10 2.5 Isotopes and Relative Abundance

Practice questions

Multiple choice questions

Select the option that best defines ‘atomic mass unit’ and that gives its correct abbreviation?

1. The atomic mass unit is defined as one-twelfth of the mass of a carbon − 14 atom. Its abbreviation is AM
2. The atomic mass unit is defined as one-twelfth of the mass of a carbon − 12 atom. Its abbreviation is amu.
3. The atomic mass unit is defined as one-twelfth of the mass of a carbon − 12 atom. Its abbreviation is AM.
4. The atomic mass unit is defined as one-sixth of the mass of a carbon − 12 atom. Its abbreviation is amu.

Short-answer questions

1. Estimate the mass, in whole numbers, of each isotope.
   1. hydrogen
   2. boron
   3. iron
2. Estimate the mass, in whole numbers, of each isotope.
   1. phosphorus
   2. carbon
   3. americium
3. Determine the atomic mass of each element, given the isotopic composition. Provide answers to 3 decimal places.
   1. Lithium, which is 92.4 % lithium-7 (mass 7.016 amu) and 7.60% lithium-6 (mass 6.015 amu). What is the atomic mass of lithium?
   2. Oxygen, which is 99.76 % oxygen-16 (mass 15.995 amu), 0.038% oxygen-17 (mass 16.999 amu), and 0.205% oxygen-18 (mass 17.999 amu). What is the atomic mass of oxygen?
4. Determine the atomic mass of each element, given the isotopic composition. Provide answers to 3 decimal places
   1. Neon, which is 90.48% neon-20 (mass 19.992 u), 0.27% neon-21 (mass 20.994 u), and 9.25% neon-22 (mass 21.991 u). What is the atomic mass of neon?
   2. Uranium, which is 99.27 % uranium-238 (mass 238.051 u) and 0.720% uranium-235 (mass 235.044 u). What is the atomic mass of uranium?

Solutions

Multiple choice questions

1. (b)

Short-answer questions

1. 1. 1
   2. 10
   3. 56
2. 1. 31
   2. 14
   3. 241
3. 1. The atomic mass of lithium=6.940 amu/ 6.940 Da
   2. The atomic mass of oxygen=16.000 amu/ 16.000 Da
4. 1. The atomic mass of neon= 20.180 u
   2. The atomic mass of uranium=238.006 u