Question

What is an Avogadro number?

Answer 1

A number used to specify the number of micro-sized particles (atoms, molecules, or ions) when their atomic or molecular mass is expressed in grams.

Complete answer:
Micro-sized particles such as atoms, molecules, or ions cannot be seen or measured directly by physical balance neither can they be counted like everyday things. In the early nineteenth century, Amedeo Avogadro found that equal volumes of gases under the same conditions of temperature and pressure have equal numbers of molecules (Avogadro’s law). It was found that at ${0^0}C{\text{ }}and{\text{ }}1{\text{ }}atm$ pressure (Standard Temperature and Pressure, STP), they all occupied $22.4{\text{ }}L{\text{ }}or{\text{ }}d{m^3}$ of volume and their mass under the same conditions was their molecular mass expressed in grams. The number of particles was equal in any gas under these conditions. This number was found to be $6.022 \times {10^{23}}$ and is known as Avogadro’s number.
For example:

Gas	Volume at STP (in L)	Mass at STP (Mol. wt. in g)	Number of particles
Hydrogen	22.4	2g	$6.022 \times {10^{23}}$
Oxygen	22.4	32g	$6.022 \times {10^{23}}$
Carbon dioxide	22.4	44g	$6.022 \times {10^{23}}$
Nitrogen	22.4	28g	$6.022 \times {10^{23}}$

The molecular mass expressed in grams is known as gram-molecular mass and the amount of any substance having $6.022 \times {10^{23}}$ particles is referred to as 1mole. Therefore, gram-molecular mass is also referred to as molar mass and the volume of 22.4L at STP is referred to as molar volume.Later on, it was found that not only gases but pure solids and liquids also have $6.022 \times {10^{23}}$ particles when their mass equal to gram-molecular mass is taken. For example, 23g of Sodium will contain $6.022 \times {10^{23}}$ atoms of sodium and 98g ${H_2}S{O_4}$ (which has a molecular mass of 98 will contain $6.022 \times {10^{23}}$ molecules of ${H_2}S{O_4}$. In these amounts, both Na and ${H_2}S{O_4}$ will be said to have amounts equal to 1mole.

Note: The relationship of Avogadro’s number with mass, moles and volume must be mentioned properly and the fact that 1mole of any substance (and not just gases) contain particles equal to Avogadro’s number. The particles may be atoms, molecules or ions.