Stoichiometry Principles and Chemical Formulae

Stoichiometry and the Law of Conservation of Mass

• Stoichiometry is used to calculate the amounts of reactants and products based on balanced chemical equations.

• It is governed by the Law of Conservation of Mass, which states mass is neither created nor destroyed; therefore, total reactant mass must equal total product mass.

• Stoichiometric coefficients indicate the mole ratio between substances in a reaction.

Chemical Formulae of Elements and Compounds

• Elements can exist as aggregates of atoms represented by symbols (e.g., $Na$, $Ca$, $C$, $Fe$) or as discrete molecules (e.g., $H_2$, $O_2$, $O_3$).

• Ionic compounds consist of ions in a crystal lattice and are represented by formula units showing the simplest ratio (e.g., $NaCl$, $CaCl_2$, $KBr$, $BaCl_2$).

• Covalent compounds exist as discrete molecules represented by molecular formulae indicating the actual number of atoms (e.g., $H_2O$, $NH_3$, $CH_4$, $H_2SO_4$, $C_6H_6$).

Empirical and Molecular Formulae

• The empirical formula represents the simplest ratio of atoms in a compound.

• The molecular formula represents the actual number of each type of atom in a single molecule.

• The relationship between the two is defined by: $\text{Molecular formula} = n \times (\text{Empirical Formula})$.

• The value of $n$ is calculated as: $n = \frac{\text{Molar Mass}}{\text{Empirical Formula mass}}$.

• Some compounds, like $H_2O$, have identical empirical and molecular formulae, while others, like benzene ($C_6H_6$) and acetylene ($C_2H_2$), share the same empirical formula ($CH$).

Formulae of Binary Ionic Compounds

• To write an ionic formula, the charges of cations and anions must result in an electrically neutral compound.

• The crisscross method is applied: the numerical value of one ion's charge becomes the subscript of the other ion.

• Examples include Aluminium oxide ($Al_2O_3$) and Magnesium nitride ($Mg_3N_2$).

• Common ions include $H^+$, $Na^{1+}$, $Li^{1+}$, $K^{1+}$, $Mg^{2+}$, $Ca^{2+}$, $Ba^{2+}$, $Cu^{1+/2+}$, $Fe^{2+/3+}$, $Zn^{2+}$, $Sn^{2+/4+}$, $Al^{3+}$, $Ni^{2+}$, $O^{2-}$, $N^{3-}$, $Cl^{1-}$, $Br^{1-}$, and $I^{1-}$.

Avogadro's Number and the Mole

• Avogadro's number ($N_A$) is defined as $6.022 \times 10^{23}$ particles.

• A mole is the amount of substance containing $N_A$ particles (atoms, molecules, or ions).

• Molar mass is the mass of one mole of a substance, expressed in $g\,mol^{-1}$.

• The relationship between grams and atomic mass units is: $1.00\,g = 6.022 \times 10^{23}\,amu$.

Chemical Equations and Calculations

• Chemical equations represent changes using symbols, with reactants on the left and products on the right.

• State symbols specify physical forms: $(s)$ for solid, $(l)$ for liquid, $(g)$ for gas, and $(aq)$ for aqueous solutions.

• Reversible reactions are indicated by the symbols $\rightleftharpoons$ or similar directional indicators.

• Stoichiometry allows for mass-mass and mole-mole conversions. For example, burning $1.80\,moles$ of ethyl alcohol ($C_2H_5OH$) requires $5.4\,moles$ of $O_2$ and produces $3.6\,moles$ of $CO_2$.