How to Find the Molar Mass of Oxygen Isotopes
If all the particles in oxygen's nucleus were stuck together like puzzle pieces, it would be easy to determine its molar mass. You could simply add the molar masses of all the protons and neutrons to come up with a total. However, nuclei are held together with binding forces that vary from element to element and isotope to isotope. Some of the protons' and neutrons' mass is converted into energy to create this binding force, making the mass of the nucleus less than the sum of its parts. The difference between the expected and actual mass of the nucleus is called the mass excess.
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Instructions
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Determine the total number of protons and neutrons in the isotope. This is the mass number measured in amu. For example, oxygen-18 has 18 protons and neutrons.
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Find the mass excess on a chart of atomic masses. Often, it will be expressed in kilo-electron volts (KeV). The mass excess of oxygen-18 is -781.522 KeV.
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Divide the number of kilo-electron volts by 1,000 to convert it to mega-electron volts (MeV). For example, -781.522 keV equals -0.781522 MeV.
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Divide the number of mega-electron volts by 931.5 to convert it to amu. The mass excess equals -0.000838993 amu.
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Add this number to the mass number to determine the atomic mass of the isotope. This is also the molar mass in grams. The adjusted mass of oxygen-18 equals 18 - 0.000838993, or approximately 17.999 amu. This means that it has a molar mass of 17.999 grams per mole.
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References
- H2physics.org: Revision Notes on Nuclear Physics
- University of Waterloo: Nuclide Stability
- Wesleyan University: The Behavior of Isotopes of Hydrogen and Oxygen: Heavy and Light Rain
- UnitConversion.org: Kiloelectron-Volts to Megaelectron-Volts Conversion Calculator
- NASA: The Many Faces of Energy
- Cavalcade O' Chemistry: How to Calculate the Molar Masses of Chemical Compounds
