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### midterm question 4B

Posted: Sat Dec 07, 2019 12:59 pm
If 100KJ of energy was absorbed by the gas sample, how many photons in total caused electronic excitations? How many moles of hydrogen were excited assuming one photon interacted with one unique hydrogen atom??

Even though I already looked over the solutions to the exam, I'm still quite confused about solving this question

### Re: midterm question 4B

Posted: Sat Dec 07, 2019 1:11 pm
Step 1: Convert kJ to J -> 100,000 J.
Step 2: We know the energy per photon is equal to 1.936... x 10^-18 from part a of the question. That 100,000 J of incoming energy is the total energy of all the photons, so if we divide 100,000/1.936...x10^-18, we get the number of photons. This also makes sense unit-wise if that helps you to think about it: J/(J/photon) = photon. The answer is 7 x 10^22 photons.
Step 3: Okay so the key here is understanding the concept that 1 photon interacts with 1 H atom. # photons x (1 H atom/1 photon) puts our units in H atoms now. Now we have the number of H atoms the light excited.
Step 4: Time to convert to moles!We use avogadro's number because our units are in atoms and avogadro's number's units are (atoms/mol). Since we are trying to get to moles, we flip this ratio to cancel out the atoms units, giving us the final number of moles. 0.116237 moles, and 0.1 moles with sig figs.