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Homework Help: [College Chem] kinetic energy problem

  1. Oct 1, 2017 #1
    1. The problem statement, all variables and given/known data

    atom traveling at 1.00% the speed of light has a KE of 2.016x10-16 kJ. Identify the element.


    2. Relevant equations

    KE = .5mv^2
    Hv= binding energy + KE



    3. The attempt at a solution

    Ok. So I plugged into velocity and me to find my mass and got like 4.48E-26 g. So I tried to convert to grams/mol but it doesn't match with any of the elements on the periodic table... What do I do?
     
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  3. Oct 1, 2017 #2
    I also have the solution but still want to know how to get the answer.

    Answer : 27g/mol Aluminum
     
  4. Oct 1, 2017 #3

    I like Serena

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    Homework Helper

    Hi Lori! Welcome to PF! :)

    The mass should be 4.48E-26 kg (kilograms), which we should divide by the average mass of a proton and a neutron, the so called unified atomic mass unit (u), which is 1.66E-27 kg/u.
    The result is 27.0 u, which corresponds to the atomic mass of Aluminium, and which is also its number of grams per mol.
     
  5. Oct 1, 2017 #4
    Shouldn't the mass be 4.48E-29 kg?

    [2(2.016E-16)] / [0.01* (3.00E8)] = 4.48E-29kg

    the units for KE is kgm^2/s^2 if i'm correct and so the kg is left and i get 4.48E-29kg
     
  6. Oct 1, 2017 #5

    I like Serena

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    Note that the energy is given in kilo joules. There is a hidden factor 1000 in there. And that's while the kilogram (including the kilo) is part of the joule.
    In other words, we have: 1 kJ = 1000 kg.m2/s2.
     
  7. Oct 1, 2017 #6
    Thank you. I did not know that!
     
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