Molecular mass of C is: 12.01 g/mol
1 mole C: 0.012 kg
Molecular mass of H is: 1.08 g/mol
1 mole H: 0.01 kg
Reduced mass: Mr = m1m2/(m1+m2) = 0.012*0.01/(0.022) = 5.45x10-³ kg
f = (1/2π)√(k/Mr) = (1/2π)√{500/5.45x10-³} = (1/2π)10²√{9.166} = 48.1 Hz. I hope this helps