A sound wave passes from air into water and then into steel.with each change in medium, the speed of the wave will.

1 mol of Xe has a mass of 131 grams
x mol of Xe has a mass of 12 grams.

1/x = 131/12 Set up proportion from the information above. Cross multiply
131x = 12 Divide by 13`
x = 12/131
x = 0.0916 mol <<<<

Just for fun
I don't know if a calculator would even tell me the number of miles in 12 grams of Xe

The diameter is 216 * 10^-12 meters. Maybe.
1 mol of anything is 6.02 * 10^23
0.0912 mols contain x atoms

1/0.0912 = 6.02 * 10^23 / x
x = 0.0912 *  6.02 * 10^23
x = 5.49 * 10 ^ 22 This thing is going to stretch out quite a ways.

1 atom = 312 * 10^- 12 meters.
5.049 * 10^22 = x meters.
x  = 312 * 10^-12 * 5.49 * 10^22
x = 1.7 * 10 ^10 km = 1.07 * 10 ^10 miles We're talking about beyond Pluto. 

I'm sorry I should delete this. If you want to you can. Take my answer for the number of mols. It's likely correct.