(40) GCSE Chemistry Revision "Reacting Masses 1"

Understanding Moles in Chemical Reactions

• Definition of Moles: A mole is a unit that measures the amount of a substance based on the number of atoms or molecules.

• Calculating Moles for Elements:

  • Formula: Number of moles = Mass of chemical / Relative atomic mass

• Calculating Moles for Compounds:

  • Formula: Number of moles = Mass / Relative formula mass

  • Important to be able to calculate the relative formula mass before solving problems.

Example Calculation of Reactants and Products

• Example Problem: Calculate the mass of magnesium chloride (MgCl2) produced from 72 g of magnesium (Mg).

• Chemical Equation: Mg + Cl2 → MgCl2

  • Assumption: Chlorine is unlimited.

  • Interpretation of coefficients: No numbers means 1 for each substance.

Step 1: Calculate Moles of Magnesium

• Formula used: Number of moles = Mass / Relative atomic mass

• Calculation: 72 g / 24 (relative atomic mass of Mg) = 3 moles of magnesium.

Step 2: Determine Moles of Magnesium Chloride Produced

• From the equation, 1 mole of Mg produces 1 mole of MgCl2. Thus, 3 moles of Mg produces 3 moles of MgCl2.

Step 3: Calculate Mass of Magnesium Chloride

• Formula rearranged: Mass = Number of moles x Relative formula mass

• Relative formula mass of MgCl2 = 95.

• Calculation: 3 moles x 95 = 285 g of magnesium chloride.

Practice Problems

• First Practice Problem: Calculate the mass of calcium sulfate (CaSO4) produced from 80 g of calcium (Ca).

  • Given: 80 g of Ca

  • Relative atomic mass of Ca = 40

  • Moles: 80 g / 40 = 2 moles of Ca.

  • 2 moles of Ca produce 2 moles of CaSO4 (no coefficients means 1:1 ratio).

  • Relative formula mass of CaSO4 = 136.

  • Mass: 2 x 136 = 272 g of calcium sulfate.

• Second Practice Problem: Calculate mass of calcium carbonate (CaCO3) needed to produce 224 g of calcium oxide (CaO).

  • Given: 224 g of CaO

  • Relative formula mass of CaO = 56

  • Moles: 224 g / 56 = 4 moles of CaO.

  • 4 moles of CaO requires 4 moles of CaCO3.

  • Relative formula mass of CaCO3 = 100.

  • Mass needed: 4 x 100 = 400 g of calcium carbonate.

Additional Resources

• Revision Workbook: More practice problems available to aid in understanding reacting masses.