Difference between molecular and structural formula

Main difference - molecular vs. structural formula

The molecular formula and the structural formula of compounds represent atoms of various chemical elements that are present in the compounds. The sum formula gives the relationship between these elements. The structural formula indicates the relative positions of these atoms . The main difference between the molecular formula and the structural formula is that the molecular formula cannot show the relative positions of the atoms, while the structural formula shows the relative positions of the atoms.

Key areas covered

1. What is a molecular formula - definition, finding the molecular formula 2. What is a structural formula - definition, uses 3. What is the difference between molecular and structural formula - comparison of the main differences

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

Difference Between Molecular and Structural Formula - Comparative Summary

What is a Molecular Formula?

The molecular formula or chemical formula of a compound is the representation of the types of atoms and their proportions that are present in that compound. The molecular formula is indicated by symbols of the chemical elements and the numbers that represent the relationship between them. This ratio is the actual integer number of atoms (the simplest integer is obtained from 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, a ternary compound, a quaternary compound, or even more elements.

The molecular formula of a compound can be found if the necessary information is provided. The following examples show how to find the sum formula.

Question:

Find the molecular formula of a compound with C = 40%, H = 6.72%, the remainder is oxygen, and the molecular weight of the compound is found to be 180 g / mol.

Answers:

Existing elements are;

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

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

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

The simplest relationship between these elements is 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 = 1O = 3.33 mol / 6.72 mol = ½

Find the simplest integer ratio between these atoms;

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

Hence the empirical formula of the compound is CH 2 O.

In order to find the empirical formula with this empirical formula, we first have to determine the number of empirical formula units present in the compound.

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

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

Molecular formula = (CH2O) x 6 = C6H12O6

What is a structural formula?

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

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

There are a few different ways that are used to indicate the structural formula. Lewis structures show the connectivity of atoms and the lone pairs or unpaired electrons in the compound. The condensed formula is another type of structural formula that gives the relative positions of the atoms, but that doesn't help much (example: CH 3 CH 2 OH is the condensed formula of ethanol ).

Difference between molecular and structural formula

Figure 1: Lewis structure of the water molecule

An important type of structural formula in organic chemistry are skeletal formulas. Most complex organic molecules can be expressed using skeletal formulas. These kind of formulas show the functional groups and their positions. But the hydrogen atoms bonded to the carbon atoms are not indicated there.

Key difference - molecular vs. structural formula

Figure 2: Ethylene glycol skeletal formula

There are many forms of structural formulas in stereochemistry . For example, the structural formula gives 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 : The molecular formula or chemical formula of a compound is the representation of the types of atoms and their proportions that are present in that compound.

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

Details given

Molecular formula : The molecular formula indicates the ratio between the atoms present in the compound.

Structural formula: Structural formal indicates the arrangement of the atoms and the relative position of the functional groups in the compound.

Used

Molecular Formula : The 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 contains even more elements, etc.

Structural Formula: Structural formulas can be used for the nomenclature of complex molecules and for predicting the chemical properties of compounds (such as polarity) and physical properties (such as boiling point).

diploma

Molecular formula and structural formula are very important in determining the actual structure of a compound. The main difference between the molecular formula and the structural formula is that the molecular formula cannot show the relative positions of the atoms, while the structural formula shows the relative positions of the atoms.

Reference:

1. Helmenstine, Anne Marie. “Computing the composite empirical and molecular formula.” ThoughtCo, August 4, 2017, available here .2. "2.1: Structural formulas." Chemistry LibreTexts, Libretexts, June 22, 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

About the author: Madhusha

Madhusha is a BSc (Hons) graduate in Biological Sciences and is currently doing her Masters in Industrial and Environmental Chemistry. Her areas of interest for writing and research include biochemistry and environmental chemistry.