Difficulty: Intermediate | Prerequisites: Periodic Table and Bonding (Ch. 6–8)
This material teaches you the language of chemistry (naming compounds and writing formulas) and the counting system (the mole). You cannot balance equations, do stoichiometry or understand lab work without these skills. Naming rules differ for ionic compounds, molecular compounds and acids, and each set follows its own logic. The mole bridges the gap between individual atoms and laboratory-scale quantities. Percent composition and empirical formulas let you work backwards from experimental data to find what a compound actually is.
Ionic compounds are named cation-first (with Roman numerals for transition metals that have variable charges), then anion with an -ide ending for monatomic ions or the polyatomic ion name. Molecular compounds use Greek prefixes. Acids follow a pattern based on whether the anion ends in -ide, -ite or -ate. The mole (6.022 x 10²³ particles) links atomic-scale counting to grams via molar mass, and to litres via molar volume at STP (22.4 L for gases). Percent composition, empirical formulas and molecular formulas are all routes to identifying unknown compounds.
Mole
The SI unit for measuring the amount of a substance. One mole = 6.022 x 10²³ representative particles (Avogadro's number).
Avogadro's number
6.022 x 10²³. The number of representative particles in one mole of a substance.
Molar mass
The mass (in grams) of one mole of a substance. Numerically equal to the formula weight in amu.
Molar volume
The volume occupied by one mole of any gas at STP (0 °C, 101.3 kPa). It equals 22.4 L.
Percent composition
The percentage by mass of each element in a compound.
Empirical formula
The lowest whole-number ratio of atoms of each element in a compound.
Molecular formula
The actual number of atoms of each element in a molecule. It is a whole-number multiple of the empirical formula.
Polyatomic ion
A charged group of covalently bonded atoms that acts as a single ion. An -ate or -ite ending indicates the ion contains oxygen.
Binary compound
A compound composed of exactly two elements. Sulfur hexafluoride (SF₆) is a binary compound, not a polyatomic compound.
Monatomic anions end in -ide (chloride, oxide, sulfide).
Binary ionic compounds: cation on the left, anion on the right. NaCl = sodium chloride.
Transition metals with more than one common charge use a Roman numeral after the name: iron(II) chloride = FeCl₂, iron(III) chloride = FeCl₃.
PbO contains the lead(II) ion (Pb²⁺). PbCl₄ contains lead(IV).
Mn₂O₃ contains the Mn³⁺ ion (total positive charge = +6 from two Mn; total negative charge = -6 from three O²⁻; each Mn = +3).
BaI₂ is an ionic compound (Ba is a metal, I is a nonmetal). CS₂, N₂O₄ and PCl₃ are molecular.
Lithium combined with fluorine would most likely form an ionic compound (metal + nonmetal).
An -ate or -ite ending indicates the compound contains a polyatomic anion with oxygen.
-ate has more oxygen than -ite (e.g. sulfate SO₄²⁻ vs sulfite SO₃²⁻).
Potassium sulfite = K₂SO₃.
Use Greek prefixes to indicate the number of each atom: mono-, di-, tri-, tetra-, penta-, hexa-.
Sulfur hexafluoride (SF₆) is a binary compound (two elements, not a polyatomic compound).
If H⁺ is the cation, the compound is an acid.
Anion ends in -ide → prefix hydro-, suffix -ic acid (HCl = hydrochloric acid).
Anion ends in -ite → suffix -ous acid (H₂SO₃ = sulfurous acid).
Anion ends in -ate → suffix -ic acid (H₂SO₄ = sulfuric acid; H₃PO₄ = phosphoric acid).
An -ite or -ate ending on a polyatomic ion means oxygen is in the formula.
The mole is the SI unit for amount of substance. One mole = 6.022 x 10²³ particles.
Avogadro's number of representative particles equals one mole.
