Most Probable Speed - Atoms From An Oven

Hi there,

I'm looking to find the most probable speed of atoms ejected from an oven.

Temp (T) is 900°C (so 1173.15K).

The material is aluminium with molar mass 0.027kg mol^-1

I've got the formula v_mp=√(2kT/m)

I'm fine with everything above, but I'm not sure what value to use as the Boltzmann constant k.

Can anyone please help me here? Do I have the right equation and what value for the constant should I use (sounds silly, but it changes depending upon units. I've tried a couple but get silly tiny speeds out. I think I should expect something in the region of a few hundred metres per second).

Many thanks

Bob