Mole Concept

Concept

This is our common experience that when we go to market to buy something, a few things we always get in definite numbers. For example, eggs we get in a number of 12 which is called a dozen , playing cards we get in a number of 52 which is called a pack, papers we get in a number of 480 sheets which is called a ream. Dozen, pack and ream are units to describe those items. Following chart shows certain items which we get in definite numbers and the units in which these items are described.

In Latin, mole means ‘massive heap’ of material. In chemistry, it is a unit which is used to describe an amount of atoms, ions and molecules. It enables chemists to count these particles by weighing. According to 14th conference of National Institutes of Standards and Technology ( NIST ) held in 1971, the definition of mole is given as follows.

When the mole is used, the elementary entities must be specified and they may be atoms, molecules, ions, electrons, other particles or specified group of such particles.

Visualizing a mole as a pile of particles, however, is just one way to understand this concept. A sample of a substance has mass, volume ( generally used with gases ) and number of particles that is proportional to the chemical amount ( measured in moles ) of the sample. For example, one mole of oxygen ( O2 ) occupies a volume of 22.4 liters at

standard temperature and pressure ( STP = O0 C and 1 atm ), has a mass of 31.998 grams and contains about 6.022 x 1023 molecules of oxygen. Measuring one of these quantities allows the calculation of the others and this is frequently done in stoichiometry.

One interpretation : A specific number of particles

When a quantity of particles is to be described, mole is a grouping unit analogous to groupings such as pair, dozen or gross, in that all of these words represent specific numbers of objects. The main difference between the mole and the other grouping units is the magnitude of the number represented and how that number is obtained. One mole is an amount of substance containing Avogadro’s number of particles. Avogadro’s number is equal to 6.02214199 x 1023.

Unlike pair, dozen and gross, the exact number of particles in a mole can not be counted. There are several reasons for this. First, the particles are too small and can not be seen even with a microscope. Second, as naturally occurring carbon contains approximately 98.90 % carbon – 12, the sample would need to be purified to remove every atom of carbon – 13 and carbon – 14. Third, as the number of particles in a mole is tied to the mass of exactly 12 grams of carbon – 12, a balance would need to be constructed that could determine if the sample was one atom over or under exactly 12 grams. If the first two requirements were met, it would take one million machines counting one million atoms each second more than 19,000 years to complete the task. So, practically it can be treated as impossible though is theoretically possible.

Obviously, if the number of particles in a mole can not be counted, the value must be measured indirectly and with every measurement there is some degree of uncertainty. Therefore, the number of particles in a mole, Avogadro’s constant ( NA ) can only be approximately found through experimentation and thus its reported values will vary slightly ( at the tenth decimal place ) based on the method used for measurement. . Most methods agree to four significant figures , so NA is generally said to equal 6.022 x 1023 particles per mole and this value is usually sufficient for solving common problems.

Another key point is that the formal definition of a mole does not include a value for Avogadro’s constant and this is probably due to the inherent uncertainty in its measurement. As for the difference between Avogadro’s constant and Avogadro’s number, they are numerically equivalent but the former has the unit of mol-1 whereas the latter is a pure number with no unit. Avogadro’s constant is related to the number of atoms or molecules present in the volume occupied by one mole of a gas at S.T.P. conditions or it is the number of atoms or molecules present in one mole of a substance. Avogadro’s number is a pure number just as dozen or gross and it is dimensionless. It equals the number of particles ( electrons, atoms, ions, molecules etc. ) of anything which is numerically equivalent to the number of atoms present in 0.012 kg of carbon – 12.