Counting atoms with the mole and calculating reaction quantities
Chemistry depends on precise measurement. Significant figures show how certain a measurement is, and calculations must follow rules: in addition the answer keeps the fewest decimal places, and in multiplication it keeps the fewest significant figures. Laws of chemical combination, including the law of multiple proportions and Gay-Lussac’s law of gaseous volumes, led to Avogadro’s law that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.
The mole is the central unit: one mole of any substance contains 6.022 × 10²³ entities and has a mass equal to its molar mass in grams. At STP, one mole of an ideal gas occupies 22.4 L. Empirical formula gives the simplest whole-number ratio of atoms, and the molecular formula is a whole-number multiple of it. Stoichiometry uses balanced equations to find quantities; the limiting reagent is used up first and decides how much product forms. Concentration can be expressed as molarity, molality or mole fraction.
Why it matters
Stoichiometry is used in industry to calculate raw material needs and yields, in medicine to prepare drug solutions of exact strength, and in environmental testing to measure pollutants. Getting significant figures right keeps laboratory results honest. Every later physical chemistry topic, from solutions to thermodynamics, uses the mole concept.
Key terms
- Molar mass
- The mass of one mole of a substance, expressed in grams per mole.
- Empirical formula
- The simplest whole-number ratio of atoms of each element in a compound.
- Limiting reagent
- The reactant that is completely consumed first and limits the amount of product formed.
- Molarity
- The number of moles of solute dissolved per litre of solution.