1.2 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.  

Understandings:

• 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 (Ar) and  relative formula/molecular mass (Mr).
• 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.

Guidance:

• The value of the Avogadro’s constant (L or NA) 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.

Aims: