Mole concept  (Page 2/4)

6: We conclude that quantity in grams numerically equal to atomic weight contains $N_o$ number of a particular element. This provides us with another definition of mole. In the case of an element, a mole of an atom means $N_o$ numbers of that atom having mass in grams numerically equal to atomic weight. Important to note here is that term like "mole or moles of atoms" is a valid term. It can be emphasized that the term mole appears to be connected to molecules only, but actually it is a general concept which determines quantity of substance - atoms/molecules/ions/entities.

7: Extending the concept to molecule, a mole means $N_o$ numbers of molecules having mass in grams numerically equal to molecular weight.

8: Further extending the concept in general, a mole of identical entities means No numbers of that entity having mass equal to $N_o$ times mass of one entity.

The official SI definition of mole is :

Mole

The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

Gram moles (n)

This term emphasizes the mass aspect of mole. One “gram mole” expresses mass of $N_o$ elements, molecules, ions (as the case be) in grams. It is equal to mass in grams numerically equal to molecular weight.

If the mass of a chemical entity is “g” grams, then the given mass contains “n” gram-moles of the entity :

$$\text{gram moles(n)}=\frac{\text{Mass in gram}}{\text{Molecular weight}}=\frac{g}{M_o}$$

The gram mole or simply mole in the given grams of a sample means that there are “n” moles of substance present. This formula is widely used to express grams of substance in terms of gram moles and vice-versa.

Note that we use symbol Mo to denote molecular weight as we reserve the symbol “M” to denote molarity in the study of stoichiometry.

Gram atoms

It is a special case of gram moles in which chemical entity is an element. In this case, “gram atoms” substitutes “gram moles” and “atomic weight” replaces “molecular weight”.

$$\text{gram-atoms}\left(n_a\right)=\frac{\text{Mass in gram}}{\text{Atomic weight}}=\frac{g}{A}$$

The atomic and molecular weights of oxygen ( $O_2$ ) are 16 and 32 respectively. If we consider a sample of 32 grams of oxygen, then

$$\text{gram moles(n)}=\frac{g}{M_o}=\frac{32}{32}=1$$

Clearly, there are No molecules in the given 32 grams of oxygen sample. On the other hand, gram-atoms is :

$$\text{gram-atoms}\left(n_a\right)=\frac{g}{A}=\frac{32}{16}=2$$

Thus, there are 2 $N_o$ atoms of oxygen in the given 32 grams of oxygen sample. These results are consistent with our understanding of the constitution of diatomic oxygen gas. The important point to keep in mind is that we need to employ the concept of “gram- atoms (g-atoms)” to elements only irrespective of whether it is mono-atomic or polyatomic.