Decomposition of H2O2 follows a first order reaction. In fifty minutes the concentration of H2O2 decreases from 0.5 to 0.125 M in one such decomposition. When the concentration of H2O2 reaches 0.05 M, the rate of formation of O2, will be:

2H2O2→2H2O+O2
In 50 minutes, the concentration is reduced to one fourth(≅0.5/0.125=4)
Hence 2 half life periods correxponds to 50 minutes.
t1/2=25min
The rate constant k=0.693/t1/2=0.693/25
Rate of decomposition of H2O2=k[H2O2]=0.693/25 × 0.05=1.39 × 10⁻⁷mol/min
Rate of formation of oxygen is one half the rate of decomposition of H2O2.
It is 1/2 × 1.39 × 10⁻⁷=6.93 × 10⁻⁸mol/min