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Chemistry - Photons

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Water is exposed to infrared radiation of wavelength 2.5×10−4 cm. Assume that all the radiation is absorbed and converted to heat.

How many photons will be required to raise the temperature of 2.5 g of water by 2.5 K?


I tried solving this problem on my own, and I got 3.3 x 10^-30 photons, which was incorrect.

  • Chemistry - Photons -

    What is the amount of energy needed to raise the T of H2O.
    q = mass H2O x specific heat H2O x (Tfinal-Tinitial)
    q in J = 2.5g x 4.184J/g*K x 2.5K = about 26 J but you should do it more accurately.
    How much energy do we obtain from one photon. That's E = hc/wavelength
    E= joules/photon = 6.626E-34 J.s x 3E8 m/2.4E-6 m = about 8E-20 J.
    So 8E-20J/photon x no. photons = 26 J
    Solve for #photons.

  • Chemistry - Photons -

    I am not sure how to solve for photons. I tried dividing the 8E-20 J/photon and 26 J, but I was incorrect.

  • Chemistry - Photons -

    Did you redo the 26 J? That's isn't the exact number. Did you redo 8.2E-20 J/photon? That isn't the exact number.
    After you correct the numbers, then
    #photons = 26?/8.2E-20? = xx

  • Chemistry - Photons -

    I was having problems fining the "exact" value of 8.2E-20. I actually got 8.285E-32. As for the 26, the exact value is 26.15. I divided those two numbers together, and my response was still wrong.

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