Plutonium dioxide: "11.852% is oxygen and 2.413% is "other" (described as "small amounts of long-lived actinides and stable impurities" in NASA's June 1995 Environmental Impact Statement (EIS) for the Cassini mission). The rest is plutonium. Here is the full breakdown of the plutonium components, from page 2-18 of the 1995 EIS:
Fuel component (Weight % at launch) (Half-life):
Pu 236 (0.0000010%) (2.851 years)
Pu-238 (70.810%) (87.75 years)
Pu-239 (12.859%) (24,141 years)
Pu-240 (1.787%) (6,569 years)
Pu-241 (0.168%) (14.4 years)
Pu-242 (0.111%) (375,800 years)”
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A chemistry student should be able to tell us the density of the starting mix with the Oxygen included.
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link to en.wikipedia.org]
The density of plutonium dioxide is 11.5 gm / cm3