Answers

1. Copper, a metal known since ancient times, is used in electrical cables and pennies, among other things. The atomic masses of its two stable isotopes ⁶³Cu (69.09 %) and ⁶⁵Cu (30.91%) are 62.93 amu and 64.9278 amu, respectively. Calculate the average atomic mass of copper. The relative abundances are given in parentheses.

(0.6909)(62.93 amu) + (0.391)(64.9278 amu) = 63.55 amu

2. The atomic masses of the two stable isotopes of boron ¹⁰B (19.78%) and ¹¹B (80.22%) are 10.0129 amu and 11.0093 amu, respectively. Calculate the average atomic mass of boron.

(.1978)(10.0129 amu) + (.8022)(11.0093 amu) =10.81 amu

3. The atomic masses of ⁶Li and ⁷Li are 6.0151 amu and 7.0160 amu, respectively. Calculate the natural abundances of these two isotopes. The average atomic mass of Li is 6.941 amu.

Let x = relative abundance of ⁶Li

1-x= relative abundance of ⁷Li

(6.0151 amu)(x) + 7.0160 amu(1-x) = 6.941 amu

x= 7.5%

1-x= 92.5 %