Calculation of Molecular Mass Moles Atomic Mass || Chemistry

Molecular mass: it is defined as the total mass of atoms that dwell in molecules.

For example; if you are asked to calculate the molecular mass of CO2 then we will it as following?

Solution; C=12x1=12

                  O=16 x2=32

                                    44 (

Moles: The moles are defined as the atomic masses, formula mass, and molecular mass expressed in grams.

Example 1; calculate the number of moles in 22g of CO2?



Example 2; Calculate the mass of 2 moles of CO2?


DATA;  Mass=??




 Formula mass: The total sum of the atomic masses of the atoms present in the empirical formula of the compound is called formula mass.

Example; calculate the formula mass of NaCl?

Solution;  Na=23 X1=23

                   Cl= 35X1=35

                                     58 (a.m.u)

Avogadro’s number: The Avogadro’s is defined as the one mole of any substance contains 6.02X10-23 it represented by NA. Avogadro’s number is used to calculate the number of atoms or calculate the no of molecules.

For example; calculate the number of atoms of Al in 22 g?

Data; no; of atoms=??

           Given mass= 22 g

           Molecular mass= 27

            NA= 6.02X10-23


Molecular formula

The formula which shows the actual number of molecules is known as the molecular formula. For instance, C6H6 is the molecular formula of benzene.

Empirical formula:

The formula which does not shows the actual number of molecules only shows the simplest ratio is known as the empirical formula. For instance, CH is the empirical formula of benzene whose chemical formula is (C6H6).


Conservation of mass:

The mass of the reactant before the reaction and the mass of the product after the reaction remains the same.  The initial weight of the reactant and final weight of the product remain fixed reaction may be changed. For example, H2O—> H+OH the mass of water before the reaction is 18 after the decomposition of water the mass remains 18. from the above example it is concluded that the reaction may be changed but the mass remains constant.

Atomic mass:

Atomic mass is defined as the sum of the total number of protons and neutrons. It is denoted by’ A’. The mathematical formula of atomic mass is A=N+Z.

Atomic number

The number of protons present in an atom. For instance, 6 is the atomic number of the carbon chemical symbol (C). It is represented by ‘Z’.




LET’S DO it yourself….


Calculate the molecular mass of the following in a.m.u?

   N2O2, C2H2, CH2, NO2


Calculate the formula mass of the following

MgCl, HCl, NO, CO


Calculate the moles in 22 g of each?

O2, C6H12O6, CH4, NH4Cl


Calculate the mass in 5 moles of each?

Cl2, N2, H2O, CH


Calculate the number of atoms in 20 g of each?

Na, Ca, Mg, N2