1B.9 HW Question

[Jessica Esparza 2H]

A lamp rated at 32W emits violet light of wavelength 420nm. How many photons of violet light can the lamp generate in 2 seconds? How many moles of photons are emitted in that time interval?
I'm wondering how to solve this problem/ how to convert photons to moles?

[Kellylin_4D]

First, convert wavelengths to photons. Since in one second, 32W is emitted, how many W is emitted in 2 seconds? Also, one photon is like one atom, or one molecule, or one electron, you can use Avogadro's number to convert number of photons to moles. Hope that helps!

[Zaynab Hashm 2I]

You first convert the Watts to Joules of energy by multiplying 32 Watts (J.s^-1) by time which is 2 seconds; the units cancel and you're left with Joules of total energy (lamp). Then use E = hc/λ to find the energy of a photon. Now you can divide the total energy (64 J) by the energy of a photon (given in Joules/photon); the units cancel and you're left with the number of photons. Lastly, use Avogadro's number to find the number of moles of photons; to do so divide the number of photons by Avogadro's number.

Hope this makes it easier!

[Sion Hwang 4D]

You have the light on for two seconds. Meaning, the total energy output will be 64J.

Then, you can use the equation:
E = hc/λ.

H and c are known constants. The wavelength of violet light is also given, but remember to convert nm to m.
Then, find the E, which is the energy per photon.

Take total energy, which was 64 J, and divide it by the energy per photon to get the number of photons.

Photons, like atoms, are quantized. Thus, you can use Avogadro's rule to convert photons into moles (as you would convert atoms into moles).

Then, you have the moles of photons produced in the 2 seconds.

[Drake Choi_1I]

When converting from photons to moles, would it be 6.022 x 10^23 photons to 1 mole?

[Sean Cheah 1E]

When converting from photons to moles, would it be 6.022 x 10^23 photons to 1 mole?

Yes, that is correct. Whenever you are converting to moles you simply divide the amount of whatever you have by Avogadro's number.

[Drake Choi_1I]

Thank you! This helped a lot :)

[Victoria Otuya 4F]

How did you get the total energy of 64J? I am really confused. Is it by multiplying 32 by 2 seconds?

[A Raab 1K]

Yes exactly! 64 J is the energy expended over 2 seconds (2 * 32)