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)
Memory Aid
- The formula triangle: Draw a triangle with m (mass) at the top, n (moles) bottom-left, and Mr bottom-right. Cover what you want: cover n → m over Mr → n = m ÷ Mr. Think: "Mr Mole finds the MASS."
- For percentage yield: "ACTUAL over THEORETICAL, times a hundred" — A/T × 100. Remember: actual yield is always lower than theoretical in real experiments.
- For atom economy: "DESIRED over ALL, times a hundred" — the product you WANT divided by ALL products formed. High atom economy = green chemistry.
- Avogadro's number: 6.02 × 10²³ — roughly 600 billion trillion. Think of it as "a million million million million" — impossibly big, which is why atoms are weighed in grams, not counted individually.