Here's why:

* Molar mass: The molar mass of an element is the mass of one mole of that element. It's numerically equal to the atomic mass of the element, but the units are grams per mole (g/mol).

* Calcium's atomic mass: The atomic mass of calcium is approximately 40.08 atomic mass units (amu).

* Conversion: Since 1 mole of any element contains Avogadro's number of atoms (6.022 x 10^23 atoms), the mass of 1 mole of calcium is 40.08 g/mol.