Calculating Moles of a Compound (HT)

Understanding Moles

• A mole is a large number used in chemistry.

• Previous video: using mass of an element to calculate moles.

• Formula: Number of moles = mass (g) / relative atomic mass.

Relative Formula Mass (Mr)

• Important to calculate moles of a compound.

• Example: Carbon dioxide (CO₂)

  • 1 atom of Carbon (C): 12

  • 2 atoms of Oxygen (O): 16 each

  • Relative formula mass of CO₂: 12 + 16 + 16 = 44

• Mr has no unit.

Calculating Moles of a Compound

• Formula: Number of moles = mass (g) / relative formula mass (Mr).

Example 1: Calcium Carbonate

• Given mass: 300 g

• Formula: CaCO₃

  • 1 atom of Calcium (Ca): 40

  • 1 atom of Carbon (C): 12

  • 3 atoms of Oxygen (O): 16 each

  • Relative formula mass of CaCO₃: 40 + 12 + 48 = 100.

• Calculation: 300 g / 100 = 3 moles of CaCO₃.

Example 2: Magnesium Chloride

• Given mass: 380 g

• Formula: MgCl₂

  • 1 atom of Magnesium (Mg): 24

  • 2 atoms of Chlorine (Cl): 35.5 each

  • Relative formula mass of MgCl₂: 24 + 71 = 95.

• Calculation: 380 g / 95 = 4 moles of MgCl₂.

Example 3: Lithium Sulfate

• Given mass: 990 g

• Formula: Li₂SO₄

  • 2 atoms of Lithium (Li): 7 each

  • 1 atom of Sulfur (S): 32

  • 4 atoms of Oxygen (O): 16 each

  • Relative formula mass of Li₂SO₄: 14 + 32 + 64 = 110.

• Calculation: 990 g / 110 = 9 moles of Li₂SO₄.

Example 4: Barium Hydroxide

• Given mass: 64.5 g

• Formula: Ba(OH)₂

  • 1 atom of Barium (Ba): 137

  • 2 atoms of Oxygen (O): 16 each

  • 2 atoms of Hydrogen (H): 1 each

  • Relative formula mass of Ba(OH)₂: 137 + 32 + 2 = 171.

• Calculation: 64.5 g / 171 ≈ 0.38 moles of Ba(OH)₂.

Conclusion

• Ability to calculate moles of a compound.

• Additional resources: Revision workbook available.