(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.