This topic summary covers Knowledge Organiser: Moles and Quantitative Chemistry within Moles & Calculations for GCSE Chemistry. Revise Moles & Calculations in Quantitative Chemistry for GCSE Chemistry with 27 exam-style questions and 20 flashcards. This topic shows up very often in GCSE exams, so students should be able to explain it clearly, not just recognise the term. It is section 17 of 17 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Knowledge Organiser: Moles and Quantitative Chemistry
Key Terms
- Mole: 6.02 × 10²³ particles
- Mr: sum of all Ar values; no units
- n = m ÷ Mr: moles formula
- % Yield: (actual ÷ theoretical) × 100
- Atom economy: (desired Mr ÷ all Mr) × 100
- Conservation of mass: total mass in = total mass out
Must-Know Facts
- Avogadro's constant = 6.02 × 10²³ mol⁻¹
- 1 mole weighs Mr grams (molar mass)
- Coefficients in equations = mole ratios
- % yield always < 100% in practice
- High atom economy = greener process
- 1 dm³ = 1000 cm³ (critical unit conversion)
- Concentration c = n ÷ V (mol/dm³)
Key Equations
- n = m ÷ Mr (moles = mass ÷ relative formula mass)
- % yield = (actual yield ÷ theoretical yield) × 100
- Atom economy = (Mr of desired product ÷ sum of Mr of all products) × 100
- c = n ÷ V (concentration = moles ÷ volume in dm³)
Common Mistakes
- Using cm³ instead of dm³ in concentration calculations: Always convert — divide cm³ by 1000 to get dm³ before using c = n ÷ V
- Forgetting mole ratios from equation coefficients: The ratio of moles is given by the coefficients — e.g. 2H₂ + O₂ → 2H₂O means 2 mol H₂ reacts with 1 mol O₂
- Calculating % yield with the wrong masses: % yield uses actual (experimental) mass divided by theoretical (calculated) mass — not the other way round
- Confusing atom economy with % yield: Atom economy is about the equation (unavoidable); % yield is about how well the experiment worked (variable)
Practice questions for Moles & Calculations
One mole of any substance contains how many particles?
Explain why the percentage yield of a reaction is never 100% in practice.
Quick recall flashcards
Define 'one mole'
The amount of substance containing 6.02 × 10²³ particles
One mole of any element weighs exactly its Ar in grams
What is Avogadro's constant?
6.02 × 10²³ particles per mole
This is the number of particles in one mole of any substance.