Physical chemistry - 1. Atoms, molecules and stoichiometry


I Relative masses of atoms and molecules

II The mole, the Avogadro constant

III The determination of relative atomic masses, Ar , and relative molecular masses, Mr , from mass spectra

IV The calculation of empirical and molecular formulae

V Reacting masses and volumes (of solutions and gases)

Learning outcomes

[The term relative formula mass or Mr will be used for ionic compounds]

Candidates should be able to:

(a) define the terms relative atomic, isotopic, molecular and formula masses, based on the 12C scale

(b) define the term mole in terms of the Avogadro constant

(c) *analyse mass spectra in terms of isotopic abundances and molecular fragments [knowledge of the working of the mass spectrometer is not required]

(d) calculate the relative atomic mass of an element given the relative abundances of its isotopes, or its mass spectrum

(e) define the terms empirical and molecular formulae

(f) calculate empirical and molecular formulae, using combustion data or composition by mass

(g) *write and/or construct balanced equations

(h) perform calculations, including use of the mole concept, involving:

When performing calculations, candidates’ answers should reflect the number of significant figures given or asked for in the question. When rounding up or down, candidates should ensure that significant figures are neither lost unnecessarily nor used beyond what is justified (see also Practical Assessment, Paper 31/32, Display of calculation and reasoning)

(i) deduce stoichiometric relationships from calculations such as those in (h)