Chemical formulas were first developed by determining the masses of all the elements that combine to form a molecule, which led to the invention of two types of chemistry formulae: Molecular formula and Empirical formula.

The empirical formula gives the simplest ratio of the number of various atoms present, but the molecular formula gives the exact number of every single atom present in a molecule. If the formula is simplified, it is an empirical formula. The molecular formula is typically employed since it is a multiple of the empirical formula.

What is Molecular Formula Meaning?

The molecular formula is a number that represents the total number of individual atoms in a compound’s molecule. The number of each type of atom in a molecule of the substance is indicated by a subscript in the molecular formula.

Gram molecular masses, which are simple whole number multiples of the empirical formula mass, are linked to molecular formulas.

The molecular formula is a formula generated from molecules that represents the total number of individual atoms in a compound’s molecule.

A subscript in a molecular formula indicates the number of each type of atom in a molecule of the substance. Molecular formulas are linked to gram molecular masses, which are simple whole number multiples of the empirical formula mass.

Relationship Between Empirical and Molecular Formula

The empirical formula, defined as the ratio of subscripts of the least whole number of elements in the formula, is the simplest formula for a compound. It’s also known as the most fundamental formula.

An empirical formula is a substance’s formula given with the fewest integer subscripts.

The empirical formula gives the number of atoms in a given ratio in the compound. A compound’s empirical formula is directly proportional to its percent composition.

What is Meant by Molecular Formula?

Molecular Formula  = n × Empirical Formula

Solved Examples

Example 1: Caffeine has the following composition: 49.48% of carbon, 5.19% of hydrogen, 16.48% of oxygen and 28.85% of nitrogen. The molecular weight is 194.19 g/mol. Find out the molecular and empirical formula.

Solution: 

Multiply percent composition with the molecular weight,

Carbon – 194.19 x 0.4948 = 96.0852

Hydrogen – 194.19 x 0.0519 = 10.07846

Oxygen –  194.19 x 0.1648 = 32.0025

Nitrogen – 194.19 x 0.2885 = 56.0238

Divide each value by the atomic weight,

Carbon : 96.0852 / 12.011 = 7.9997

Hydrogen : 10.07846 / 1.008 = 9.998

Oxygen : 32.0025 / 15.9994 = 2.000

Nitrogen : 56.0238 / 14.0067 = 3.9997

Round off the values to closest whole number,

8: Carbon

10: Hydrogen

2: Oxygen

4: Nitrogen

Hence, the molecular formula is C8H10N4O2.

As we know 2 is the common factor of 8, 10, 4 and 2.

The empirical formula is C4H5N2O.

Example 2: The empirical formula is BH3 of boron hydride. Calculate the molecular formula given the measured mass is 27.66.

Solution:

Atomic mass  = B + 3(H) = 10.81 + 3(1) = 13.81u

The measured molecular mass for an atom is given as 27.66u.

Molecular formula = n × empirical formula

n = molecular formula/empirical formula = 27.6613.81 = 2

Putting the value of n = 2 in the empirical formula,

Molecular formula = 2(BH3) = B2H6.

Molecular Mass

The molecular mass of a compound is the sum of the atomic masses of the atoms that make it up. The molecular mass of a molecule is used to determine its mass. Atomic mass is another name for it. The mass of each atom is usually multiplied by the number of atoms of the element present in the molecule. Water, for example, is made up of two hydrogen atoms and one oxygen atom. Water molecules have a mass equal to two or more times the atomic mass of oxygen multiplied by the standard atomic mass of hydrogen. The molecular mass of an element is determined by the atoms that make up the molecule.

The molecular masses of molecules can be changed in the following ways:

Mass Spectrometry: The mass of tiny molecules is frequently determined using this method. This is explained using monoisotopic mass.

In a hydrodynamic method, the weight is resolved using Mark-Houwink relations. This technique is sometimes referred to as a relative atomic weight determination procedure because it requires calibration.

Static Light Scattering: The molecular weight is calculated using the Zimm method based on the amount of light scattered.

Solved Example

Question: What is the molar mass of sulfur trioxide SO3, an atomic compound? 

Solution: By definition, the molar mass of SO3 equals its atomic mass in grams. Hence, the calculation of the molar mass of SO3 is needed first. 

The molecular mass (also called formula mass) of a compound refers to the total atomic masses of the apparent multitude of atoms in the molecule. The molecular mass of  SO3  with 4 atoms in its molecule, is given by,

1S = 1 x 32.07 amu 

= 32.07amu 

3O = 3 x 16.00 amu

= 48.00amu 

Therefore, the molecular mass of  SO3 is 80.07 amu.

Benzene is an important organic chemical compound with the chemical formula C6H6. Its molecule is made up of six carbon atoms arranged in a ring, each with one hydrogen atom attached.

Conclusion

A molecule is formed when the masses of all the atoms unite. The empirical formula provides a ratio of the number of atoms present, but the molecular formula provides the precise number of each and every atom in a molecule. The molecular formula represents the total number of individual atoms in a compound’s molecule.