Molecular Formula and Empirical Formula for JEE

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

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.