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Homework Help: Mass Spectrometry

  1. Oct 9, 2008 #1
    1. The problem statement, all variables and given/known data

    Natural carbon consists of two different isotopes (excluding 14C, which is present in only trace amounts). The isotopes have different masses, which is due to different numbers of neutrons in the nucleus; however, the number of protons in the same, and subsequently the chemical properties are the same. The most abundant isotope has an atomic mass of 12.0000 u. When natural carbon is placed in a mass spectrometer, two lines are formed on the photographic plate. The lines show that the more abundant isotope moved in a circle of radius 9.00 cm, while the rarer isotope moved in a circle of radius 9.36 cm. What is the atomic mass of the rarer isotope? (The ions are accelerated through the same potential difference before entering the magnetic field.)

    2. Relevant equations

    r = mv^2 / qvB

    3. The attempt at a solution

    Charges, velocities, and magnetic fields are the same. Set up ratio:

    radius abundant / radius rarer = mass abundant / mass rarer

    9 cm / 9.36 cm = 12 u / x

    x = 12.48 u

    This answer shows as incorrect. Any thoughts? Thanks.
  2. jcsd
  3. Oct 9, 2008 #2
    If they are accelerated through the same potential difference they don't have the same velocity.
  4. Oct 9, 2008 #3
    Thanks! I got it.
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