Calculating the Number of Moles of a Compound

Introduction

• Focus on calculating the number of moles of a compound.
• Useful for higher-tier chemistry students.
• Previous knowledge: Understanding of what a mole is and how to calculate moles for elements.

Basic Concept

• Mole: A very large number used in chemistry to quantify atoms or molecules.
• Calculation for elements: (\text{Number of moles} = \frac{\text{mass in grams}}{\text{relative atomic mass}}).

Relative Formula Mass ((M_r))

• A necessary concept to understand when calculating moles of compounds.
• Example: Carbon dioxide (CO2)
  • Carbon (C): Relative atomic mass = 12
  • Oxygen (O): Relative atomic mass = 16
  • Relative formula mass ((M_r)): 12 + 2(16) = 44
• (M_r) is unitless.

Calculating Moles of a Compound

• Equation: (\text{Number of moles} = \frac{\text{mass in grams}}{\text{relative formula mass (Mr)}}).

Example Calculations

Example 1: Calcium Carbonate (CaCO3)

• Given: 300 grams
• Relative Formula Mass Calculation:
  • Calcium (Ca): (M_r = 40)
  • Carbon (C): (M_r = 12)
  • Oxygen (O): (M_r = 16)
  • Total (M_r = 40 + 12 + 3(16) = 100)
• Moles Calculation:
  • (\text{Number of moles} = \frac{300}{100} = 3) moles

Example 2: Magnesium Chloride (MgCl2)

• Given: 380 grams
• Relative Formula Mass Calculation:
  • Magnesium (Mg): (M_r = 24)
  • Chlorine (Cl): (M_r = 35.5)
  • Total (M_r = 24 + 2(35.5) = 95)
• Moles Calculation:
  • (\text{Number of moles} = \frac{380}{95} = 4) moles

Example 3: Lithium Sulfate (Li2SO4)

• Given: 990 grams
• Relative Formula Mass Calculation:
  • Lithium (Li): (M_r = 7)
  • Sulfur (S): (M_r = 32)
  • Oxygen (O): (M_r = 16)
  • Total (M_r = 2(7) + 32 + 4(16) = 110)
• Moles Calculation:
  • (\text{Number of moles} = \frac{990}{110} = 9) moles

Example 4: Beryllium Hydroxide (Be(OH)2)

• Given: 64.5 grams
• Relative Formula Mass Calculation:
  • Beryllium (Be): (M_r = 9)
  • Oxygen (O): (M_r = 16)
  • Hydrogen (H): (M_r = 1)
  • Total (M_r = 9 + 2(16) + 2(1) = 43)
• Moles Calculation:
  • (\text{Number of moles} = \frac{64.5}{43} = 1.5) moles

Conclusion

• Practice more calculations using additional problems.
• Reference: Revision workbook available for further practice.