Answer:
The volume occupied by the oxygen gas is 0.5353 L.
Step-by-step explanation:

Moles mercury(II) oxide =

According to reaction, 2 mol of mercury(II) oxide gives 1 mol of oxygen.
Then 0.0478 mol of mercury(II) oxide will give:

At STP, the 1 mol gas occupies = 22.4 L
Then 0.0239 mol of oxygen at STP will occupy volume =

The volume occupied by the oxygen gas is 0.5353 L.