Average Atomic Mass Practice Problems 1. What is the atomic mass of hafnium if, out of every 100 atoms, 5 have a mass of 176, 19 have a mass of 177, 27 have a mass of 178, 14 have a mass of 179, and 35 have a mass of 180.0? 2. Calculate the average atomic mass of lithium, which occurs as two isotopes that have the following atomic masses and abundances in nature: 6.017 u, 7.30% and 7.018 u, 92.70%. 3. Hydrogen is 99% 1H, 0.8% 2H, and 0.2% 3H. Calculate its average atomic mass. 4. Calculate the average atomic mass of magnesium using the following data for three magnesium isotopes. Isotope mass (u) relative abundance Mg-24 23.985 0.7870 Mg-25 24.986 0.1013 Mg-26 25.983 0.1117 5. Lithium has two naturally occurring isotopes: lithium-6 and lithium-7. If the average atomic mass of lithium is6.941 amu, which isotope is the most abundant? How do you know? 6. Calculate the average atomic mass of chromium. (not in percents) Isotope Mass (amu) Relative Abundance Chromium – 50 49.946 0.043500 Chromium – 52 51.941 0.83800 Chromium – 53 52.941 0.095000 Chromium – 54 53.939 0.023500 7. The average atomic mass of copper is 63.55 amu. If the only two isotopes of copper have masses of 62.94 amu and 64.93 amu, what are the percentages of each? (Think algebra) 85 87 8. Rubidium is a soft, silvery-white metal that has two common isotopes, Rb and Rb. 85 87 If the abundance of Rb is 72.2% and the abundance of Rb is 27.8%, what is the average atomic mass of rubidium? 9. Uranium is used in nuclear reactors and is a rare element on earth. Uranium has 234 235 three common isotopes. If the abundance of U is 0.01%, the abundance of U is 238 0.71%, and the abundance of U is 99.28%, what is the average atomic mass of uranium? 46 47 48 10. Titanium has five common isotopes: Ti (8.0%), Ti (7.8%), Ti (73.4%), 49Ti (5.5%), 50Ti (5.3%). What is the average atomic mass of titanium? 11. Calculate the percentages of each isotope present in a mixture of: 113 115 a) In and In has an average mass of 114.8 u. 28 29 30 b) Naturally occurring silicon consists of three isotopes, Si, Si, and Si, whose atomic masses are 27.9769, 28.9865, and 29.9838, respectively. The most 28 abundant isotope is Si, which accounts for 92.23% of naturally occurring silicon. Given that the observed atomic mass of silicon is 28.0855, calculate the 29 30 percentages of Si and Si in nature. .