1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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?
     
  2. jcsd
  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

    User Avatar
    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

    User Avatar
    Homework Helper

    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!
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted