Question: I am trying to find the mass density $\rho$ of plutonium dioxide, $\ce{PuO2}$. I could calculate it, but the mass density of oxygen found in the literature is the density of gas oxygen.

So I was wondering where can one find mass densities for compounds that come from experimental measurements?

Answer: 
[A quick Google search](https://www.google.com/?gws_rd=ssl#q=PuO2+density) shows the density to be $11.5~\mathrm{g~cm^{-3}}$.

You might have had trouble finding it since it is actually called **plutonium (IV) oxide**, as plutonium is an f-block metal (specifically an actinide) displaying multiple oxidation states ([commonly III, IV, V, and VI, while VII is rare](https://en.wikipedia.org/wiki/Plutonium#Compounds_and_chemistry)).
This can be done if you know the unit cell dimensions of plutonium dioxide. You also need to know the locations of the atoms in the unit cell. The unit cell for plutonium dioxide is the same as uranium dioxide and calcium(II) flouride in terms of the fractional coordinates.

First calculate the volume of the unit cell in cubic meters

Next work out the number of plutonium atoms inside the cell, you will need to use the following rules.

1. If the atom is totally inside the unit cell then it is not shared between more than one one unit cell.
2. If an atom is touching the face of the unit cell then it is shared between two unit cells, so count it as half an atom.
3. If an atom is touching an edge (the line where two faces meet) then it is shared between four cells, count it as a quarter atom.
4. If the atom is at a vertex of the cell then it is shared between eight cells, count it as an eighth.

Repeat this process for the oxygen atoms.

Next work out the mass in amu of the atoms inside the unit cell.

Convert this into kilos

Divide the mass by the volume of the cell, this will give the density.

I have used this as an undergraduate (first year) teaching excerise, it is something which takes a few minutes on a blackboard.