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Homework Help: Estimate atomic masses

  1. Apr 6, 2004 #1
    Here's a verbatim section of the problem:
    " The source contains carbon isotopes of mass numbers 12, 13, 14 from a long-dead piece of a tree. (To estimate atomic masses, multiply by 1.67x10^-27kg). "

    My question is isn't the 12, 13, & 14 in the atomic mass units already (Carbon-12 has an amu of 12)? Why would I need to multiply? And if I do need to multiply, should I leave it in kg or convert to g?

    Thank you.
  2. jcsd
  3. Apr 6, 2004 #2


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    I'm afraid you haven't given enough of the problem statement for us to know what exactly you are going to be doing with the carbon masses so that we can know if you really need to convert to some unit such as kg. Since whoever wrote the question took the effort to provide a conversion factor for you, it is pretty likely that you really do need to make use of the conversion factor.

    To an approximation, an atomic mass unit is kinda sorta the mass of a neutron or the mass of a proton. This glosses over the fact that the proton's mass is a little different than the neutron's mass. Also, there are issues of binding energy when protons and neutrons bind together to form a nucleus.

    Way back when I was a kid, I am pretty sure they liked to call either the mass of a certain isotope of oxygen a perfect 16.00000 amu, or else maybe it was that the mass of the carbon-12 isotope was taken to be a perfect 12.00000 amu, I can't remember which.
  4. Apr 6, 2004 #3
    I was afraid that would not be enough info, the entire question is as follows:

    Suppose the e field between the e plates in the mass spec is 2.48x10^4 v/m and the magnetic fields B=B'=0.68T. The source contains carbon isotopes of mass #s 12, 13, 14 from a dead tree. (estimate masses by x by 1.67x10^-27) How far apart are the lines formed by the singly charged ions of each type on the photographic film?

    I thought I would need to multiply since they bothered mentioning it. However, I do not know if I should convert to grams or leave it in kg. The eq I used is r=(mE)/(qBB')

  5. Apr 6, 2004 #4
    Nevermind, Telsa is in kg so for it to cancel I have to use kg.

  6. Apr 6, 2004 #5


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    Ah yes.

    One Tesla is one kilogram per second squared per ampere.
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