Calculate the oscillation frequency f of the h2 molecule. use meff=m/2 as the "effective mass" of the system, where m in the mass of a hydrogen atom.

For the given question above, there are other details that is missing. Here is what I got from my research. 

Mass of a hydrogen atom as 1.008 u, 
where 1 u=1.661 times 10^-27 kg.

The frequency of a mass on a spring is: 
f = omega / 2pi = sqrt (k/meff) / 2pi 

The meff you use is the effective mass. If two bodies with mass M and m are both free to move: 
Meff = Mm / (M+m) 

If M=m, then Meff = m/2 (as you were given) 

So the answer is: 
f = sqrt (2k/m) / 2pi