The Mole and Molecular Weight

The Mole and Molecular Weight:

Mole and Molecular weight plays an important role while dealing with problems. Let's discuss these two terms briefly

Mole:

Mole is defined as a certain amount of amount of material that contains a specified amount of atoms, molecules, ions or specified types of particles. In the SI system, for our convenience, we take 6.02*10^23 molecules as a gram mole, 6.02*10^23*454 molecules as pound mole and kg mol composed of 1000 moles and so on.
One important calculation at which you should get become skilled is to convert moles into mass and mass into moles. To do this you can use the following formula in which molecular weight is used:
Molecular weight (MW) = Mass/Molar Mass
Based on this formula of MW we can use
g mol = mass in g/molecular weight
lb mol = mass in lb/molecular weight
Therefore, using this formula we can convert moles into mass or mass into moles using molecular weight. For historical reason, atomic weight or molecular weight is usually used instead of atomic mass and molecular mass.
Let us take an Example of converting mass into moles.

Example:
If a bucket holds 2 lb of NaOH
(a) How many pound moles of NaOH does it contain?
(b) How many gram moles of NaOH does it contain?
Solution:
Here we can convert the mass of NaOH to moles by using the formula mentioned above
(a)
= 2lb*1lb mole/40lb = 0.050 lb mole of NaOH (40lb is molecular weight of NaOH)
(b)
= 2lb*1lbmol/40lb*454 g mole/1 lb mol = 22.7 g mol

Values of the molecular weight are built up from values of atomic weights based on a scale of relative masses of elements. The atomic weight of an element is the mass of an atom that is based on a scale that assigns a mass of exactly Carbon Isotope of atomic weight 12.
A compound is composed of more than one atoms and molecular weight of a compound is nothing more than the addition of atomic weights of the elements of which a compound is composed of. For example, H2O is a compound consisting of two atoms of Hydrogen and one atom of Oxygen. So we can calculate the molecular weight of water by simply adding atomic weights of these elements by multiplying with their corresponding number of atoms. In the case of water (H2O) molecular weight will be:
Molecular Weight of Water (H2O) = 2*1+1*18 = 18 approximately.
You can calculate Average Molecular Weight as well for the mixture of a constant composition even though they are not chemically bonded and you know the compositions accurately.

Mole Fraction and Mass Fraction:

Mole Fraction is nothing more than the number of moles of a specific substance divided by the total number of moles present in mixture or solution. This definition holds for gases, liquids, and solids. Similarly, Mass Fraction is defined as the mass of a particular substance divided by the total mass of a solution or mixture. Below are given formulas of these two definitions to calculate Mole Fraction and Mass Fraction. Let us consider there is a mixture of two substances A and B, then
Mole Fraction of A = Moles of A/Total Moles
Mass Fraction of B = Mass of B/Total Mass