Chemistry: The Mole Concept and Conversions

The Mole Concept

• Avogadro developed a method to count atoms and molecules.
• If elements are weighed in the same proportions as their relative atomic masses, the same number of atoms will result.
• The constant number of atoms in the elements listed is 6 x 10²³, called Avogadro's number (or constant), denoted by L.
• One mole of a substance contains 6 x 10²³ particles of that substance.
• The mass of one mole of an element is equal to its relative atomic mass in grams.
• Example: One mole of sodium is 23 grams; one mole of oxygen is 16 grams.

Converting Moles to Grams

• The mass of one mole of a compound is equal to its relative molecular mass in grams.
• Example: The relative molecular mass of water (H₂O) is 18, so one mole of water is 18 grams.

Converting Grams to Moles

• The number of moles of an element is calculated by dividing the mass of the element by its relative atomic mass.
• Example: To find the number of moles in 36 grams of carbon, divide 36 by 12 (carbon's relative atomic mass), resulting in 3 moles.

Calculations Involving Avogadro's Number

• One mole of a substance contains 6 x 10²³ particles.
• To find the number of atoms or molecules in a given number of moles, multiply the number of moles by Avogadro's number.
• Example: 0.25 moles of sodium contain 1.5 x 10²³ atoms.

Converting Numbers of Atoms or Molecules to Moles

• To convert a number of atoms or molecules to moles, divide the given number by Avogadro's number (6 x 10²³).
• Example: 9 x 10²² atoms of aluminum are equal to 0.15 moles.

Converting Numbers of Atoms or Molecules to Grams

• To determine the mass of a given number of atoms or molecules, first calculate the number of moles, then multiply by the relative atomic or molecular mass.
• Example: 3 x 10²³ atoms of zinc have a mass of 0.0325 grams.