May 26, 2016

Homework Help: Chem 1

Posted by Rossi on Saturday, October 21, 2006 at 9:43am.

It takes 208.4 kJ of energy to remove 1 mole of electrons from an atom on the surface of rubidium metal. What is the maximum wavelength (in nm) of light capable of removing a single electron from an atom on the surface of rubidium metal?

Ok, I know I have to find how many kJ will be needed to remove one electron. Do I find this number by dividing 208.4kJ by 6.022 x 10^23? I have been doing it that way and I get the wrong answer. How do I find how many kJ will be needed to remove ONE electron?

Once I find the energy, I divide hc by the energy, to get lambda, right?

The is the right approach. You dont want kJ, however, you want it in Joules.

Then, use Energy=Plancksconst*speedlight/lambda

solve for lambda. You have it correct, but watch for the correct units.

Well, this is what I have done:

208.4 kJ/ 1 mol x 1 mol/ 6.0221x10^23 = 3.461x10^-22 kJ = 3.461x10^-19 J


6.626x10^-34 x 2.9979x106^8 / 3.461x10^-19 = 5.740x10^-7m = 574.0 nm (final answer)

This is an online homework problem, and each time I submit that answer it tells me I am wrong... that is why I believe I am not doing this problem correctly. Is there an error I am making?

I dont get exactly you answer on Joules per electron, and it does affect the last digit of the wavelength answer. Otherwise, I dont see anything wrong.

I've tried with rounding to the correct sig figs throughout the problem and then not rounding to the correct sig figs until the end, and no matter what form I put, it tells me I am wrong. This is stressing... thank you though
  • Chem 1 - Anonymous, Monday, October 12, 2015 at 6:02pm

    the answer should be 574.4

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