[alanjz]

The question I'm struggling with is from the 2011 USNCO National Exam:

The first ionization energy of Na is 495.9 kJ*mol^-1. What is the longest wavelength of light that could remove an electron from a Na atom?
(A) 2.41x10^-7 m
(B) 2.41x10^-4 m
(C) 4.14 m
(D) 4.14x10^3 m

I tried:
E=hv=h(c/λ)
495.9x10^3=(6.626x10^-34)(2.998x10^8/λ)
λ=4.006x10^-31 m

I think this is the correct equation to use so could someone explain to me what I'm doing wrong?

[Borek]

Watch your units and it should become obvious.

[alanjz]

495.9x10^3  =(6.626x10^-34)(2.998x10^8)/λ
We have J or kg*m²*s^-2=(kg*m²*s^-1)(m*s^-1)/m so it looks like everything matches up.
I'm still not seeing it.

[Corribus]

You're forgetting to include one of the units in the piece of information provided.

[alanjz]

That helped me finally figure it out. I had to multiply by 6.02x10^23 since it said per mole. Thank you!