Chemistry
posted by Sarah .
Ionization energy is defined as the minimum energy required to remove an electron from the ground state (n0) to infinity (n∞). Determine the wavelength of radiation required to ionize the hydrogen electron from the n = 2 energy level. Calculate the energy (Joules) associated with this photon. (1 cm1 = 1.986 x 1023 J)
do I use E = RH / n^2 ?
I don't know why I'm given the cm to J conversion, or why they ask for the wavelength first.

I would do it the other way around.
delta E = 2.180 x 10^18 J*[(1/n1^2)(1/n2^2)]. With the Balmer series, n1 is 2 and n2 is infinity.
Then delta E = hc/wavelength.
The reason reciprocal cm is given (cm^1) is because wave number = 1/wavelength and many spectroscopists prefer to use wave number instead of wavelength. Also, if you use the Rydberg constant, then
1/wavelength = R[(1/N1^2)  (1/N2^2)] and you don't need to convert to wavelength first to get energy in joules. 
ok so 1/lambda = (2.18e19) (.25  0)
1/lambda = 5.45e19
then i set up a proportion to convert it into joules?
1 cm1/1.986e23 joules = 5.45e19 cm1/x
I got x = 1.082e41 joules and that it so not right.. 
thank you i got it now!!
thanks so much for your help!!