[lboldtfhs]

Okay, so I'm in Hnrs. Chem in HS, and I am completely lost. I was given a worksheet the other day with practice problems, and I haven't been able to understand any of them. I'll start with the basics...

1. A photon of green light has a frequency of 5.82 x 10¹⁴S^-1. Calculate:
    a. Wavelength
    b. Energy in Joules/Photon
    c. Energy in kJ/mol
So, for a, I attempted and plugged into my calculator the speed of light/frequency, so 2.998x10⁸/5.82 x 10¹⁴. My answer was outrageous, I know it's not right, it had E in there and everytrhing.
For b and c, I don't even know where to start. I was completely lost in the whole conversions unit as well, and I really need help.  

[jdurg]

Okay, let me see if I can help you there.  Remember that the energy of light is quantized.  That means that all light has a very specific energy associated with it.  As the wavelength of a light goes down, the amount of energy the light has goes up.  X-Rays and Ultraviolet Rays have VERY short wavelengths.  As a result, they have a very high amount of energy.  Infrared and radiowaves have VERY long wavelengths, so the amount of energy that the 'light' has is very small.  A pretty smart physicist named Max Planck noticed this and came up with an equation to calculate  the energy of a wave.  The equation is E = hv.  E is the energy of the wave, v is the wavelength, and h is known as Planck's constant.  (6.626e-34 J/s).  So if you know the wavelength of light, you can calculate the energy.

To help answer a), remember that ALL light travels at the same speed (~3.0e8 m/s).  (The notation 'e' means 10^ in case you were wondering).  So if you know the frequency, you can figure out the wavelength and vice versa.  (As the Wavelength (in meters) times the frequency (in s^-1) gives you meters/second which should be equal to the speed of light).  c = (Wavelength)*(Frequency).  So the number you got with the 'e' in there was correct!  Visible light has a VERY small wavelength.  Your answer of 5.15e-7 m is right.  

For answer b), you can then use the wavelength you just calculated in the E = hv equation, along with Planck's constant, to figure out the energy of one photon of that light.

Finally, for answer c) you just need to do some unit conversion and figure out the energy in a mole of those photons.

So just post what answers you've come up with and I'll let you know if you're right.  

[xiankai]

1. what is S^-1? frequency is supposed to be hertz, Hz.

[Borek]

1. what is S^-1? frequency is supposed to be hertz, Hz.

And Hz is s^-1.

[sdekivit]

1. what is S^-1? frequency is supposed to be hertz, Hz.

what borek said follows from the definition of 'frequency': the number waves in 1 second.

thus if you know the time when 1 wave has completed, then out of the formula f = 1/T it follows the amount of waves that has been completed in 1 second. The dimension of T = seconds, so frequency has the dimension s^-1 or Hz.

[xiankai]

i see, thanks. the capital S got me confused.