Mole Concept and Stoichiometry

The One Thing

The mole provides the quantitative link between particles, mass and gas volume. Balanced chemical equations express the fixed mole ratios required for stoichiometric calculations, while vapour density helps determine the relative molecular mass of a gaseous substance.

Reactions, Processes and Experiments

Key Terms

• Mole: The amount of substance containing 6.022 × 10^23 elementary particles such as atoms, molecules or ions.
• Avogadro constant: The number of particles in one mole of a substance: 6.022 × 10^23 mol^-1.
• Relative atomic mass: The average mass of an atom compared with one-twelfth of the mass of a carbon-12 atom.
• Relative molecular mass: The sum of the relative atomic masses of all atoms present in one molecule.
• Molar mass: The mass of one mole of a substance, expressed in grams per mole (g mol^-1). Its numerical value is equal to the relative molecular or formula mass.
• Mole fraction: The ratio of the number of moles of one component to the total number of moles in a mixture.
• Molar volume: The volume occupied by one mole of a gas at standard temperature and pressure; in school-level calculations it is commonly taken as 22.4 dm^3 or 22.4 litres at STP.
• Vapour density: The ratio of the mass of a certain volume of a gas or vapour to the mass of an equal volume of hydrogen under the same conditions of temperature and pressure.
• Relative molecular mass from vapour density: For gases, relative molecular mass is twice the vapour density: relative molecular mass = 2 × vapour density.
• Stoichiometry: The quantitative study of the reactants and products involved in a chemical reaction.
• Balanced chemical equation: An equation having the same number of atoms of each element on both sides, in accordance with the law of conservation of mass.
• Limiting reactant: The reactant that is completely used up first and therefore determines the maximum amount of product formed.
• 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 of a compound.
• Percentage composition: The percentage by mass of each element present in a compound.
• Law of conservation of mass: Matter is neither created nor destroyed during a chemical reaction.
• Law of constant proportions: A pure compound always contains the same elements in the same fixed proportion by mass.
• Law of multiple proportions: When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in simple whole-number ratios.

Easily Confused

• Relative molecular mass and molar mass: Relative molecular mass is a numerical value without units, whereas molar mass is expressed in g mol^-1; their numerical values are equal.
• Molecular mass and formula mass: Molecular mass applies to molecules, whereas formula mass applies to ionic compounds and is calculated from one formula unit.
• Empirical formula and molecular formula: The empirical formula gives the simplest whole-number ratio of atoms, whereas the molecular formula gives the actual number of atoms in one molecule.
• Coefficients and mass ratios: Coefficients in a balanced equation represent mole ratios, not necessarily mass ratios.
• Limiting reactant and excess reactant: The limiting reactant is used up first and determines the product amount; an excess reactant may remain after the reaction.
• Mole fraction and number of moles: Mole fraction is a ratio involving one component’s moles and the total moles in a mixture; it is not the absolute amount of a component.

What Gets Asked

• Calculate particles, moles or mass using the relationships between number of particles, moles and molar mass. A common error is confusing “number of moles = given mass ÷ molar mass” with “mass = number of moles × molar mass.”
• Calculate gas moles or volume at STP. The relevant volume is 22.4 dm^3 or 22.4 litres per mole, and units must remain consistent.
• Determine relative molecular mass or vapour density. The required relationship is relative molecular mass = 2 × vapour density, or vapour density = relative molecular mass ÷ 2.
• Determine percentage composition, an empirical formula or a molecular formula. Marks are lost by failing to convert percentage or mass data into moles, divide by the smallest mole value, or apply the molecular-mass factor.
• Complete mass-to-mass or volume-to-volume stoichiometric calculations. The equation must first be balanced, and its coefficients must be used as mole ratios rather than mass ratios.
• Identify the limiting reactant and calculate the maximum product formed. The limiting reactant is the one completely used up first; another reactant may remain in excess.