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2.1Mass of Atoms and Molecules

2.1    Mass of Atoms and Molecules

Concept of relative mass

  • Relative mass is an indication of how heavy is an atom compared to another atom which is used as a standard
  • Relative mass is expressed in atomic mass unit(a.m.u).
  • C-12 was chosen to be the standard model because:
    1. it is the most abundant isotope of  carbon
    2. it is a solid, easy to handle and easily available
  • C-12 was assigned a mass of exactly 12 a.m.u.. This is known as C-12
  • For example, an atom which is 3.5 times heavier than a C-12 atom would have a relative mass of (3.5 x 12) = 42 a.m.u.. That means, this atom is 42 times heavier than the mass of (1/12 x the mass of C-12 atom).

 

Relative isotopic mass

1) Relative isotopic mass is the mass of an isotope measured on a scale in which a carbon-12 atom has a mass of exactly 12 units.

 

Relative atomic mass, Ar

1) Relative atomic mass, Ar is the weighted average relative masses of all its isotopes measured on a scale in which a carbon-12 atom has a mass of exactly 12 units.

Example:

Ratio of Cl-35 to Cl-37 is 3:1. If you have 4 typical atoms of chlorine, total mass is (35 x 3) + (37 x 1) = 142. So, the average mass of the isotopes is 142/4 = 35.5.

 

This implies that 35.5 is the relative atomic mass of  chlorine while 35 is the

relative mass of Cl-35 and 37 is the relative mass of Cl-37.

Relative molecular mass, Mr

  • Relative molecular mass, Mr is the weighted average of the masses of the molecules measured on a scale in which a carbon-12 atom has a mass of exactly 12 units
  • It should only be applied to substances which exist as molecules.
  • It is found by adding up all the relative atomic masses of all the atoms present in the molecule
  • Examples:
  1. Mr (H2O) = 2(1) + 16 = 18
  2. Mr (CHCl3) = 12 + 1 + 3(35.5) = 119.5

 

Relative formula mass, Mr

  • Relative formula mass, Mr is the weighted average of the masses of the formula units measured on a scale in which a carbon-12 atom has a mass of exactly 12 units.
  • It works for both ionic and covalent compounds
  • Examples:
  1. Mr (NaCl) = 23 + 35.5 = 58.5
  2. Mr (CuSO4 • H2O) = 64 + 32+ 4(16) +5[2(1) + 16] = 249.5