The equilibrium bond length of 1

Question

H35Cl is 126 pm. What is wavelength of the required electromagnetic radiation to excite the rotational state from j=1 to j=2?
The masses of 1H and 35Cl are 1.0078 amu and 34.9689 amu, respectively)

Answer 1

Here is a site that spells that out.
Using m1 and m2 you can calculate the reduced mass,u, with u and R you can calculate I (moment of inertia), knowing I allows you to calculate B. Knowing J and B allows you to calculate E. Then E = hc/wavelength and wavelength can be determined.
http://hyperphysics.phy-astr.gsu.edu/%E2%80%8Chbase/molecule/rotrig.html#c3