# Chemical Forums

## Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Hemidol on October 16, 2009, 12:23:01 AM

Title: Various electromagnetic calculations
Post by: Hemidol on October 16, 2009, 12:23:01 AM
Planck’s constant h = 6.6256 x 10^-34 J•sec; speed of light c = 2.9979 x 10^8 m/s
1. For electromagnetic radiation with a wavelength of 261.2 nm:
(a) What is the frequency of the radiation (in s-1)?
(b) What is the energy (in J) of one photon of the radiation?
(c) What is the energy (in kJ) of one mole of photons of the radiation?

Here is my work...

(a) Here is the nm conversion first:

261.2nm * (1 * 10-1 m / 1 nm) = 2.612 * 10-7 m

v = speed of light / wavelength = ( 2.9979 * 108 m/s / 2.612 * 10-7 m) = 1.147741194 * 1015

(b) E = h * (speed of light / wavelength)
= 6.6256 x 10-34 J•sec * ( 2.9979 * 108 m/s / 2.612 * 10-7 m) = 7.604474058 * 10-19 J

(c) I think this is the technique... 7.604474058 * 10^-19 J / 1000 J = 7.604474058 * 10-22 kJ

Edit: corrected unclosed tags
Title: Re: Various electromagnetic calculations
Post by: Borek on October 16, 2009, 03:05:38 AM
261.2nm * (1 * 10-1 m / 1 nm) = 2.612 * 10-7 m

Correct result, but incorrect as written, typo perhaps; you have messed tags too.

In b methods looks OK, no idea about result - for sure writing all these insignificant digits doesn't make sense.

Quote
(c) I think this is the technique... 7.604474058 * 10^-19 J / 1000 J = 7.604474058 * 10-22 kJ

You have just converted to kJ, but you have ignored "mole of photons".
Title: Re: Various electromagnetic calculations
Post by: Hemidol on October 16, 2009, 08:38:37 PM
261.2nm * (1 * 10-1 m / 1 nm) = 2.612 * 10-7 m

Correct result, but incorrect as written, typo perhaps; you have messed tags too.

In b methods looks OK, no idea about result - for sure writing all these insignificant digits doesn't make sense.

Quote
(c) I think this is the technique... 7.604474058 * 10^-19 J / 1000 J = 7.604474058 * 10-22 kJ

You have just converted to kJ, but you have ignored "mole of photons".

Yeah sorry that should've said nm at the top. I'll have to clean up the sig figs. For (c) though:

7.604474058 * 10-19 J / 1000 J = 7.604474058 * 10-22 kJ * 6.022 * 1023 photons

= 457.9414278 kJ

Is that better?
Title: Re: Various electromagnetic calculations
Post by: Borek on October 17, 2009, 05:25:51 AM
Yes and no.

Yes - this seems to be correct way of calculating energy of mole of photons (apparently sig figs are still way above your head; while I don't like them too much as they are faulty by design, I hate it when someones put all these random digits in the answer).

No - take a look at what you wrote:

7.604474058 * 10-19 J / 1000 J = 7.604474058 * 10-22 kJ * 6.022 * 1023 photons

Basically it means 1 = 6.022*1023. This may seem like nitpicking, but doing things this way is a sure way of getting into troubles sooner or later.