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Chemistry - energy and photons, Lewis structures

  1. Dec 11, 2006 #1
    I'm doing a practice exam that I have answers to, but I'm not sure how to go about getting to the right answer for some.

    1. The problem statement, all variables and given/known data
    A nitrogen laser produces pulses of light of wavelength 337.1 nm. If each pulse contains 10.0 mJ of energy, how many photons are contained in one pulse?

    2. Relevant equations
    E= energy
    h= 6.626 x 10-34 Js
    c= 2.998 x 108

    3. The attempt at a solution
    Don't have one, as I don't know what to do. I know I have to use the equation given above, but it seems that I can plug everything in, leaving me nothing to solve for. I don't know how to relate energy of a pulse to photons. I know the answer is 1.7 x 1016.

    1. The problem statement, all variables and given/known data
    Identify which of the choices below (attachment) gives the best Lewis diagram for the gaseous species represented by the given chemical formulas.

    2. Relevant equations

    3. The attempt at a solution
    This is a multiple choice question. In the attachment I have drawn two of the choices accompanied by their chemical formulas; one (AlCl3) is the correct choice, the other (OCN-) is the one I thought was correct.

    Both show the correct number of electrons, both have correct formal charge, however the octet is not satisfied for Al in AlCl3. I'm guessing Al is an exception to the octet rule, but that what about the OCN- diagram is incorrect?

    I've just noticed an acceptable Lewis diagram would be to triple bond the N to C, so the O-C bond becomes a single bond, and the -1 formal charge is now on the O. Is this more acceptable because O is more electronegative?

    Thanks for your help.

    Attached Files:

  2. jcsd
  3. Dec 11, 2006 #2
    1) E=h(c/Λ) gives the amount of energy for one photon. You know the amount of energy in one pulse. Now, can you figure out the number of photons in one pulse?

    2) If you think of aluminum chloride as an ionic compound then it does not violate the octect rule.

    I can't see your diagram for OCN so it's hard to know exactly what you are talking about, but what you said here:

    agrees with the formal charge rules.
  4. Dec 11, 2006 #3
    1.) Thanks, that helps!

    2.) The diagram in the attachment had O with two lone pairs double bonded to C double bonded to N with two lone pairs and a formal charge of -1 on N.

    Thanks for the help.
  5. Dec 12, 2006 #4


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    2) The actual structure of this radical is best described as a superposition (or resonance hybrid) of those two possible structures (i.e., OCN- and NCO-), but the latter structure has a greater weight associated with it (i.e., it is closer to the hybrid), for the reason you gave (the greater electronegativity of O compared to N).
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