Molar Mass, Empirical Formulas, and Nomenclature of Compounds

Molar Mass of Compounds

Definition and Calculation

The molar mass of a compound is the mass of exactly one mole of that compound, expressed in units of grams per mole (g/mol). This value numerically matches the sum of the atomic masses of the constituent atoms, but the units differ.

• Molar mass formula:

$\text{Molar mass} = \text{mass of one mole of compound (in g/mol)}$

• Example: To find the molar mass of calcium nitrate, Ca(NO3)2, sum the atomic masses of all atoms in the formula.

Using Molar Mass to Count Molecules and Atoms

Conversions Involving Moles, Molecules, and Atoms

Molar mass, combined with Avogadro's number, allows chemists to determine the number of molecules or atoms in a given mass of a substance.

• Convert mass to moles using molar mass.

• Convert moles to molecules using Avogadro's number ($6.02 \times 10^{23}$ molecules/mol).

• Convert molecules to atoms by considering the number of each atom per molecule.

General conversion sequence:

$\text{grams} \rightarrow \text{moles} \rightarrow \text{molecules} \rightarrow \text{atoms}$

Example: Find the number of oxygen atoms in 0.350 mol CO2.

• Each CO2 molecule contains 2 oxygen atoms.

• Number of O atoms = $0.350 \text{ mol CO}_2 \times \frac{6.02 \times 10^{23} \text{ molecules}}{1 \text{ mol}} \times \frac{2 \text{ O atoms}}{1 \text{ molecule}}$

Example: How many oxygen atoms are in 2.00 g of H2CO3?

Mass Percentage Calculations

Determining Mass Percent Composition

The mass percent of each element in a compound can be determined from the chemical formula or from experimental mass analysis.

• Formula for mass percent:

$\text{Mass \% of Element X} = \frac{\text{mass of 1 mol of element X in the compound}}{\text{mass of 1 mol of the compound}} \times 100\%$

• Percentages may not always total exactly 100% due to rounding.

Example: Calculate the mass percent of oxygen in acetic acid (C2H4O2):

$\text{Mass \% of Oxygen} = \frac{32}{60} \times 100\% = 53.33\%$

Application of Mass Percent

Example: The recommended daily allowance of iodine is 150 μg. If ingested as potassium iodide (KI), how much KI should be consumed?

• Calculate mass percent of iodine in KI:

$\text{Mass \% Iodine} = \frac{126.90 \text{ g/mol}}{166.00 \text{ g/mol}} \times 100\% = 76.45\%$

• Amount of KI needed: $150 \mu g = (\mu g \text{ KI}) \times 0.7645 \implies 196 \mu g$ KI must be eaten.

Empirical and Molecular Formulas

Empirical Formula

The empirical formula is the simplest, whole-number ratio of each element in a compound. It can be determined from elemental analysis or percent composition data.

• Represents a ratio of atoms or moles of atoms.

Steps to Find an Empirical Formula

1. If given percentages, convert to grams (assume 100 g sample).

2. Convert grams to moles for each element using molar mass.

3. Divide each mole amount by the smallest number of moles calculated.

4. Adjust ratios to whole numbers by multiplying as needed.

Example: A compound produces 165 g C, 27.8 g H, and 220.2 g O. Calculate the empirical formula.

Example: Ibuprofen has C 75.69%, H 8.80%, O 15.51%. Find the empirical formula.

Molecular Formula

The molecular formula is a multiple of the empirical formula and matches the compound's actual molar mass.

• Find the empirical formula mass.

• Divide the molecular mass by the empirical formula mass to find the multiplier.

• Multiply the subscripts in the empirical formula by this number.

Example: A compound with molecular weight 58 g/mol, 82.76% C, remainder H. Empirical formula = CH2, empirical mass = 29 g/mol, so molecular formula = C2H4.

Ionic Compounds: Formulas and Names

Charges of Ions

• Main group elements: charge predicted from group number, form only one type of ion.

• Transition metals: may form multiple types of ions; charge must be specified.

Naming Type I Ionic Compounds

• Type I: Metal forms only one type of ion (fixed charge).

• Binary compounds: name of cation (metal) + base name of anion (nonmetal) + -ide.

• Examples: KCl = potassium chloride, CaO = calcium oxide.

Metal	Ion	Group Number
Li	Li+	1A
Na	Na+	1A
K	K+	1A
Mg	Mg2+	2A
Ca	Ca2+	2A

Nonmetal	Symbol	Base Name	Anion Name
Fluorine	F-	fluor	Fluoride
Chlorine	Cl-	chlor	Chloride
Bromine	Br-	brom	Bromide
Iodine	I-	iod	Iodide

Naming Type II Ionic Compounds

• Type II: Metal forms more than one type of ion (usually transition metals).

• Name: cation name (metal) + charge in Roman numerals in parentheses + base name of anion + -ide.

• Examples: FeCl2 = iron(II) chloride, FeCl3 = iron(III) chloride, CrBr3 = chromium(III) bromide.

Metal	Ion	Older Name
Iron	Fe2+	Ferrous
Iron	Fe3+	Ferric
Copper	Cu+	Cuprous
Copper	Cu2+	Cupric

Polyatomic Ions

Definition and Examples

Polyatomic ions are ions composed of multiple covalently bonded atoms with an overall charge. They commonly combine with metals from groups 1A and 2A to form ionic compounds.

• NO3-: Nitrate ion

• CO32-: Carbonate ion

• PO43-: Phosphate ion

Polyatomic Formula	Name	Polyatomic Charge
NH4+	Ammonium ion	+1
OH-	Hydroxide ion	-1
CH3COO-	Acetate	-1
NO3-	Nitrate ion	-1
CO32-	Carbonate ion	-2
SO42-	Sulfate ion	-2
PO43-	Phosphate ion	-3

Naming Ionic Compounds with Polyatomic Ions

• Name the cation first, then the polyatomic ion.

• Example: NaNO3 is sodium nitrate.

Molecular Compounds: Formulas and Names

General Rules

• Molecular compounds are composed of two or more nonmetals.

• The formula cannot always be predicted from the elements alone, as multiple compounds may form from the same elements.

• Prefixes indicate the number of each atom present.

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

• If only one atom of the first element, the prefix mono- is omitted.

• Examples: CO = carbon monoxide, CO2 = carbon dioxide, N2O5 = dinitrogen pentoxide.

Acids: Formulas and Names

Types of Acids

• Binary acids: Contain H+ and a nonmetal anion.

• Oxyacids: Contain H+ and a polyatomic anion (usually containing oxygen).

Naming Binary Acids

• Add the prefix hydro- to the base name of the nonmetal, change the ending to -ic, and add the word acid.

• Example: HCl (aq) = hydrochloric acid.

Naming Oxyacids

• If the polyatomic ion ends in -ate, the acid name ends with -ic acid.

• Example: HNO3 (nitrate) = nitric acid.

• If the polyatomic ion ends in -ite, the acid name ends with -ous acid.

• Example: HNO2 (nitrite) = nitrous acid.

Summary Table: Inorganic Nomenclature Flow Chart

The flow chart summarizes the rules for naming ionic, molecular, and acid compounds, guiding the user through the decision process based on the types of elements and ions present.

Additional info: For more practice, see: Compound Naming Practice.