Stoichiometry and Atomic Mass Concepts

Stoichiometry

• Stoichiometry is the calculation of reactants and products in a chemical reaction.
• Formulae of elements and compounds must be known.
• Molecular formula: the number and type of atoms in a molecule.
• Empirical formula: simplest whole number ratio of elements in a compound.
• Simple compound formula can be derived from models or diagrams.
• Word and symbol equations show reactant-product transformations, including state symbols.
• Ionic compound formulae are derived from the relative number of ions, and their charges.
• Polyatomic ions are groups of atoms that behave as a single unit, for example: carbonate (CO₃²⁻), nitrate (NO₃⁻), ammonium (NH₄⁺), hydroxide (OH⁻), and sulfate (SO₄²⁻).
• Examples of metals: Group 1 (K⁺, Na⁺), Group 2 (Ca²⁺, Mg²⁺), Group 3 (Al³⁺), Transition metals (Zn²⁺, Cu⁺, Cu²⁺, Ag⁺, Fe²⁺, Fe³⁺, Cr³⁺).

Relative Masses of Atoms and Molecules

• Relative atomic mass (A_r) is the average mass of isotopes compared to ¹/₁₂ of ¹²C.
• Relative molecular mass (M_r) of covalent compounds is the sum of relative atomic masses.
• Relative formula mass (M_r) of ionic compounds is the sum of relative atomic masses.
• Calculating reacting masses involves simple proportions, without moles.

The Mole and Avogadro Constant

• The mole (mol) is the unit of amount of substance.
• One mole contains 6.02 x 10²³ particles (Avogadro constant).
• Amount of substance (mol) = mass (g) / molar mass (g/mol)
• Calculations can find molar mass, amount of substance, masses, and the number of particles.
• Molar gas volume of a gas under room temperature and pressure is 24 dm³.

Gas Calculations

• 1 mole of any gas occupies 24 dm³ at room temperature and pressure (r.t.p. = 25°C, 1 atm).
• Gas volume calculations involve stoichiometry.

Percentage Yield

• Percentage yield is the ratio of the actual mass obtained to the theoretical mass, multiplied by 100%.
• Actual yield is the mass of a product obtained in a chemical reaction.
• Theoretical yield is the maximum mass of product that could be produced.