We are given with
120 g Calcium
The molar mass of Calcium is 40 g Ca / mol Ca. Using this to determine the number of moles
120 g (1 mol / 40 g) = 3 mol
We use the Avogadro's number to convert moles to number of atoms
3 mol (6.022 x 10^23 atoms / 1 mol) = 1.81x10^24 atoms
