a 51.6 litre sample of a pure allotrope of oxygen has a mass of 100.0 grams. (25 degrees celsius and 100 kPa)

Show, by calculation, which allotrope of oxygen is present.

Use PV = nRT and solve for n.

Then n = grams/molar mass. You know grams and mols(n), solve for molar mass.
Then molar mass/16 = x where Ox is the allotrope.