Calculation of Relative Atomic Mass given the percentage abundances of the stable isotopes is quite straightforward if you are confident with "crunching numbers". 


The sort of question you might face would be something like you were given the percentage abundance of each isotope and be asked to calculate from that the relative atomic mass. These questions are usually used to explain why elements such as chlorine appear to have "half a neutron" with a relative atomic mass of 35.5. Of course we know that this cannot be the case!


Q1 . The element Chlorine has two main, stable isotopes. Chlorine-35 with a percentage abundance of 75.77% and Chlorine-37 with a percentage abundance of 24.23%. (Chlorine does in fact have 25 isotopes ranging from Chlorine-28 to Chlorine-52 but these do not concern us because they are so short lived and of interest to theoreticians only). Using the information given for the two stable isotopes, calculate the relative atomic mass of chlorine to its usual 1 decimal place.


Q2. The element Uranium has three main isotopes, Uranium-234 (0.0054%), Uranium-235 (0.72%) and Uranium-238 (99.274%). Using the percentage abundance data given, calculate the relevant atomic mass of Uranium.


Q3. Hydrogen exists in three main isotopes, Hydrogen-1 (protium), Hydrogen-2 (deuterium) and Hydrogen-3 (tritium). The percentage abundances of protium equals 99.98%, deuterium equals 0.0156% and tritium equals 0.0044%. Given this data calculate the relative atomic mass of Hydrogen.


Q4. The Group 1 Alkali Metal Potassium exists in 3 main stable isotopes, Potassium-39 at 93.3%, Potassium-40 at 0.012% and Potassium-41 at 6.688%. From this information, calculate the relative atomic mass of Potassium.


Given the equation:

Where Ar is the relative atomic mass of the element, M1 and M2 are the isotopic/atomic masses of the isotopes and x / 1-x are algebraic representations of the relative abundances for a 2 isotopes system, attempt the following questions.


Q5. The relative atomic mass of the element Nitrogen is 14.007. Nitrogen exists (for the purposes of this question) in 2 isotopic forms, Nitrogen-14 and Nitrogen-15. Using the algebraic expression above, calculate the percentage abundances of these 2 isotopes.


Q6. The 2 main isotopes of the element Copper are Copper-63 and Copper-65, with atomic masses of 62.9296 (Copper-63) and 64.9278 (Copper-65). Given that the relative atomic mass of copper is 63.546, use the information given to calculate the percentage abundance of each isotope.



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