Relative Atomic Mass (Ar)

• This is the average mass of atoms of an element relative to mass of an atom of Carbon-12
• We can calculate RAM using % abundance or using ratios
• Using % abundance

  -Multiply the Mass. No of each isotope with its respective % abundance, then get the sum total of all the isotopes present.

  
  

  Example: Chlorine exists as two isotopes, Cl-35 & Cl-37 with the

  % abundance, 75% & 25% respectively. Calculate the R.A.M of Chlorine.

  
  

  R.A.M= (35 x 75/100) + (37 x 25/100)= 35.5
• Using ratios

  -Multiply the mass.no of each isotope with its respective ratios, then get the sum total of all the isotopes present and finally divide by the total ratio.

  Examples: Lithium exists as two isotopes, Li-4 & Li-6 in the ratio 3:1. Calculate the R.A.M of Lithium

  R.A.M= (4 x 3) + (6 x 1)/3 + 1= 4.5