Sapling Q 7

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Cecilia Cisneros 1F
Posts: 127
Joined: Wed Sep 30, 2020 9:45 pm

Sapling Q 7

Postby Cecilia Cisneros 1F » Mon Oct 19, 2020 8:37 pm

How many photons with a wavelength of 857 nm are needed to melt 417 g of ice? The enthalpy of fusion of water can be found in this table.

I solved this problem by multiplying the enthalpy of fusion with the mass of ice to get the energy needed to melt the ice. From there I solved for the energy of the radiation with the wavelength provided in the question. Then I multiplied:
Energy needed to melt water x 1 photon/energy per photon = 5.99 x 10^21 photons

However my answer was wrong and I am not sure what I am doing wrong. If anyone could help that would be great!!

Nane Onanyan 1G
Posts: 71
Joined: Wed Sep 30, 2020 9:43 pm
Been upvoted: 1 time

Re: Sapling Q 7

Postby Nane Onanyan 1G » Mon Oct 19, 2020 9:33 pm

Hello! Your method is correct, however at the end you must divide the energy required to melt the ice by the energy of the photon.
The units would cancel out like this:
J/(J/photon) -> J(photon/J)=photons

Juliet Cushing_2H
Posts: 72
Joined: Wed Sep 30, 2020 9:39 pm

Re: Sapling Q 7

Postby Juliet Cushing_2H » Mon Oct 19, 2020 9:55 pm

I multiplied grams of ice by the joules required to melt 1 gram of ice to find the total Joules required for this example. Then take Joules and multiply by (1 photon / joules in P of wavelength x) and you get # photons required :)


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