Skip to content

2.1.3 Amount of substance

Definitions

Term Definition
Mole A mole is the amount of a substance that contains the Avogadro number of elementary particles.
Molar mass (\(M_r\)) The mass in grams in each mole of the substance, measured in \(g \cdot mol^{-1}\).
Hydrated A crystalline compound that contains water (e.g. \(CuSO_4 \cdot 5H_2O(s)\)).
Anhydrous A crystalline compound containing no water (e.g. \(CuSO_4(s)\)).
Water of crystallisation Water molecules that form part of the crystalline structure of a compound (e.g. \(H_2O\) in \(CuSO_4 \cdot 5H_2O(s)\)).
Stoichiometry The relative quantities of substances in a reaction.
Standard solution A solution of known concentration.
Limiting reagent The reactant that is not in excess and will be used up in the reaction.

Amount of substance

Amount of substance

  • Symbol: \(n\)
  • Measured in moles (symbol \(mol\))
  • Always use decimals (not fractions) in every step of a calculation

Avogadro constant (\(N_A\))

  • \(6.02 \times 10^{23} mol^{-1}\)
  • The number of particles per mole

Concentration (\(c\))

  • Unit = \(mol \cdot dm^{-3}\) (aka molar / M) or \(g \cdot dm^{-3}\)
  • \(mol \cdot dm^{-3}\): \(c = \frac{n}{V} = \frac{\text{number of moles}}{\text{volume } (dm^3)}\)
  • \(g \cdot dm^{-3}\): \(c = \frac{\text{mass } (g)}{\text{volume } (dm^3)}\)
  • Conversion: \(\text{Conc } (mol \cdot dm^{-3}) = \frac{\text{Conc } (g \cdot dm^{-3})}{M_r}\)

Room temperature and pressure (RTP)

  • Temp = 20°C / 293 K
  • Pressure = 1 atm or \(1.01 \times 10^5\) Pa

Standard temperature and pressure (STP)

  • Temp = 0°C / 273 K
  • Pressure = 1 atm or \(1.01 \times 10^5\) Pa

Molar gas volume (\(V_m\))

  • The volume per mole of gas at a stated temperature and pressure
  • Under RTP: 1 mol = 24 \(dm^3\) = 24,000 \(cm^3\)
  • Under STP: 1 mol = 22.4 \(dm^3\) = 22,400 \(cm^3\)

Experimental techniques to measure the amount of substances

Variable measured Method
Mass - Use a digital mass balance
- Choose a balance with a suitable resolution for the experiment
Volume of solution - Use a measuring cylinder
- Standard solution: use volumetric flask
Gas produced - Use a gas syringe / measure mass lost on a balance and calculate the number of moles of gas produced

Percentage yield

  • \(\text{Percentage yield} = \frac{\text{actual yield}}{\text{theoretical yield}} \times 100\%\)
  • Actual yield: the amount of the product obtained from a reaction
  • Theoretical yield: the yield resulting from complete conversion of reactants into products
  • Reasons for < 100% percentage yield
    • Reaction did not go to completion (reversible reaction)
    • Side reactions may have taken place along the main reaction
    • Purification of the product may result in the loss of some products

Atom economy

  • \(\text{Atom economy} = \frac{\text{sum of molar masses of useful products}}{\text{sum of molar masses of all products or reactants}} \times 100\%\)