1 cm^3 = 1 mL
So if density is always 13.6 g/mL, that means at ANY volume, you will have x number grams of Hg to make the density 13.6 g/mL.
Since density is mass over volume, you find the mass by doing 13.6 g/mL times 2.88 mL. This gives you 39.168 g.
Now, you have to convert from grams to moles, so take your 39.168 g sample and multiply it by the molar mass, which, in order to cancel the units of grams means you would end up dividing by 200.59 g/mol.
This gives you 0.195 moles of Hg, now to get the number of atoms, use Avogadro's number, which is 6.022x10^23. This gives you 117,429,000,000,000,000,000,000 atoms of Hg.
To convert this to scientific notation, simply do 1.1749 x 10^23