Moles to particles: multiply by 6.022 x 10²³. 2.10 mol CO₂ = 2.10 x 6.022 x 10²³ = 1.26 x 10²⁴ molecules.
Moles to grams: multiply by molar mass. 5.90 mol C₈H₁₈ (molar mass = 114.2 g/mol) = 5.90 x 114.2 = 673 g.
Grams to moles: divide by molar mass. 432 g Ba(NO₃)₂ (molar mass = 261.3 g/mol) = 432 / 261.3 = 1.65 mol.
Molar mass of (NH₄)₂CO₃: 2(14 + 4) + 12 + 3(16) = 96 g/mol.
The atomic masses of any two elements contain the same number of atoms (one mole of each).
One mole of any gas occupies 22.4 L at STP (standard temperature and pressure: 0 °C, 101.3 kPa).
This applies to gases only, not liquids, solids or compounds in general.
0.500 mol C₃H₈ gas at STP = 0.500 x 22.4 = 11.2 L.
If a noble gas has density 1.783 g/L at STP: molar mass = 1.783 x 22.4 = 39.9 g/mol → argon (Ar).
Percent composition = (mass of element in compound / total mass of compound) x 100.
If 60.2 g Hg combines with 24.0 g Br: percent Hg = 60.2 / (60.2 + 24.0) x 100 = 71.5%.
Percent carbon in acetone (C₃H₆O, molar mass 58.08): (3 x 12.01) / 58.08 x 100 = 62.1%.
The empirical formula is the lowest whole-number ratio of elements.
A compound that is 40% S and 60% O: convert to moles (40/32 = 1.25 mol S; 60/16 = 3.75 mol O), divide by the smallest (1.25): ratio S:O = 1:3, giving empirical formula SO₃.
Students confuse -ate and -ite: -ate has MORE oxygen, not less.
The 22.4 L molar volume applies only to gases at STP, not to liquids or solids.
Empirical and molecular formulas are not the same thing. The molecular formula is a whole-number multiple of the empirical formula.
Roman numerals are used for transition metals with variable charges, not for all metals.
⚠️ Know the acid naming pattern: -ide → hydro___ic acid; -ite → ___ous acid; -ate → ___ic acid.
⚠️ Be able to convert between moles, grams and particles in both directions.
⚠️ Molar mass calculations: add up atomic masses carefully, especially for compounds with parentheses like (NH₄)₂CO₃.
⚠️ Phosphoric acid = H₃PO₄. This is a commonly tested formula.
⚠️ Know that BaI₂ is ionic (metal + nonmetal), while CS₂ and PCl₃ are molecular.
⚠️ Percent composition questions require you to know the formula and molar mass of the compound.
Fill in the blank: One mole of any gas at STP occupies ______ L.
True or False: An -ate ending on a polyatomic ion means nitrogen is present.
Fill in the blank: The SI unit for amount of substance is the ______.
True or False: Roman numerals are used in naming all ionic compounds.
Fill in the blank: The empirical formula gives the ______ whole-number ratio of elements.
Q: What is the formula for potassium sulfite?
A: K₂SO₃.
Q: How many molecules are in 2.10 mol CO₂?
A: 1.26 x 10²⁴ molecules (2.10 x 6.022 x 10²³).
Q: What is the mass in grams of 5.90 mol C₈H₁₈?
A: 673 g (5.90 x 114.2 g/mol).
Q: What is the molar mass of (NH₄)₂CO₃?
A: 96 g/mol.
Q: What is the empirical formula of a compound that is 40% sulfur and 60% oxygen by weight?
A: SO₃ (mole ratio S:O = 1:3).
Q: When naming acids, the prefix hydro- is used when the anion ends in what?
A: -ide.
Naming skills are used every time you write a chemical equation (Ch. 11). The mole concept is the foundation of stoichiometry (Ch. 12): every mole ratio in a balanced equation depends on it. Percent composition and empirical formulas connect to analytical chemistry and identifying unknown substances in lab.
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