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Homework Help: What is this element, carbon, hydrogen, oxyden or aluminum?

  1. Jan 20, 2004 #1
    8. [CJ6 14.P.001.] A mass of 135 g of an element is known to contain 30.1 1023 atoms. What is the element?

    This is the question on my Physics practice test for my exam tomorrow. i don't know where to even begin with this one. we haven't really gone over anything like this in class...

  2. jcsd
  3. Jan 20, 2004 #2
    Can u find the no. of moles
  4. Jan 20, 2004 #3
  5. Jan 20, 2004 #4
    1 mole contains 6.02 * 1023 entities here atom
  6. Jan 20, 2004 #5
    ok so i am guessing that the equation that i need to use is n=N/NA

    but how do we figure out N?
  7. Jan 20, 2004 #6
    yup, N is given to u 30.1 1023
  8. Jan 20, 2004 #7
    ok so we have 5 moles but now what? how do we figure out what element it is?
  9. Jan 20, 2004 #8
    Do u now approx mol wt of above given elements

    Find the Mol wt with the help of Mole equation

    [tex]n=\frac{g}{M_0}[/tex] where M0 is Mol wt and g is the given wt
  10. Jan 20, 2004 #9
    so i feel u got M0=135/5=27

    Which belongs to ????(WHAT)
  11. Jan 20, 2004 #10
    so using the equation you gave me, i have n = 5 and g = 135, so i find Mo, which came out to be 27, and in the periodic table 27 is Co. what element is Co? but i think its wrong...
  12. Jan 20, 2004 #11
    now that i know the mol wt, i matched it with the periodic table and it comes out to Co, but which one of the choices are Co.
  13. Jan 21, 2004 #12
    U checked in for Atomic Number Rather than atomic wt

    Atomic wt for Al is 27 (Aluminum)

    Co is Cobalt
  14. Jan 21, 2004 #13
    hehe, that could help if i looked for the right thing. Thanks alot, i really appreciate your help.

  15. Jan 21, 2004 #14


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    Science Advisor

    The whole point of a "mole" of an element is that it's weight in grams is equal to the weight of an individual atom in "atomic units". Once you know that you can identify the element.
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