Posted by Amy on Tuesday, December 4, 2012 at 10:45pm.
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  DrBob222, Wednesday, December 5, 2012 at 1:08am
What is the amount of energy needed to raise the T of H2O.
q = mass H2O x specific heat H2O x (TfinalTinitial)
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 = 6.626E34 J.s x 3E8 m/2.4E6 m = about 8E20 J.
So 8E20J/photon x no. photons = 26 J
Solve for #photons. 
Chemistry  Photons  Amy, Wednesday, December 5, 2012 at 3:48pm
I am not sure how to solve for photons. I tried dividing the 8E20 J/photon and 26 J, but I was incorrect.

Chemistry  Photons  DrBob222, Wednesday, December 5, 2012 at 9:21pm
Did you redo the 26 J? That's isn't the exact number. Did you redo 8.2E20 J/photon? That isn't the exact number.
After you correct the numbers, then
#photons = 26?/8.2E20? = xx 
Chemistry  Photons  Amy, Thursday, December 6, 2012 at 10:18pm
I was having problems fining the "exact" value of 8.2E20. I actually got 8.285E32. As for the 26, the exact value is 26.15. I divided those two numbers together, and my response was still wrong.