Avogadro’s Number and the Mole

• Because atoms are so small and nearly impossible to count, scientists have developed a unit called the mole to make counting atoms more practical.
• A mole (mol) is the amount of substance contained in 6.022 x 10²³ particles. This is called Avogadro’s number.
• 1 mol of He atoms = 6.022 x 10²³ He atoms
• 1 mol of H₂O molecules = 60.22 x 10²³ H₂O molecules
• 1 mole of NaCl formula units = 6.022 x 10²³ NaCl formula units
• Because the mole contains such a huge number of particles, it is only used to describe things that are very small.
• Example Problem: How many atoms are in a 4.5 mol sample of helium? 2.7 x 10²⁴ atoms