# Homework Help: Calculte the Energy of 1 mole of photons - Help

1. Oct 25, 2008

### lucky_star

Calculte the Energy of 1 mole of photons - Help!!!

1. The problem statement, all variables and given/known data

Calculate the energy, in kilojoules, of one mole of photons of yellow laser light of wavelength 570 nm.

2. Relevant equations
Ephoton= hc/wavelength

3. The attempt at a solution

Step by step:
1. Convert the wavelength of 570nm to m:

10-9m= 1 nm, so:

570nm * (10-9m) = 5.7E-7 m

2. Ephoton= hc/wavelength

Ephoton= (6.63E-34 * 3.E8)/5.7E-7 m

Ephoton= 3.49E-19 J

3. Finally, I convert to kJ:

Ephoton= 3.49E-19J/1000J = 3.49E-22 kJ

Can someone help me with this problem, I tried many times but I still get the same result.

2. Oct 26, 2008

### rock.freak667

Re: Calculte the Energy of 1 mole of photons - Help!!!

3. Oct 26, 2008

### lucky_star

Re: Calculte the Energy of 1 mole of photons - Help!!!

So, I would have:
3.49E-22 kJ * the Avogadro's constant, which is 6.022 × 10E23 mol-1
And I got the answer. yoohoo! Thank you so much. Please explain to me why I have to multiply my answer with the Avogadro's constant to get the final answer. I appreciate your help!

4. Oct 26, 2008

### rock.freak667

Re: Calculte the Energy of 1 mole of photons - Help!!!

One mole should contain 6.02E23 elementary units. You found the energy of 1 elementary unit.

5. Oct 26, 2008

### lucky_star

Re: Calculte the Energy of 1 mole of photons - Help!!!

Oh, I forgot that concept. Thanks for reminding me. One mole of any substance contains this many [6.022 × 10E23] units. :D Cheers!