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Mole Concept:

Wilhelm Ostwald coined the term "mole" around $$1896$$, originating from the Latin word moles, which means "heap" or "pile." Any material may be considered as a set of atoms or molecules.

Wilhelm Ostwald

In $$1967$$, the unit mole was adopted as a straightforward way to report a huge number–the vast heap of atoms and molecules in a sample.
One mole of any particles such as atoms, molecules, or ions is that quantity in number having a mass equal to its atomic or molecular mass in grams.
The one mole of any substance is a constant value. That is, the number of particles (atoms, molecules or ions) present in one mole of any substance is constant, with a value of  $6.022×{10}^{23}$.

This value $6.022×{10}^{23}$ is known as the Avogadro number or Avogadro constant, which is obtained experimentally. Avogadro number can be represented as ${N}_{0}$ .

Therefore, one mole of particles (atoms, molecule, or ion) $$=$$ $6.022×{10}^{23}$

Mole:
Relative atomic or molecular mass in grams is equal to the mass of  one mole of a substance. The mass of one atom of an element is measured in atomic mass units by its atomic mass (u).
How to find the gram molecular mass or molar mass of a molecule?

To get the mass of one mole of an atom of that element (molar mass), we have to take the same numerical value by changing the units from $$‘u’$$ to $$‘g’$$.
Example:

The atomic mass of hydrogen $$= 1u$$. One g of hydrogen has one mole of atoms, which is equal to $6.022×{10}^{23}$ atoms of hydrogen.

$$16u$$ Oxygen has one atom of Oxygen. $$16g$$of Oxygen has one mole of atoms, which is equal to  $6.022×{10}^{23}$ atoms of Oxygen.
Molecular Mass:

In atomic mass, we get the mass of the atom alone, but in the molecular mass, we get the mass of the total molecule.
Molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of  a substance. It is, therefore, the relative mass of a molecule expressed in atomic mass units (u).

For instance, the molecular mass of $$H_2O$$ is two times the mass of one H-atom plus the mass of one O-atom.

Formula unit mass:

For the substances whose constituent particles are ions, the formula unit mass is used.  It is the number of the atomic masses of all the atoms in a compound's formula.

For example, unit mass of ionic $$NaCl$$, is .
Note:To calculate or to find

1. Mass from a mole of molecule or atom - Use $m\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}M\phantom{\rule{0.147em}{0ex}}×n$

2. Atomic mass from atomic number -  Use $m\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}M\phantom{\rule{0.147em}{0ex}}×n$, where n is
$\mathit{Number}\phantom{\rule{0.147em}{0ex}}\mathit{of}\phantom{\rule{0.147em}{0ex}}\mathit{moles}\phantom{\rule{0.147em}{0ex}}\left(n\right)\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}\frac{\mathit{Given}\phantom{\rule{0.147em}{0ex}}\mathit{mass}\phantom{\rule{0.147em}{0ex}}\left(m\right)}{\mathit{Molar}\phantom{\rule{0.147em}{0ex}}\mathit{mass}\phantom{\rule{0.147em}{0ex}}\left(M\right)}$

3. Number of particles from mass -  Use
$\mathit{Number}\phantom{\rule{0.147em}{0ex}}\mathit{of}\phantom{\rule{0.147em}{0ex}}\mathit{moles}\phantom{\rule{0.147em}{0ex}}\left(n\right)\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}\frac{\mathit{Given}\phantom{\rule{0.147em}{0ex}}\mathit{mass}}{\mathit{molar}\phantom{\rule{0.147em}{0ex}}\mathit{mass}}×\mathit{Avogadro}\phantom{\rule{0.147em}{0ex}}\mathit{number}$
$N\phantom{\rule{0.147em}{0ex}}=\frac{m}{M}×{N}_{O}$
Mole