IB Chemistry home > Syllabus 2016 > Stoichiometry > The microscopic world

Syllabus ref: 1.2

The microscopic world of particles produces problems of scale in terms of their almost infintesimally small size and the enormous numbers of them.

This problem was dealt with specifically by Amedeo Avogadro, who developed the work of French chemist Joseph Louis Guy-Lussac (1778-1850) with the 1811 publication of his hypothesis, and the idea that gases are made up of atoms or combinations of atoms (molecules) and can be quantified. Although his work was largely ignored during his lifetime, by the 1880s it was universally accepted, thanks to Stanislao Cannizzaro, who created a table of atomic weights based on Avogadro's work.

This section deals with this quantification and its utility to modern chemists.

Nature of science:

Making quantitative measurements with replicates to ensure reliability - definite and multiple proportions.

Concepts - the concept of the mole developed from the related concept of 'equivalent mass' in the early 19th century.


Essential idea: Physical and chemical properties depend on the ways in which different atoms combine.

Atoms of different elements combine in fixed ratios to form compounds, which have different properties from their component elements. Mixtures contain more than one element and/or compound that are not chemically bonded together and so retain their individual properties.

Mixtures are either homogeneous or heterogeneous.

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

Deduction of chemical equations when reactants and products are specified.

Application of the state symbols (s), (l), (g) and (aq) in equations.

Explanation of observable changes in physical properties and temperature during changes of state.

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.

In Chapter 3.1