## Introduction to Fundamental Concepts of Chemistry -Molecular Formula

9/24/2015 CBSE

Molecular Formula

Definition

The formula which shows the actual number of atoms of each element present in
a molecule is called molecular formula.

OR

It is a formula which represents the element ratio between the elements and
actual number of atoms of each type of
elements present per molecule of the compound.

Examples

�The molecular formula of benzene is "C6H6". It indicates that

�1. Benzene molecule is composed of two elements carbon and hydrogen.

�2. The ratio between carbon and hydrogen is 1:1.

�3. The number of atoms present per molecule of benzene are 6 carbon and 6
hydrogen atoms.

�The molecular formula of glucose is "C6H12O6". The formula represents that

�1. Glucose molecule is composed of three elements carbon, hydrogen and oxygen.

�2. The ratio between the atoms of carbon, hydrogen and oxygen is 1:2:1.

�3. The number of atoms present per molecule of glucose are 6 carbon atoms. 12
hydrogen atoms and 6 oxygen atoms.

Determination of Molecular Formula

�The molecular formula of a compound is an integral multiple of its empirical
formula.

�Molecular formula = (Empirical formula)n

�Where n is a digit = 1, 2, 3 etc.

�Hence the first step in the determination of molecular formula is to calculate
its empirical formula by using the procedure
as explained in empirical formula. After that the next step is to calculate the
value of n

�n = Molecular Mass / Empirical Formula Mass

Example

�The empirical formula of a compound is CH2O and its molecular mass is 180.

�To calculate the molecular formula of the compound first of all we will
calculate its empirical formula mass

�Empirical formula mass of CH2O = 12 + 1 x 2 + 16

�= 30

�n = Molecular Mass / Empirical Formula Mass

�= 180 / 30

�= 6

�Molecular formula = (Empirical formula)n

�= (CH2O)6

�= C6H12O6

Molecular Mass

Definition

The sum of masses of the atoms present in a molecule is called as molecular
mass.

OR

�It is the comparison that how mach a molecule of a substance is heavier than
1/12th weight or mass of carbon atom.

Example

�The molecular mass of CO2 may be calculated as

�Molecular mass of CO2 = Mass of Carbon + 2 (Mass of Oxygen)

�= 12 + 2 x 16

�= 44 a.m.u

�Molecular mass of H2O = (Mass of Hydrogen) x 2 + Mass of Oxygen

�= 1 x 2 + 16

�= 18 a.m.u

�Molecular mass of HCl = Mass of Hydrogen + Mass of Chlorine

�= 1 + 35.5

�= 36.5 a.m.u

Gram Molecular Mass

Definition

The molecular mass of a compound expressed in gram is called gram molecular
mass or mole.

Examples

�1. The molecular mass of H2O is 18. If we take 18 gm H2O then it is called 1 gm
molecular mass of H2O or 1 mole of water.

�2. The molecular mass of HCl is 36.5. If we take 36.5 gm of HCl then it is
called as 1 gm molecular mass of HCl or 1 mole of HCl.