Test Your Knowledge
Key Idea 3: Determining Atomic Weight
Consult the following mass spectrum to answer questions 12 and 13.
12) The mass spectrum for a sample of atoms is shown above. Which of the following values best represents the atomic weight of the sample? (Hint: remember that the atomic weight takes into account relative abundances.)
- 37
- 35
- 35.5
- 72
13) What is the approximate atomic mass of the isotope with the highest relative abundance for the sample in question 12?
- 35
- 37
- 36
- 35.5
14) On a mass spectrum, the number of peaks represents the _______of isotopes and the relative intensity of those peaks indicates the ___________.
15) On a mass spectrum, it is common to arbitrarily set the highest intensity peak to 100 %, and then scale the other peaks relative to this. Consult the mass spectrum of boron to the left. What do you predict the relative abundances of the isotopes of boron to be closest to?
- 25 and 75
- 0.20 and 0.80
- 25 and 100
- 10 and 11
16) Carbon has 15 experimentally detected isotopes, and yet only 2 of them are used in the calculation of the atomic weight of carbon. Why might some of the isotopes be excluded from the calculation?
- The relative abundance of the excluded isotopes is too small
- The excluded isotopes are short-lived radioisotopes
- The excluded isotopes do not have common practical applications
- a and b
17) Three isotopes of potassium are used in the calculation of atomic weight.
Calculate the atomic weight of potassium using the information below. Open the Atomic Weight Calculator to check your work.
18) What’s the difference between the mass number of an isotope and the relative atomic mass of an isotope?