Avogadro's number is defined as 6.022 × 10²³ of particles (atoms, molecules, ions, etc.) present in one mole of a substance. It is usually denoted by the symbol "Nₐ".

1. Calculation: To calculate the number of particles in a sample, multiply the amount of the substance in moles by Avogadro's number. For example, if you have 2 moles of water, the number of water molecules present would be 2 × 6.022 × 10²³ = 1.2044 × 10²⁴ molecules.

2. Relationship to molar mass: Avogadro's number is also related to the molar mass of a substance. The molar mass is the mass of one mole of a substance in grams. The molar mass of a substance can be calculated by dividing the mass of a sample by the number of moles present, which can then be used to calculate the number of particles using Avogadro's number.

3. Importance: Avogadro's number is a crucial concept in many areas of chemistry, including stoichiometry, gas laws, and thermodynamics. It provides a way to relate the number of particles in a sample to its macroscopic properties, such as volume and pressure.

4. Units: Avogadro's number has units of particles per mole (or molecules per mole).

5. Experimental determination: The value of Avogadro's number can be determined experimentally by a variety of methods, including X-ray crystallography, electron microscopy, and measurements of the density and viscosity of gases.

6. Use in the mole concept: Avogadro's number is an integral part of the mole concept, which is used to relate the number of particles in a sample to its mass and volume.

7. Applications: Avogadro's number is used in many applications in chemistry, including in the calculation of reaction stoichiometry, the determination of molecular weight, and the analysis of gas properties.