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**Wavelengths and dissociation??**

I needed some help with this problem.

By using photons of specific wavelengths, chemists can dissociate gaseous HI to produce H atoms with accurately known speeds. When HI dissociates, the H atoms move away rapidly, wheras the relatively heavy I atoms move little.

a.) What is the longest wavelength that can dissociate a molecule of HI.

b.) If a photon of 254 nm is used, what is the excess energy (in J) over that needed for the dissociation?

c.) If all this excess energy is carried away by the H atom as kinetic energy, what is the speed(in m/s)?

i did part b but i got the wrong answer. I used the equation E=hc/wavelength. But i got the wrong answer.

Thanks.