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Homework Help: Exercise about determining energy of light that breaks bonds

  1. Jan 7, 2016 #1
    1. The problem statement, all variables and given/known data
    Here is the quote of the exercise:
    Excessive exposure to sunlight increases the risk of
    skin cancer because some of the photons have
    enough energy to break chemical bonds in biological
    molecules. These bonds require approximately
    250–800 kJ / mol of energy to break. The energy of a
    single photon is given by E = hc / lambda where E is the
    energy of the photon in J, h is Planck’s constant 6.626 * 10^(-34) J s
    and c is the speed of light 3 * 10^8 m/s
    Determine which kinds of light
    contain enough energy to break chemical bonds in
    biological molecules by calculating the total energy
    in 1 mol of photons for light of each wavelength.
    (a) infrared light (1500 nm)
    (b) visible light (500 nm)
    (c) ultraviolet light (150 nm)

    Of course, everyone knows that it's ultraviolet, but I can't come up with a correct math. Please, explain
    my mistakes:
    (6.626 * 10^(-34) J s * 3 * 10^8 m/s) / (150 * 10^(-9)) = 1.3252 * 10^(-17)
    which I don't see how to fit in 250–800 kJ / mol of energy

    Thank you!
  2. jcsd
  3. Jan 7, 2016 #2


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    Science Advisor
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    Gold Member

  4. Jan 7, 2016 #3
    Do you mean that 1.3252 * 10^(-17) is for one photon, and I have to multiply this number by avogadro number?
    I didn't quite understand your remark.
  5. Jan 7, 2016 #4


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    Staff: Mentor

    That's exactly what you are asked to calculate, aren't you?
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