Stoichiometry

The One Thing

Stoichiometry uses balanced chemical equations to relate the amounts of reactants and products. Relative masses, moles, concentrations and gas volumes provide the quantitative links needed to calculate these amounts.

Reactions, Processes and Experiments

Key Terms

• Chemical formula: A symbolic representation showing which elements are in a substance and the ratio of their atoms or ions.
• Empirical formula: The simplest whole-number ratio of atoms of each element in a compound.
• Molecular formula: The actual number of atoms of each element in one molecule.
• Structural formula: A formula showing how atoms are arranged or bonded in a molecule.
• Balanced chemical equation: An equation with the same number of atoms of each element on both sides, obeying conservation of mass.
• State symbol: A symbol showing physical state: (s) solid, (l) liquid, (g) gas or (aq) aqueous solution.
• Relative atomic mass (Ar): The weighted average mass of an atom of an element compared with one-twelfth of the mass of a carbon-12 atom.
• Relative formula mass (Mr): The sum of the relative atomic masses of all atoms shown in a formula.
• Mole: The amount of substance containing approximately 6.02 × 10^23 particles, such as atoms, molecules or ions.
• Avogadro constant: The number of particles in one mole: approximately 6.02 × 10^23 mol^-1.
• Molar mass: The mass of one mole of a substance, measured in g mol^-1; its numerical value equals Mr for a compound.
• Concentration: The amount of solute dissolved per unit volume of solution.
• Limiting reactant: The reactant that is completely used up first and therefore limits the amount of product formed.
• Excess reactant: A reactant present in more than the amount required, so some remains after the reaction.
• Percentage yield: The actual amount of product obtained expressed as a percentage of the theoretical maximum amount.
• Percentage composition by mass: The percentage of a compound's total mass contributed by a particular element.
• Titration: A method used to find the concentration of a solution by reacting it with a solution of known concentration.
• Conservation of mass: Atoms are rearranged during a chemical reaction but are not created or destroyed.
• Stoichiometric calculation: A calculation using a balanced equation to determine the quantitative relationship between reactants and products.
• Mass concentration: The mass of solute dissolved per unit volume of solution, commonly expressed in g dm^-3.

Easily Confused

• Empirical formula vs molecular formula: The empirical formula gives the simplest whole-number ratio of atoms; the molecular formula gives the actual number of atoms in one molecule.
• Relative atomic mass vs relative formula mass: Relative atomic mass applies to an individual element's atoms; relative formula mass is the sum for all atoms represented in a formula.
• Moles from mass vs mass from moles: Use n = mass ÷ molar mass to calculate moles from mass, but mass = moles × molar mass to calculate mass from moles.
• Limiting reactant vs excess reactant: The limiting reactant is completely used up first; the excess reactant remains after the reaction.
• Concentration in mol dm^-3 vs concentration in g dm^-3: Molar concentration uses moles per dm^3; mass concentration uses grams per dm^3.
• Volume in cm^3 vs volume in dm^3: Concentration calculations require consistent units; 1 dm^3 = 1000 cm^3, so volume in dm^3 = volume in cm^3 ÷ 1000.
• Theoretical yield vs actual yield: The theoretical yield is the maximum calculated amount; the actual yield is the amount obtained experimentally.
• Coefficients vs subscripts: When balancing equations, change coefficients placed before formulae; never change subscripts within formulae.
• Gas volume vs solution volume: At room temperature and pressure, one mole of gas occupies approximately 24 dm^3; solution volumes are used in concentration equations.

What Gets Asked

• Balancing chemical equations: Questions require the same number of atoms of each element on both sides. Changing subscripts within formulae instead of coefficients loses marks.
• Relative mass calculations: Questions may require Mr = sum of (Ar × number of each atom), including the example Mr of CaCO3 = 40.1 + 12.0 + (3 × 16.0) = 100.1.
• Mole and stoichiometric calculations: The required sequence is to balance the equation, convert the known quantity to moles, use the equation ratio, and convert to the required quantity. The equation coefficients must be used as mole ratios.
• Concentration and titration calculations: Questions require volumes in dm^3, calculation of moles of the known solution, application of the balanced-equation ratio and calculation of the unknown concentration.
• Limiting-reactant problems: Students must compare available mole amounts with the balanced-equation requirements and identify the reactant producing the smaller amount of product as limiting.
• Yield, atom economy and composition: Questions may require Percentage yield = (actual yield ÷ theoretical yield) × 100%, Atom economy = (Mr of desired product ÷ total Mr of reactants) × 100%, or percentage composition by mass. Experimental yields may be below theoretical yields because of incomplete reactions, side reactions, product loss during separation or measurement uncertainty.