Relying on Wiki (Wikipedia) for starters. Please use a chemistry or physics textbook!
Pu-238 should have a molar mass of ~238 gm/g-mole!
- with two O16
atoms would have a molar mass of 32 g/g-mole
So the molar mass of Pu238
should be about 238 + 32 = 270. If it's heavier, by 6 g/g-mol, then it's probably got some heavier isotopes of Pu or other TUs.
Also, one mole of PuO2
has one (1) mole of Pu and 2 moles of O, or one mole of O2
For atomic masses see
or NIST - http://www.nist.gov/pml/data/comp.cfm