well if each decay gives 8.2x10-13 Joules, yes you would need to use P=e/t, and dN/dt = A = LambdaN.
(
)The rate of decay x the energy per decay would give you the power. So set up the equation A x 8,2x10-13=2.5,
Mr is always stated a grams per mol, so 0.239 is the mr of Plutonium in kg