Quantitative Practical Skills: Percentage Yield From Experimental Data (Copy)

Quantitative Practical Skills

Percentage Yield From Experimental Data

Purpose of Percentage Yield Calculations in ATP

• Percentage yield shows:
  • How much product was actually obtained compared to the theoretical maximum
• ATP questions test:
  • Ability to calculate moles of reactants and products
  • Correct use of stoichiometry
  • Correct application of percentage yield formula
• Examiners reward:
  • Stepwise method
  • Clear working
  • Proper units and significant figures

Core Examiner Rule

• Percentage yield formula:

  Percentage yield = (Actual yield ÷ Theoretical yield) × 100

• Use:
  • Mass of product for actual yield
  • Calculated mass from moles for theoretical yield
• Show all steps; final answer with correct significant figures

Step 1: Identify Actual Yield

• Actual yield = mass of product obtained experimentally
• Often given in question
• Example:
  • Actual yield of CuSO₄·5H₂O = 2.10 g

Step 2: Calculate Moles of Limiting Reactant

• Use balanced chemical equation
• Use formula: Moles = Mass ÷ Relative Formula Mass (RFM)
• Example: Reaction:
  Cu + H₂SO₄ → CuSO₄ + H₂
• Cu used: 2.00 g
• RFM Cu = 63.5 g/mol
• Moles Cu = 2.00 ÷ 63.5 = 0.0315 mol

Step 3: Calculate Theoretical Mass of Product

• Use stoichiometry from balanced equation
• Moles of product (CuSO₄) = moles of limiting reactant = 0.0315 mol
• RFM CuSO₄·5H₂O = 249.5 g/mol
• Theoretical mass = 0.0315 × 249.5 = 7.85 g

Step 4: Calculate Percentage Yield

• Percentage yield = (Actual ÷ Theoretical) × 100
• Example:
  (2.10 ÷ 7.85) × 100 = 26.8%

Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course

Step 5: Expressing the Answer Correctly

• Use correct units if required (usually % has none)
• Round to 2 significant figures unless stated otherwise
• Box the final answer

Common Examiner Traps

• Using experimental mass instead of theoretical mass in denominator
• Using wrong stoichiometric ratio
• Ignoring water of crystallisation in hydrated salts
• Using wrong RFM
• Incorrect significant figures

High-Yield ATP Tips for Percentage Yield

• Always identify the limiting reactant
• Use moles to calculate theoretical mass
• Convert all masses to grams consistently
• Apply formula carefully
• Check units and significant figures

Stepwise Example Table for Clarity

Core Scientific Principle

• Percentage yield measures efficiency of reaction
• Low yield can be due to:
  • Side reactions
  • Loss during filtration or transfer
  • Incomplete reactions
• ATP marks focus on:
  • Correct calculation process
  • Accurate use of stoichiometry
  • Presentation with proper significant figures