The mole concept

Nature of science:

Concepts—the concept of the mole developed from the related concept of “equivalent mass” in the early 19th century. (2.3)

• The mole is a fixed number of particles and refers to the amount, n, of substance.
• Masses of atoms are compared on a scale relative to ¹²C and are expressed as relative atomic mass (A_r) and relative formula/molecular mass (M_r).
• Molar mass (M) has the units g mol-1.
• The empirical formula and molecular formula of a compound give the simplest ratio and the actual number of atoms present in a molecule respectively.

Applications and skills:

• Calculation of the molar masses of atoms, ions, molecules and formula units.
• Solution of problems involving the relationships between the number of particles, the amount of substance in moles and the mass in grams.
• Interconversion of the percentage composition by mass and the empirical formula.
• Determination of the molecular formula of a compound from its empirical formula and molar mass.
• Obtaining and using experimental data for deriving empirical formulas from reactions involving mass changes.

• The SI system (Système International d’Unités) refers to the metric system of measurement, based on seven base units.
• The International Bureau of Weights and Measures (BIPM according to its French initials) is an international standards organization, which aims to ensure uniformity in the application of SI units around the world.

Theory of knowledge:

• The magnitude of Avogadro’s constant is beyond the scale of our everyday experience. How does our everyday experience limit our intuition?

Utilization:

• Stoichiometric calculations are fundamental to chemical processes in research and industry, for example in the food, medical, pharmaceutical and manufacturing industries.
• The molar volume for crystalline solids is determined by the technique of X- ray crystallography.
  • Syllabus and cross-curricular links:
  • Topic 2.1—the scale of atoms and their component particles
  • Topics 4.1, 4.3 and 4.5—lattice structure of ionic compounds, molecular structure of covalent compounds and metallic lattice
  • Topics 5.1 and 15.2—standard enthalpy and entropy changes defined per mole Topic 19.1—mole ratios of products in electrolysis

• The value of the Avogadro’s constant (L or N_A) is given in the data booklet in section 2 and will be given for paper 1 questions.
• The generally used unit of molar mass (g mol^-1) is a derived SI unit.

• Aim 6: Experiments could include percent mass of hydrates, burning of magnesium or calculating Avogadro’s number.
• Aim 7: Data loggers can be used to measure mass changes during reactions.