Molecular Formula and Empirical Formula for JEE

VSAT 2022

A Brief Intro about Molecular and Empirical Formulas

At first, chemical formulas were calculated by determining the masses of all the elements that combine to form a molecule, and subsequently  this arose to get into two important types of chemistry formulas: molecular formulas and empirical formulas. The empirical formula of a compound gives the simplest ratio of the number of different atoms present, whereas the molecular formula gives the actual number of each different atom present in a molecule. Let us learn more about these two concepts in this article. 


What is a Molecular Formula?

  • Molecular Formula is a term used in Chemistry to describe the number of atoms in a molecule. This number can be represented by the chemical formula. The molecular formula is important for many reasons, including determining the properties of a molecule and predicting how it will react with other molecules.

  • It is a representation of a molecule that uses chemical symbols to indicate the types of atoms followed by subscripts to show the number of atoms of each type in the molecule. (A subscript is used only when more than one atom of a given type is present.)

 

What is an Empirical Formula?

  • Empirical formula is a mathematical formula that represents the composition of a compound. The term "empirical" refers to the fact that these formulae are not derived from any set of predetermined rules or principles of Chemistry but are instead based on empirical observations of chemical reactions.

  • An empirical formula for a compound is the formula of a substance written with the smallest integer subscript.


Steps to Determine  Empirical Formula of a Compound 

  • Determine the relative masses of the elements in the compound.

  • Convert the relative atomic masses into moles. This can be done by dividing each element's relative mass by its molar mass.

  • Divide each element's mole value by the smallest mole value from all of the elements in the compound. This will give you the ratio of each element in the compound.

  • Round off these ratios to whole numbers; this is your empirical formula for the compound!


Steps to Determine  Molecular Formula of a Compound 

  • In order to determine the molecular formula of a compound, one must first identify the relative masses of the elements present in the compound.

  • Once the relative masses have been determined, we can also calculate the molar mass of the compound. The molar mass is simply the sum of all of the molar mass of the atoms of every element present in a compound.

  • Once the molar mass has been calculated, it can be used to determine how many moles of each element are present in one mole of the compound. This is done by dividing the mass of the element present in one mole compound by molar mass of the element.

  • The resulting values will give you the number of atoms for each element in one molecule of the compound.

  • These values can then be used to write out the molecular formula for the compound.


Let's take an example and understand the above concepts. Here is an example.

An unknown chemical compound  has the following composition: 29.48% of carbon, 3.19% of hydrogen, 15.48% of oxygen, and 18.85% of nitrogen. The molecular weight is 194.19 g/mol. Find out the molecular and empirical formula.

 

Solution:

Step 1

Multiply percent composition with the molar mass of the unknown compound.

Carbon – $194.19~g\times 0.2948=57.2472~g$

Hydrogen – $194.19~g\times 0.0319=6.1946~g$

Oxygen –  $194.19~g\times 0.1548=30.0606~g$

Nitrogen – $194.19~g\times 0.1885=36.6048~g$

 

Step 2

Divide the relative mass of each element in an unknown compound with their corresponding molar masses.

Carbon: $\dfrac{57.2472~g}{12.01~g/mol}=4.766~mol$

Hydrogen: $\dfrac{6.1946~g}{1.088~g/mol}=5.6935~mol$

Oxygen: $\dfrac{30.0606~g}{15.994~g/mol}=1.8794~mol$

Nitrogen: $\dfrac{36.6048~g}{14.0067~g/mol}=2.61333~mol$

 

Step 3

Round off the values to the closest whole number.

5: Carbon

6: Hydrogen

2: Oxygen

3: Nitrogen

Hence, the molecular formula is C5H6N3O2.

 

Step 4

Since 1 is the common factor among 5, 6, 3, and 2.

The empirical formula is C5H6N3O2.


Difference Between Molecular Formula and Empirical Formula

Molecular Formula

Empirical Formula

1.  The molecular formula tells us the exact number of atoms of each element present in a molecule.

1. The empirical formula gives us the relative proportions of different elements.

2. It gives the exact number of atoms in a molecule.

2. It gives the simplest ratio of type of atoms in a molecule.

3. Example: For Acetylene, the empirical formula is ${{C}_{2}}{{H}_{2}}$

3. Example: For Acetylene, the empirical formula is CH.


Relationship Between Molecular Formula and Empirical Formula

The empirical formula of a molecule is the simplest whole number ratio of atoms of each element in the molecule. A molecular formula is the actual number of atoms of each element in the molecule. For instance, let us take Glucose. The molecular formula of glucose is ${{C}_{6}}{{H}_{12}}{{O}_{6}}$ and the empirical formula of glucose is CH2O. We can derive a relation between the molecular formula and the empirical formula of glucose.


${{C}_{6}}{{H}_{12}}{{O}_{6}}=6\times C{{H}_{2}}O$

We can derive a general expression as

Molecular formula = n × empirical formula where n is a whole number.

Sometimes, the empirical formula and molecular formula both can be the same.


Summary

In this article, we discussed two important concepts, i.e., empirical formula and molecular formula. The empirical formula is the simplest formula for a compound which is defined as the ratio of subscripts of the smallest possible whole number of the elements present in the formula. The molecular formula is the formula derived from molecules and is representative of the total number of individual atoms present in a molecule of a compound.

FAQs on Molecular Formula and Empirical Formula for JEE

1. How is the empirical formula used in practice?

Empirical method for determining the empirical formula can be applied to almost any real-world problem. The empirical formula helps in showing what are the elements present in a compound. This also indicates the ratio in which we need to combine the elements to form the compound. Knowing what a reaction will produce makes the reaction safer and more efficient for the consumer. In some problems, it also helps in finding the molecular formula of the compound.

2. Which is more informative: empirical formula or molecular formula?

The molecular formula is most useful when we want to know how many atoms of the elements are present in the compound. It gives more information than the empirical formula and is, therefore, more common. The molecular formula is especially important when working with Organic Chemistry where most elements almost have the same empirical formula, for example, the empirical formula of ethyne (C2H2) and benzene(C6H6) is CH. The empirical formula gives an overview of a molecule composition.

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