Atoms and Molecules

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08 Gram Atomic Mass and Gram Molecular Mass



Gram Atomic Mass:

The amount of a substance whose mass in gram is numerically equal to its atomic mass,  is called gram atomic of that substance.

                     Atomic mass of oxygen, O = 16 u

Gram atomic mass of oxygen, O = 16 u grams

The gram atomic mass of a substance represents the mass of 1 mole of atoms

(6.022 × 1023)

of that substance. The molar mass of an element is the mass of 1 mole of its atoms. The molar mass of an element has

(6.022 × 1023)

atoms of the element in it.


Gram Molecular Mass:

The amount of a substance, whose mass in gram is numerically equal to its molecular mass, is called gram molecular mass of that substance.

Molecular mass of oxygen,

O2 =32 u 

Gram molecular mass of oxygen,

O2 =32


The gram molecular mass of a substance represent the mass of 1 mole of molecules

(6.022 × 1023 molecules)

of that substance.


  • Measurements and calculation of various parameters determine large number of physical and chemical experiments.
  • Measurement always has two parts: numerical part and a unit.
  • For e.g. if the mass of a ball is 2 kg, 2 is numerical part and kg is unit. 1 kg is a well-defined unit.
  • Similarly for most of other measurements there are well defined units, like Kelvin for temperature, meter for length etc.
  • Counting does not have S.I unit but nevertheless, there are units of counting like dozen which is equivalent to 12.
  • Such unit for counting was required when the sub-atomic particles were involved.
  • This was because the scientists started to encounter very large numbers frequently.
  • Consequently, concept of mole was introduced in the study for counting very large numbers.
  • Mole is a link between the mass of atoms (or molecules) and the number of atoms (or molecules).
  • A group of
    6.022 × 1023

    particles (atoms, molecules or ions) of a substance is called a mole of that substance.

  • This number of
    6.022 × 1023

    , which represents a mole, is known as Avogadro number (NA).

  • Mole concept is one of the most convenient ways of expressing amount of reactants and product in reaction.
  • The word “mole” was introduced around 1896 by Wilhelm Ostwald who derived the term from the Latin word moles meaning a ‘heap’ or ‘pile.



Mole of Atoms:

  • 1 mole of atoms of an element has a mass equal to the gram atomic mass of the element.


Mole of Molecules:

  • 1 mole of molecules of a substance has a mass equal to the gram molecular mass of the substance.


Applications of mole concept:

If mass is given and moles or number need to be calculated follow the below steps:

  • STEP 1:- Establishing relationship between molar mass and the number (NA) or moles of that particular entity (atom, molecule or ion).


  • STEP 2:- Use unitary method to calculate what is asked in the question.


Conversion of Moles to Mass and Vice Versa:

The key concept used in these kind of problems is that a mole of any substance contains gram formula mass or molecular mass of that substance.

For example, molecular mass of Hydrogen is 2 a.m.u. so mass of 1 mole of hydrogen which is also known as molar mass will be 2 gram. Similarly if we have 2 moles of hydrogen, it will weigh grams which is equal to 4 grams.



  1. During chemical changes, the sum of masses of all the reactants remains equal to the mass of products. This is known as Law of conservation of mass.
  2. In a compound, the elements are always present in definite proportion of mass. This is known as Law of definite proportions.
  3. According to Dalton’s atomic theory, all matter whether element or compound are composed of small particles called atoms or molecules.
  4. Ion is charged particles and can be positively or negatively charged.
  5. The chemical formula of a molecular compound is determined by the valency of the combining elements.
  6. The number of atoms present in 12 of carbon  -12  is
    6.022  × 1023

     which is called Avogadro number or Avogadro constant.

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