Practice Problems: Atomic Mass (Answer Key)
- The element bromine has three naturally-occurring isotopes. A mass spectrum
of molecular Br2 shows three peaks with mass numbers of 158 u,
160 u, and 162 u. Use this information to determine which isotopes of Br occur
in nature.
79 u, 81 u
- Calculate the elemental atomic mass of Mg if the naturally occurring isotopes
are 24Mg, 25Mg and 26Mg. Their masses and
abundances are as follows:
Isotope |
Atomic Mass |
Isotopic Abundance |
24Mg |
23.98504 u |
78.70% |
25Mg |
24.98584 u |
10.13% |
26Mg |
25.98259 u |
11.17% |
24.31 u
- Lithium has an elemental atomic mass of 6.941 u and has two naturally occurring
isotopes, 6Li and 7Li. Their masses are 6.0151 u and
7.0160 u respectively. What are the natural abundances (to 2 decimal places in percentage)
of the isotopes of Lithium?
6Li: 7.49%,
7Li: 92.51%
- How many peaks would be observed on a mass spectrum for H2S+
ion? Hydrogen has two stable isotopes, 1H and 2H, and
sulfur has 4 stable isotopes, 32S, 33S, 34S,
and 36S. Assume that the ion does not decompose into smaller fragments.
Seven
- A compound made of C, H, and Cl shows two peaks on a mass spectrum, one
at 52 u and the other at 50 u. What is a reasonable molecular formula for
this compound? Assume that there are only 1H, 12C, 35Cl
and 37Cl in the compound.
CH3Cl
- What is the nuclear binding energy in joules (to 4 significant figures)
of 19F if the experimental mass is found to be 18.9984 u?
2.367 x 10-11 J
- What is the nuclear binding energy in joules (to 4 significant figures)
of 127I if the experimental mass is found to be 126.9004 u?
1.724 x 10-10 J