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21 Jun 2007 22:09:57 IST

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how was the value of avagadro number derived???

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aravindh ramaswamy

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21 Jun 2007 22:19:13 IST

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Hi!

The Avogadro constant is named after the early 19th century italian scientist Amedeo avogadro who is credited with being the first to realize that the volume of a gas (strictly, of an deal gas) is proportional to the number of atoms or molecules..Avogadro never attempted to measure the constant: the numerical value was first estimated by the Austiran physicist johann josef Loschmidt in 1865 using the Kinetic theory of gases. In German-speaking countries, the constant may still be referred to as the Loschmidt constant or Loschmidt's number: However this name is more correctly reserved for the number of particles in a given volume of an ideal gas.

$n_circ = rac{RT}{p} ,$

equal to (2.686 7773 ± 0.000 0047)×10²⁵ m^?3 at 273.15 K and 101.325 kPa with R the gas constant, T the temperature and p the pressure.

This constant is related to the Avogadro constant by the relation:

$R = N_Ak_B ,$

with k_B the Boltzmann constant hence

$N_A = rac{n_circ p}{k_B T} ,$

The connection with Loschmidt is the explanation for the symbol L, often used instead of N_A to refer to the Avogadro constant.

Before 1960, there were conflicting definitions of the mole, and hence of the Avogadro number (as it was known at the time), based on 16 grams of oxygen: physicists generally used oxygen-16 while chemists generally used the "naturally occurring" isotope ratio. Switching to 12 grams of carbon-12 as the basis ended this dispute and had other advantages.

At this time, the Avogadro number was defined as the number of atoms in 12 grams of carbon-12, that is as a dimensionless quantity, while a mole was defined as one Avogadro number of atoms, molecules or other entities. When the mole entered the SI system in 1971 as the base unit of amount of substance, the definitions were interchanged: what had previously been a number became a physical constant with the unit of reciprocal moles (mol^-1).

The genitive form "Avogadro's constant (number)" is often used but not recommended, particularly as Avogadro never attempted to measure the constant himself.

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