subject: Molar Molecular Weight [print this page] Introduction to molar molecular weight Introduction to molar molecular weight
The mass of a carton is sum of boxes packed inside it.Further more it is constituted of items inside the packets.
Molecules are made up of atoms and atoms are in turn made up of protons,neutrons and electrons.
The mass of an atom is mass of the protons, neutrons and electrons.
Thus the mass of a molecule is the sum of masses of the atoms. The unit of this is atomic mass unit,often represented by 'u'.
The molar mass, as it is called, when expressed in grams instead of atomic mass unit, is called a mole.
Thus one mole of a substance is molar mass in grams or gram equivalent.
It is also called as gram molecule.
Difference between Molar Mass and Molecular Weight
Many a times a lot of confusion is created while defining the molar mass and molecular weight. Many mistakenly treat it as similar terms. However they are not same.The difference between the two is enumerated below.
An element may exist in number of isotopes.The isotopes have different mass numbers.If a particular molecule has particular isotope of an element,and the same has been considered while calculating the total mass of the molecule,it is called molecular weight.
However,in most of the cases,when the average of the masses of the isotopes as found in the nature are considered,while calculating the mass of the molecules,it is called as molar mass.
e.g. Molar mass of HCl is 35.5 + 1=36.5
In reality the mass number can never be in fraction i.e.35.5, however it is a figure arrived at by taking into account the average of isotopic distribution of chlorine found in nature.So 36.5 rightfully represents the molar mass.
The molar mass of hydrogen is 2. Therefore one mole is 2g.
The molar mass of oxygen is 32. Therefore one mole is 32g.
The molar mass of nitrogen is 28. Therefore one mole is 28g.
The molar mass of carbon is 12. Therefore one mole is 12g.
The molar mass of water is 18. Therefore one mole is 18g.
The molar mass of sodium hydroxide is 40. Therefore one mole is 40g.
Relevance between Molar Mass and Molecular Weight
How and why did the concept of molar molecular weight originate?
Originally it was believed that molecules when take part in a reaction, they do so by mass.
However this led to failure to explain many stoichiometric problems. Finally it was Avogadro, who proved that when substances react, they react by molar molecular weights and not the absolute masses.
e.g.
2H2 +O2 ----------> 2 H2O
Originally it was thought that the reaction is by mass like 2g of hydrogen will react with 1 gram of oxygen to give 2g of water.
But the observations were something else. According to Avogadro's contribution,
2x 2 g of hydrogen would react with 32 g of oxygen to give 2 x 18 grams of water .
This held quiet true for observations. Since they react by molecules obviously the molar molecular weights and not the absolute masses would be the participating factors.
He further was also successful in finding total number of molecules in a molecule. According to Avogadro's number, a mole contains 6.022 X 10^23 molecules.
In case of gases too,it was established that at STP the volume of one mole of a gas is 22.4 L.
These properties have been widely used to measure volume of certain gases in all chemical research and manufacturing.
The molar molecular weights are deciding factor in titrations and expressing the concentrations of compounds.
