Remember:
one ampere = 1 coulomb/second or one amp.s = 1 coul
charge of one electron is 1.602 x 10^{19} coulomb
Avogadro's number  



Where,
Z = Number of atoms within the crystal unit cell,
M = Average atomic mass,
D = Density,
V = Volume.
Things to understand about Avogadro's number
• It is a number, just as is dozen , and thus is dimensionless; you can think of Avogadro's number as the chemist's dozen.
• It is a huge number, far greater in magnitude than we can visualize;
• Its practical use is limited to counting tiny things like atoms, molecules, "formula units", electrons, or photons.
• Its value can be known only to the precision that the number of atoms in a measurable weight of a substance can be estimated. Because large numbers of atoms cannot be counted directly, a variety of ingenious indirect measurements have been made involving such things as brownian motion and Xray scattering.
• The current value was determined by measuring the distances between the atoms of silicon in an ultrapure crystal of this element that was shaped into a perfect sphere. (The measurement was made by Xray scattering.) When combined with the measured mass of this sphere, it yields Avogadro's number. But there are two problems with this: 1) The silicon sphere is an artifact, rather than being something that occurs in nature, and thus may not be perfectly reproducible. 2) The standard of mass, the kilogram, is not precisely known, and its value appears to be changing. For these reasons, there are proposals to revise the definitions of both NA and the kilogram.
Example:
Calculate Avogadro constant number N for the given details.
Z: 25 g/mole
M (Average atomic mass): 30
D: 15 g/cm^{3}
V: 10 cm^{3}
Solution:
Apply Formula:
N = (Z x M) / (D x V)