Atomic Masses : A complete Guide For JEE mains , JEE advance & NEET

Guys for preparing Chemistry for JEE mains , JEE advance & NEET examination it is must to understand some key concepts in details . These are called as basic of Chemistry .

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One of the starting thing for JEE mains , JEE advance & NEET exam is to prepare 1st chapter of class 11th that is Some basic concepts . The first Topic which you will encounter is Atomic mass.

In this article i am going to explain all the important types of atomic masses & Their uses in MCQ questions in JEE mains , JEE advance & NEET exams.

Atomic mass as the name suggest mass of an atom . But in actual it is explained in different ways as :

  1. Relative Atomic mass
  2. Average Atomic mass
  3. Approximate atomic mass
  4. Exact atomic mass
  5. Gram Atomic mass

(a) Relative atomic mass is measured with respect to C-12 & was Oldest method of determining Atomic mass.

(b) Average Relative mass It generally used at school level for solving problems based upon atomic masses.

It may be defined as average relative mass of an atom as compare to 1 atom of C-12.

Avg. Atomic mass = Atomic mass of 1st Isotope x Fractional Abundance + Atomic mass of 2nd x Fractional Abundance / Sum of Abundance

Units of Atomic masses : amu & u are the two famous units of Atomic mass.

1 amu = 1 u .

1 amu may be defined as 1/12 of the mass of 1-C-Atom .

1 amu = (1/12) x mass of 1-C-Atom .

(c) Approximate Atomic mass ( Doulong's & Petites law )  It states that the Product of Atomic mass & Specific heat = 6.4 .

Atomic mass x Specific Heat = 6.4

Note - This is applicable only to the metals & Solid elements.


In which of the following the Concept of Approximate atomic mass or Doulong’s & petites is not applicable –

(a) Oxygen


(c) Hg

(d) chlorine

(e) sodium

Ans = A & D


2.An element has specific heat 0.16 , find its approximate atomic mass?

  1. 30
  2. 40
  3. 50
  4. None

Ans = B


(d) Exact Atomic mass = Valency x equivalent Mass.

This method is widely used for determination of Molecular mass.

3.Find out exact atomic mass of a trivalent atom  whose equivalent mass is 60?





Ans = C

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