What is molecular mass and atomic mass

The atomic mass is the mass of an atom of an element. The atomic mass is calculated as (approximately) the sum of the number of protons and neutrons in an atom. In case there are isotopes of an element (atoms with the same atomic number but a different mass number), the relative abundance of the isotopes is used to calculate the atomic mass.
A simple example is the sodium atom, which has an atomic number of 11 and a mass number of 23. This means that each atom of sodium has 11 protons and 12 (23-11) neutrons. Thus, its atomic mass is (approximately) 23 atomic mass units.
The molecular mass is the mass of a molecule. This is equal to the sum of the atomic mass of each atom present in the molecule multiplied by the number of atoms of that particular element present in the molecule.
For example, water (H2O) molecules consists of 2 atoms of hydrogen (atomic mass = 1 amu) and 1 atom of oxygen (atomic mass = 16 amu). Its molecular mass is 18 g/mol (= 2 x 1 + 1 x 16).
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