Question

What is the molecular mass of $CaO$ ?

Answer 1

Molecular mass of an ionic compound is the sum total of the atomic masses of all the constituent elements. If the compound contains more than one atom of an element then its atomic mass is multiplied by its stoichiometric number.

Complete answer:
A molecule is formed when two or more atoms combine through chemical bonds in a fixed ratio. Calcium oxide is an ionic compound made up of two elements namely calcium and oxygen in the ratio $1:1$ .
The atom of each element contributes to the total mass of the molecule and this contribution is numerically equal to the amount of matter contained by a single atom of that element i.e. it’s atomic mass.
Thus, the mass of calcium oxide can be obtained by adding the individual masses of calcium and oxygen atoms.
Calcium has an atomic mass equal to $40.08u$ and that of Oxygen is approximately $16.00u$ .
The units of measurement of atomic masses are called atomic mass units or unified mass units that make use of a relative measurement scale in which one unit of atomic mass is considered to be one-twelfth part of a carbon atom.
$\Rightarrow$ Thus, the mass of calcium oxide is:
$Mass(CaO) = 40.08u + 16.00u = 56.08u$ .

Note:
The molecular mass is simply the sum of atomic mass and is different from molar mass and formula mass. For ionic compounds having complex crystals the formula mass is calculated and molar mass is calculated in grams instead of atomic units.