How was Avogadro's number determined?

Question

The number is about 6.022 x 10^23 but how was is determined?

Answer 1

This is not an easy question, but a very good history of the number can be found at the site below. This should give you a much better idea of the complications involved and what had to be done to determine the value of Avogadro's number.

"The best modern values for what we now call 'Avogadro's Number' are the result of the x-ray diffraction measurement of lattice distances in metals and salts." The number has been calculated to about 8 significant digits:

6.02214199 x 10^23 mol^-1 atoms per mol.

Answer 2

The term "Avogadro’s number" was first used by French physicist Jean Baptiste Perrin.

Avogadro proposed his hypothesis in 1811. At that time there was no data at all on the number of paticles in a mole, or an agreement on any atomic weights or the standard.
The first measurements which could give an approximate value for Avogadro's number were observations of brownian motion by Robert Brown in 1827.

Cannizarro (1860) used the Avogadro's hypothesis to develop a defensible set of atomic weights based on 1/16 of the atomic weight of oxygen

The numerical value was first estimated by Johann Josef Loschmidt in 1865 using the kinetic gas theory.

Answer 3

The numerical value was first estimated by Johann Josef Loschmidt in 1865 using the kinetic gas theory. In German-speaking countries, the number may still be referred to as Loschmidt's number. Unfortunately, in a few cases (mainly in the older literature) Loschmidt's number refers to the number of atoms (or molecules) in a cubic centimeter, a usage now disparaged.

In the nineteenth century physicists measured the mass of one atom of hydrogen to be about 1/(6.023x1023) grams in an attempt to measure the number of ideal gas molecules would fit in 1 cubic centimeter at STP ... which is related to Avogadro's number via the ideal gas law.