Relative Atomic Mass


 
 
Concept Explanation
 

Relative Atomic Mass

Relative Atomic Mass: It is defined as the number of times a given atom is heavier than 1/12th of mass of 1 atom of carbon-12 (C-12) or it is the average mass of the atom as compared to 1/12th the mass of one carbon-12 atom.

The relative atomic mass, of an element is calculated from:

  • the mass number of its isotopes.
  • the abundance  of these isotopes.
  • Chlorine : 

    Chlorine natuarlly exists as two isotopes,_{17}^{35}Cl and _{17}^{37}Cl. The abundance of chlorine -35 is 75% and the abundance of chlorine-37 is 25%.

    To calculate relative atomic mass, A_r, of chlorine:

    A_r=frac{total:mass :of :atoms }{total: number :of: atoms}=frac{(35times75)+(37times25)}{75+25}

    A_r=frac{2625times925}{100}=frac{3550}{100}

    A_r=35.5

    Sample Questions
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    Question : 1

    Which of the following are correct :

    (a) The relative atomic mass, of an element is calculated from the mass number of its isotopes only.

    (b) The relative atomic mass, of an element is calculated from the abundance  of these isotopes only.

    (c) Relative atomic mass is the average mass of the atom as compared to 1/12th the mass of one carbon-12 atom.

    Right Option : C
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    Question : 2

    What is the relative atomic mass of Chlorine ?

    Right Option : B
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    Explanation
    Question : 3

    The relative atomic mass, of an element is calculated from:

    (a) The atomic number of the element.

    (b) The abundance  of these isotopes of an element.

    Right Option : B
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    Explanation
     
     


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