1.27: Units Confusion


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Isabelle De Rego 1A
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1.27: Units Confusion

Postby Isabelle De Rego 1A » Sun Apr 22, 2018 2:50 pm

A lamp rated at 32 W (1 W 1 J s 1) emits violet light of wavelength 420 nm. How many photons of violet light can the lamp generate in 2.0 s? How many moles of photons are emitted in that time interval?
I have so far: used the wavelength (converted to meters) to find the energy of the light. After multiplying 32*2 (bc you are finding how many photons in 2 sec) the solutions manual says that you do 64/4.73*10^-19. When I was first doing this problem, I did 4.73*10^-19/64, which I think still leaves me with photons because the Joules cancel out. Why is it 64 divided by the energy and not the other way around?

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Re: 1.27: Units Confusion

Postby Chem_Mod » Sun Apr 22, 2018 3:20 pm

the small number you have is energy/photon. 64 is the total energy. So, you need 64/small number to get number of photons.

paulacamara1E
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Re: 1.27: Units Confusion

Postby paulacamara1E » Mon Apr 23, 2018 10:53 am

I'm also confused as to why dividing 64J by 4.7x10^-19J/photon instead of the other way around.
Other than that, on the last part that asks us to find moles of photons, why use Avogadro's number as the number of photons in 1 mole?

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Re: 1.27: Units Confusion

Postby Chem_Mod » Mon Apr 23, 2018 3:24 pm

When you divide 64J by 4.7x10-19 J/photon, you get J x photon/J so the joules cancel and you are left with the number of photons. It is the same logic as dividing 3 by 3/4 is equal to 3 x 4/3 which is equal to 4.

For the relation between Avagadro's number and 1 mole, 1 mole = 6.02 x 1023 of something whether that be atoms, molecules, or photons. That is why you use it for this question to find the moles of photons in 2 seconds.


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