.

Details Given

Molecular Formula: Molecular formula gives the ratio between atoms present in the compound.

Structural Formula: Structural formal gives the arrangement of atoms and the relative positions of the functional groups in the compound.

Uses

Molecular Formula: Molecular formula can be used for the nomenclature of simple molecules, to determine whether the compound is a binary compound, ternary compound, quaternary compound or has even more elements, etc.

Structural Formula: Structural formula can be used for the nomenclature of complex molecules and to predict the chemical properties of compound (such as polarity) and physical properties (such as boiling point).

Conclusion

Molecular formula and structural formula are very important in determining the actual structure of a compound. The main difference between molecular formula and structural formula is that molecular formula cannot indicate the relative positions of the atoms whereas structural formula indicates the relative positions of the atoms.

Reference:

1. Helmenstine, Anne Marie. “Calculating Compound Empirical & Molecular Formula.” ThoughtCo, Aug. 4, 2017, Available here.
2. “2.1: Structural Formulas.” Chemistry LibreTexts, Libretexts, 22 June 2017, Available here.

Image Courtesy:

1. “Water-2D-flat” (Public Domain) via Commons Wikimedia
2. “Ethylene glycol” By Sander de Jong – Own work (Public Domain) via Commons Wikimedia