# Thread: Ionization energy and wavelength problem

1. ## Ionization energy and wavelength problem

I can't figure this problem out, despite using E = hc/lambda...

The first ionization energy for potassium in the vapor state is 418.8 kJ/mol. Determine the longest wavelength that could be used to ionize an atom of potassium.

2. ## Re: Ionization energy and wavelength problem

Originally Posted by Hamzawesome
I can't figure this problem out, despite using E = hc/lambda...

The first ionization energy for potassium in the vapor state is 418.8 kJ/mol. Determine the longest wavelength that could be used to ionize an atom of potassium.
$E = \dfrac{hc}{\lambda}$ only works for individual molecules. You need to find a scale factor between moles and molecules (think Avogadro) and watch the units, E in that equation is in joules whereas your value is in kJ

edit: I get an answer of 285.6nm

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### Ionisation energy and wavelength formula

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