Molecular Mass

Molecular mass of substance (elements or compound) is the relative mass of its molecule Compared with that of an atom Of carbon-12 isotope taken as 12.

In other word molecular mass of substance represent the number of times the molecule of that substance is heavier than 1/12 of  the mass of an atom of C-12 isotope.

Calculations of Molecular Mass

  • The molecular mass is equal to the sum of atomic masses of all atom present in one molecule of the substance.
  • A molecule is made up of two or more atom of the same or different elements and each has definite atomic mass ,  therefore molecular mass of a molecule of substance can be calculated by adding atomic mass Of all the atom present in one molecule of the substance.
  • Example;

1) A molecule of hydrogen has the formula H,  it contains two atom of hydrogen.  Atomic mass of hydrogen 1.0u

So,       2, hydrogen   =   (2×1. 0)

=    2.0u.

2) A molecule of oxygen has the formula O2    ,  it contains two atom of oxygen.  Atomic mass of oxygen 16u.

So, 2 ,Oxygen      =  (2×16)

32 u.

3) A molecule of ozone has the formula O3 ,  it contains three atom of oxygen.  Atomic mass of oxygen 16u.

So, 3 Oxygen   =  ( 3×16)

=    48u.

4) A molecule of carbon dioxide has the formula CO2 ,  it contains two atom of oxygen and one carbon atom. Hence the  Atomic mass of carbon- 12u and oxygen- 16u.

So, One carbon +    two oxygen =  ?

(1×12)   +  (2×16)

12  +   32   =  44u.

Question answer:

1) What is molecular mass?

Answer: The molecular mass is equal to the sum of atomic masses of all atom present in one molecule of the given substance.

2) Calculate the molecular mass of ZnO?

Answer:  molecular mass of ZnO is following.

A molecule of Zinc oxide has the formula ZnO    ,  it contains one atom of Zinc and one atom of oxygen.  Atomic mass of Zinc 65u.and Atomic mass of oxygen 16u.

So,

One atom of Zinc   (1×65) = 65

One atom of oxygen  (1×16)  =16

ZnO   ( 65+16) = 81 u.

4) Calculate the molecular mass of C12H22O11?

Answer: A molecule of sugar has the formula C12H22O11    ,  it contains  12-atom of carbon22- atom of hydrogen, and 11- atom of oxygen. Atomic mass of carbon 12u,  hydrogen 1u, and Atomic mass of oxygen 16u.

So,

12-atom of carbon C- (12×12) = 144

22-atom of hydrogen H- (22×1)  = 22

11-atom of oxygen  O-  (11×16) =176

C12H22O11: (144+22+176)   =342 u.


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