(40) GCSE Chemistry Revision "Avogadro's Constant 2"

Understanding Avogadro's Constant

• Avogadro's constant is defined as 6.02 x 10^23, representing the number of atoms, ions, or molecules contained in one mole of a substance.

Moles and Calculations

Calculation of Moles

• To calculate the number of moles from mass:

  • Formula: Number of moles = Mass (in grams) / Relative atomic mass

Example 1: Magnesium

• Given: 48 grams of magnesium

• Relative atomic mass of magnesium = 24

  • Number of moles: 48g / 24 = 2 moles

• Total number of atoms:

  • 2 moles x Avogadro's constant (6.02 x 10^23) = 1.204 x 10^24 atoms

Example 2: Lithium

• Given: 28 grams of lithium

• Relative atomic mass of lithium = 7

  • Number of moles: 28g / 7 = 4 moles

• Total number of atoms:

  • 4 moles x Avogadro's constant = 2.408 x 10^24 atoms

Calculating Atoms in Compounds

Example 3: Calcium Oxide (CaO)

• Given: 56 grams of calcium oxide

• Relative formula mass is calculated as follows:

  • Calcium (Ca): relative atomic mass = 40

  • Oxygen (O): relative atomic mass = 16

  • Relative formula mass = 40 + 16 = 56

• Number of moles:

  • 56g / 56 = 1 mole

• Total number of molecules:

  • 1 mole x Avogadro's constant = 6.02 x 10^23 molecules

• Total number of atoms:

  • Each molecule contains 2 atoms (1 Ca + 1 O)

  • 6.02 x 10^23 molecules x 2 = 1.204 x 10^24 atoms

Example 4: Water (H2O)

• Given: 54 grams of water

• Relative formula mass is calculated as follows:

  • Hydrogen (H): relative atomic mass = 1

  • Oxygen (O): relative atomic mass = 16

  • Water contains 2 H and 1 O, therefore:

  • Relative formula mass = (2 x 1) + 16 = 18

• Number of moles:

  • 54g / 18 = 3 moles

• Total number of molecules:

  • 3 moles x Avogadro's constant = 1.806 x 10^24 molecules

• Total number of atoms:

  • Each water molecule contains 3 atoms (2 H + 1 O)

  • 1.806 x 10^24 molecules x 3 = 5.418 x 10^24 atoms

Practice

• Additional practice problems can be found in the vision workbook associated with this video.