AVERAGE ATOMIC MASS OPENER

1. Neon has three isotopes of masses 22, 21 and 20. If the isotopes have the abundance 8%, 2% and 90% respectively, what is the average relative atomic mass of neon?

2. A naturally occurring sample of rubidium consists of two isotopes, one of mass 85 and one of mass 87. The ratio of abundance of these isotopes is 72:28. Calculate the average relative atomic mass of rubidium.  

3. If ⁶⁹Ga and ⁷¹Ga occur in the ratio 62:38, calculate the average relative atomic mass of gallium. 

4.  Naturally occurring lead is found to have the following isotopic relative abundance. ²⁰⁴Pb 3%, ²⁰⁶Pb 24%, ²⁰⁷Pb 20% and ²⁰⁸Pb 53%. Calculate the average relative atomic mass of Pb from the data.
 

ANSWERS:

1.  (22 x 0.08 + 21 x 0.02 +  20 x 0.90) = 20.18 amu   

2.  85 x 0.72 + 87 x 0.28 = 85.56 amu     or      (85 x 72 + 87 x 28)/100 = 85.56 amu

3.  69 x 0.62 + 71 x 0.38 = 69.76 amu

4.  (204 x 0.03) + (206 x 0.24) +  207 x 0.20)  +  208 x  0.53) = 207.2 amu