We have just learn that, mole is a quantity, and it is equal to 6.02 x 10²³. The number 6.02 x 10²³ is called the Avogadro constant.
Therefore, if we are given the number of mole of substance, and asked to find the number of particles (atoms, molecules or ions) in it, we multiply the number of mole by the Avogadro constant.
Likewise, if we are given the number of particles, and asked to find the number of mole of the particles, we divide the number of particles by the Avogadro constant.

Example
Which contains more atoms, 1 mol of helium or 1 mol of uranium? Which has a greater mass? [ RAM: He=4; U=238 ]

Answer:
1 mol of helium and 1 mol of uranium has equal number of atoms.

The mass of one Uranium atom ss greater than the mass of one helium atom.

Example
Find the number of atoms in 2.5 mol of gold.

Answer:
Number of atoms
= Number of mole x Avogadro constant
= 2.5 x 6.02 x 10²³ = 1.505 x 10²⁴

Example
How many moles of magnesium that contain 2.76 x 10²³ of magnesium atom?

Answer:
Number of mole
= Number of atoms ÷ Avogadro constant
= 2.76 x 10²³÷ 6.02 x 10²³
= 0.46 mol

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