Practice Problems: Atomic Mass

  1. The element bromine has two 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. Hint

  2. 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%

    Hint

  3. 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? Hint

  4. 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. Hint

  5. 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. Hint

  6. 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? Hint

  7. 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? Hint


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