(40) GCSE Chemistry Revision "Calculating Moles of a Compound"

Introduction

• Objective: Calculate the number of moles of a compound.

• Target audience: Higher tier students.

Recap on Moles

• A mole is a large number used in chemistry to quantify substances.

• Previous videos covered calculating moles for elements using the equation:

  • Number of moles = Mass in grams / Relative atomic mass.

Calculating Moles of Compounds

• To find the number of moles of a compound, we need to understand relative formula mass (Mr).

• Relative Formula Mass (Mr)

  • Example: Carbon Dioxide (CO₂)

    • 1 Carbon (C) = 12

    • 2 Oxygen (O) = 16 x 2 = 32

    • Total Mr of CO₂ = 12 + 32 = 44

• Note: Relative formula mass has no units.

Equation for Moles of Compounds

• Use the following equation:

  • Number of moles = Mass in grams / Relative formula mass (Mr)

• Similar to the previous equation but with compound mass.

Example Calculations

1. Calcium Carbonate (CaCO₃)

• Given mass: 300 grams

• Calculate Mr:

  • 1 Calcium (Ca) = 40, 1 Carbon (C) = 12, 3 Oxygen (O) = 16 x 3 = 48

  • Total Mr = 40 + 12 + 48 = 100

• Calculate moles:

  • Number of moles = 300 g / 100 = 3 moles

2. Magnesium Chloride (MgCl₂)

• Task: Calculate moles for 380 grams.

• Calculate Mr:

  • 1 Magnesium (Mg) = 24, 2 Chlorine (Cl) = 35.5 x 2 = 71

  • Total Mr = 24 + 71 = 95

• Calculate moles:

  • Number of moles = 380 g / 95 = 4 moles

3. Lithium Sulfate (Li₂SO₄)

• Task: Calculate moles for 990 grams.

• Calculate Mr:

  • 2 Lithium (Li) = 7 x 2 = 14, 1 Sulfur (S) = 32, 4 Oxygen (O) = 16 x 4 = 64

  • Total Mr = 14 + 32 + 64 = 110

• Calculate moles:

  • Number of moles = 990 g / 110 = 9 moles

4. Barium Hydroxide (Ba(OH)₂)

• Task: Calculate moles for 64.5 grams.

• Calculate Mr:

  • 1 Barium (Ba) = 137, 2 Oxygen (O) = 16 x 2 = 32, 2 Hydrogen (H) = 1 x 2 = 2

  • Total Mr = 137 + 32 + 2 = 171

• Calculate moles:

  • Number of moles = 64.5 g / 171 ≈ 0.38 moles

Additional Resources

• More practice questions available in the revision workbook.