I'm doing a practice exam that I have answers to, but I'm not sure how to go about getting to the right answer for some.(adsbygoogle = window.adsbygoogle || []).push({});

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

A nitrogen laser produces pulses of light of wavelength 337.1 nm. If each pulse contains 10.0 mJ of energy, how many photons are contained in one pulse?

2. Relevant equations

E=h(c/Λ)

E= energy

h= 6.626 x 10^{-34}Js

c= 2.998 x 10^{8}

Λ=wavelength

3. The attempt at a solution

Don't have one, as I don't know what to do. I know I have to use the equation given above, but it seems that I can plug everything in, leaving me nothing to solve for. I don't know how to relate energy of a pulse to photons. I know the answer is 1.7 x 10^{16}.

Another...

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

Identify which of the choices below (attachment) gives the best Lewis diagram for the gaseous species represented by the given chemical formulas.

2. Relevant equations

None.

3. The attempt at a solution

This is a multiple choice question. In the attachment I have drawn two of the choices accompanied by their chemical formulas; one (AlCl_{3}) is the correct choice, the other (OCN-) is the one I thought was correct.

Both show the correct number of electrons, both have correct formal charge, however the octet is not satisfied for Al in AlCl_{3}. I'm guessing Al is an exception to the octet rule, but that what about the OCN- diagram is incorrect?

I've just noticed an acceptable Lewis diagram would be to triple bond the N to C, so the O-C bond becomes a single bond, and the -1 formal charge is now on the O. Is this more acceptable because O is more electronegative?

Thanks for your help.

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# Chemistry - energy and photons, Lewis structures

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