Stoichiometry

1. Formulae and Equations

Chemical Formulae

  • Formula of Elements: Represents the simplest ratio of atoms (e.g., $\text{O}_2$, $\text{P}_4$).
  • Molecular Formula: Shows the actual number of atoms of each element in a molecule of a compound.
  • Deducing Formulae: Determine formulae from molecular models or diagrams.

Chemical Equations

  • Word Equations: Use names of reactants and products.
  • Symbol Equations: Use chemical symbols and formulae.
  • State Symbols:
    • $(s)$ solid
    • $(l)$ liquid
    • $(g)$ gas
    • $(aq)$ aqueous solution
  • Ionic Equations: Show only the species that change during the reaction; spectator ions are omitted.

2. Relative Masses

Relative Atomic Mass ($A_r$)

  • The weighted average mass of an atom of an element compared to 1/12th of the mass of an atom of carbon-12.

Relative Molecular Mass ($M_r$) and Formula Mass

  • Relative Molecular Mass ($M_r$): Sum of the relative atomic masses of all atoms in a molecule.
  • Relative Formula Mass: Sum of relative atomic masses for ionic compounds (where molecular formula is not applicable).

3. The Mole Concept

The Mole and Avogadro Constant

  • The Mole ($\text{mol}$): The unit for amount of substance.
  • Avogadro Constant: $6.02 \times 10^{23}$ particles per mole.
  • Calculation: $\text{Number of particles} = \text{moles} \times (6.02 \times 10^{23})$

Molar Mass Calculations

  • Formula: $$\text{mass (g)} = \text{moles (mol)} \times \text{molar mass (g/mol)}$$
  • $\text{Molar mass}$ is numerically equal to $M_r$ or $A_r$.

Empirical and Molecular Formulae

  • Empirical Formula: The simplest whole-number ratio of atoms of each element in a compound.
  • Molecular Formula: A multiple of the empirical formula.
  • Calculation: Determine empirical formula from percentage composition or mass, then use $M_r$ to find the molecular formula.

4. Stoichiometry and Reacting Masses

Simple Proportions

  • Calculate reacting masses using the ratio of $M_r$ without the mole concept.

Stoichiometric Calculations

  • Use balanced symbol equations to determine the molar ratio between reactants and products.
  • Limiting Reactant: The reactant that is completely consumed first, limiting the amount of product formed.

5. Gas and Solution Stoichiometry

Molar Gas Volume

  • At room temperature and pressure (r.t.p.), 1 mole of any gas occupies $24\text{ dm}^3$.
  • Formula: $\text{volume (dm}^3) = \text{moles} \times 24$

Solution Concentrations

  • Mass Concentration: $\text{g/dm}^3$
  • Molar Concentration: $\text{mol/dm}^3$
  • Conversion: $\text{mol/dm}^3 = \frac{\text{g/dm}^3}{\text{molar mass}}$

Titrations

  • Use titration data (volume and concentration of a known solution) to calculate the unknown concentration or volume of another solution.
  • Titration setup

6. Yield and Purity

$$\text{Percentage Yield} = \frac{\text{actual yield}}{\text{theoretical yield}} \times 100%$$

$$\text{Percentage of element} = \frac{(\text{number of atoms} \times A_r)}{\text{total } M_r} \times 100%$$

$$\text{Percentage Purity} = \frac{\text{mass of pure substance}}{\text{total mass of impure sample}} \times 100%$$