Avogadro constant

The earliest accurate method to measure the value of the Avogadro constant was based on coulometry.

A modern method to determine the Avogadro constant is the use of X-ray crystallography.

The Avogadro constant is a molar constant.

With this approach, the Avogadro constant would not only be fixed, but so too would the atomic mass of Si.

With this recognition, the Avogadro constant was no longer a pure number, but had a unit of measurement, the reciprocal mole (mol).

Note that the Avogadro constant is a physical quantity with the dimension [amount-of-substance]1, and is not a pure number.

It is the numerical value of the Avogadro constant which has the unit 1/mol, and relates the molar mass of an amount of substance to its mass.

For example(taking the Avogadro constant to be ): By turning these statements around we can show that the Avogadro constant is a physical quantity and not a pure number.

Under the current definitions of the International System of Units (SI), a measurement of the Avogadro constant is an indirect measurement of the Planck constant:

In Hahnemann's time it was reasonable to assume that remedies could be diluted indefinitely, as the concept of the atom or molecule as the Avogadro constant was just beginning to be recognized.

The accuracy of the measured value of the Avogadro constant is currently limited by the uncertainty in the value of the Planck constant-a measure relating the energy of photons to their frequency.

Note that under the SI, Avogadro constant has units and it is thus improper to refer to it as "Avogadro's number," since a "number" is supposed to be Dimensionless quantity.

While it is rare to use units of amount of substance other than the mole, the Avogadro constant can also be defined in units such as the pound mole (lb-mol) and the ounce mole (oz-mol).

At present (2006 CODATA recommended value), the precision of the value of the Avogadro constant is limited by the uncertainty in the value of the Planck constant (relative standard uncertainty of 5x10-8).

The value of the Avogadro constant was first indicated by Johann Josef Loschmidt who in 1865 estimated the average diameter of the molecules in air by a method that is equivalent to calculating the number of particles in a given volume of gas.

The Avogadro constant is named after the early 19th century Italian scientist Amedeo Avogadro, who in 1811 first proposed that the volume of a gas (at a given pressure and temperature) is proportional to the number of atoms or molecules regardless of the nature of the gas.

As may be observed in the table of 2006 CODATA values below, the main limiting factor in the precision of the Avogadro constant is the uncertainty in the value of the Planck constant, as all the other constants that contribute to the calculation are known more precisely.