Naming Molecular Compounds, Acids, and Calculating Formula Mass

Naming Molecular Compounds, Acids, and Calculating Formula Mass

Objectives

• Understand the rules for naming molecular compounds.

• Learn how to name binary acids and oxyacids (acids containing an oxyanion ending in -ate or -ite).

• Be able to write and name chemical compounds.

• Calculate formula mass for compounds.

Molecular Compounds

Definition and Characteristics

Molecular compounds are composed of two or more nonmetals bonded together. These compounds are also known as covalent compounds and do not contain metals.

• Formed by sharing electrons between nonmetal atoms.

• Examples include CO2 (carbon dioxide), SO2 (sulfur dioxide).

Naming Binary Molecular Compounds

Binary molecular compounds consist of exactly two different nonmetal elements. The naming convention uses prefixes to indicate the number of atoms of each element present.

• First element: Use the full element name. If there is only one atom of the first element, the prefix "mono-" is usually omitted.

• Second element: Use the base name of the element with the suffix "-ide" and always include a prefix to indicate the number of atoms.

Prefixes for Number of Atoms:

Number	Prefix
1	mono-
2	di-
3	tri-
4	tetra-
5	penta-
6	hexa-
7	hepta-
8	octa-
9	nona-
10	deca-

• If the prefix ends with a vowel and the element name starts with a vowel, drop the final vowel of the prefix (e.g., "monooxide" becomes "monoxide").

• Exception: "Dioxide" and "triiodide" retain both vowels.

Examples:

• CO: carbon monoxide

• CO2: carbon dioxide

• N2O: dinitrogen monoxide

• CCl4: carbon tetrachloride

• BCl3: boron trichloride

• SF6: sulfur hexafluoride

Acids

Definition and Classification

Acids are molecular compounds that release hydrogen ions (H+) when dissolved in water. The formula of an acid typically has hydrogen as the first element. Acids are classified into two main types:

• Binary acids: Contain hydrogen and one other nonmetal element.

• Oxyacids: Contain hydrogen, a nonmetal, and oxygen (usually as part of a polyatomic ion called an oxyanion).

Naming Binary Acids

• General formula: H + nonmetal

• Name: hydro + base name of nonmetal + -ic + acid

• The (aq) subscript indicates the compound is dissolved in water (aqueous phase).

Examples:

• HCl(aq): hydrochloric acid

• HBr(aq): hydrobromic acid

• H2S(aq): hydrosulfuric acid

• HF(aq): hydrofluoric acid

Oxyacids

Oxyacids are acids that contain hydrogen, oxygen, and another element (the central atom), usually as part of a polyatomic ion (oxyanion).

• General formula: HxXOy (where X is a nonmetal, often from a polyatomic ion)

• Examples of oxyanions: nitrate (NO3-), sulfate (SO42-), chlorate (ClO3-), etc.

Examples:

• HNO3(aq): one H+ ion and one NO3- ion (nitrate)

• H2SO3(aq): two H+ ions and one SO32- ion (sulfite)

• H2SO4(aq): two H+ ions and one SO42- ion (sulfate)

Naming Oxyacids

• If the oxyanion ends in -ate: acid name is base name of oxyanion + -ic acid

• If the oxyanion ends in -ite: acid name is base name of oxyanion + -ous acid

Examples:

• HNO3: nitric acid (from nitrate, NO3-)

• HNO2: nitrous acid (from nitrite, NO2-)

• H2SO4: sulfuric acid (from sulfate, SO42-)

• H2SO3: sulfurous acid (from sulfite, SO32-)

Table: Common Oxyacids and Their Oxyanions

Acid Formula	Acid Name	Oxyanion Name	Oxyanion Formula
HNO2	nitrous acid	nitrite	NO2-
HNO3	nitric acid	nitrate	NO3-
H2SO3	sulfurous acid	sulfite	SO32-
H2SO4	sulfuric acid	sulfate	SO42-
HClO2	chlorous acid	chlorite	ClO2-
HClO3	chloric acid	chlorate	ClO3-
HC2H3O2	acetic acid	acetate	C2H3O2-
H2CO3	carbonic acid	carbonate	CO32-

Calculating Formula Mass

Definition and Method

Formula mass (also called molecular mass or molecular weight) is the sum of the atomic masses of all atoms in a chemical formula. It is usually expressed in atomic mass units (amu).

• To calculate formula mass, multiply the atomic mass of each element by the number of atoms of that element in the formula, then sum the results.

Formula:

$\text{Formula mass} = \sum (\text{number of atoms of element} \times \text{atomic mass of element})$

Example: Calculate the formula mass of MgCl2:

• Mg: 1 atom × 24.31 amu = 24.31 amu

• Cl: 2 atoms × 35.45 amu = 70.90 amu

• Total formula mass = 24.31 amu + 70.90 amu = 95.21 amu

Another Example: Calculate the formula mass of H2SO4:

• H: 2 × 1.01 amu = 2.02 amu

• S: 1 × 32.07 amu = 32.07 amu

• O: 4 × 16.00 amu = 64.00 amu

• Total formula mass = 2.02 + 32.07 + 64.00 = 98.09 amu

Summary Table: Naming Flowchart for Compounds

The following flowchart can help determine the correct naming convention for a compound:

Type of Compound	Example	Naming Convention
Molecular	CO	carbon monoxide
Ionic – Type I	CaF2	calcium fluoride
Ionic – Type II	Fe(NO3)3	iron(III) nitrate
Acid – Binary	HF	hydrofluoric acid
Acid – Oxyacid (-ate)	HClO4	perchloric acid
Acid – Oxyacid (-ite)	H2SO3	sulfurous acid

Additional info: The notes reference a flowchart for nomenclature, which is a common tool in introductory chemistry to help students systematically name compounds based on their composition. The table above summarizes the main branches of such a flowchart.