Question

What is molecular mass? And how to find molecular mass?

Answer 1

Molecules are formed from two or more atoms, but a random collection of atoms never makes a molecule. Each molecule contains atoms in a fixed ratio, and the masses of these atoms add up in the same ratio to give the molecular mass.

Complete answer:
A molecule is a chemical entity formed when two or more atoms of the same or different elements combine together and form bonds. A molecule may contain ions linked with each through electrostatic forces of attraction or it may contain neutral atoms connected through shared pairs of electrons resulting in covalent interactions.
Once a molecule is formed from a given set of atoms, it no longer holds the same physical and chemical properties as that of individual elements. The molecule tends to show a unique set of properties that are independent of its constituent atoms. But certain laws govern the formation of these molecules. One such important law is the conservation of mass that suggests that even though a molecule behaves differently than its parent atoms, its mass is always the same as that of the sum of masses contributed by each atom in the molecule. Thus, no molecule can have extra mass or lesser mass as compared to its parent atoms.
Molecular mass is a measure of the amount of matter present inside a molecule. It is the sum of atomic masses of all the atoms that form the molecule.
In order to find out the molecular mass of a molecule, the atomic masses of its constituent elements must be known. The atomic masses are multiplied by the stoichiometric number $(v)$ of the atom present inside the molecule and the sum of all these masses gives the final molecular mass.
Example: Let us find out the molecular mass of $C{H_4}$.
Stoichiometric number of carbon: ${v_C} = 1$
Stoichiometric number of hydrogen: ${v_H} = 4$
Atomic masses of carbon and hydrogen are: $C \to 12.011amu$ and $H \to 1.008amu$
${\text{Molecular mass of }}C{H_4} = (12.011amu \times 1) + (1.008amu \times 4)$
${\text{Molecular mass of }}C{H_4} = 16.043amu$

Note:
The atomic masses are measured in atomic mass units $amu$. This unit is derived from carbon-12, where $1amu$ is considered to be equal to one twelfth of the $^{12}C$ atom. Instead of $amu$, unified mass can also be used which is represented by $u$.