Question

How many atoms are in a drop of mercury that has a diameter of 1.0 mm? (Volume of a sphere is 4ðr3/3; density of mercury = 13.6 g/cm3)

Answer 1

Molar mass of Mercury = 200g/mole, look it up.
Assume drop of mercury is a sphere, apply the formula pi * r^3 * 4/3 = volume.
Use the density.

Should be fairly simple.

Answer 2

(4π/3 * 0.05^3 cm^3) * 13.6g/cm^3 * 1mol/200g * 6.023*10^23atom/mol = 2.144x10^19 atoms