isotopes of masses : m1= 70.95 amu
                               Â
m2 = 72.95 amu
abundances : Â Â Â Â Â A1 = 30.9 % =
0.309
                           Â
A2 = 69.1% = 0.691
average atomic mass = ( atomic mass of isotope 1 x abundance 1 +
atomic mass of isotope 2 x abundance 2 )
                                  Â
= (70.95 x 0.309 + 72.95 x 0.691)
                                  Â
= 72.33 amu
average atomic mass   = 72.33
amu
