How many calcium atoms are in 31.2 g of calcium?

To find the number of atoms in calcium one has to multiply a given mole of Calcium by Avogadro's number since each mole of a substance contains 6.03 × 10²³.

Now, mol of Ca = mass of Ca ÷ molar mass of Ca
          mol of Ca = 31.2 g ÷ 40 g/mol
          mol of Ca = 0.78 mol

⇒ atoms of Ca = 0.78 mol × (6.02 × 10²³ atoms/mole)
                          = 4.696 × 10²³ atoms