If the molecular weight of water is 18g/mol, then the number of water molecule would be: 90g/ (18g/mol)= 5 moles.
In the reaction equation, 2 H2O molecules will result in 1 O2 molecule. Then, the amount of O2 formed would be: 5 moles * (1/2)= 2.5 moles.
In STP, an ideal gas will have 22.4 liters of volume per mol. Then, the volume of the oxygen formed would be: 2.5 moles * (22.4l/mol)= 56.0 L