Lecture 9. Composition of compounds
Thursday 15 February 2024
Formula stoichiometry and composition by mass. Determination of chemical formulas from experimental data. Combustion analysis.
Reading: Tro NJ. Chemistry: Structure and Properties (3rd ed.) - Ch.4, §4.7-4.11 (pp.184-201)
Summary
We'll finish Ch. 4 with examples of calculations relating to the composition of compounds. Mass percent composition is defined and used in conversions involving mass of a compound and of a component of that compound. Chemical formulas provide the stoichiometric relationship among the atoms that make up a molecule or formula unit. The whole number ratios, from formula subscripts, are central to calculations relating masses of a compound to those of any of its components.
Bond polarity can be thought of as arising from a difference in electronegativity between the bonded atoms (this difference in electronegativety is denoted as ΔEN). A continuum of bonding types exists between 100% ionic bonding and perfectly covalent bonding.
It is important to practice drawing Lewis structures for molecules or polyatomic ions, Initially, we'll build Lewis structures based on a careful (and therefore correct) count of total number of valence electrons shared by the atoms comprising the molecule or ion, and the so-called "octet rule", which states that atoms tend to lose, gain, or share electrons in order to reach a noble gas electron configuration (i.e. a completely filled valence shell), which for n = 2 or n = 3 constitutes a filled valence shell of eight electrons (electron configuration of ns2np6).