Writing and Predicting Chemical Formulas, Stoichiometry, and Nomenclature: Study Notes for Introduction to Chemistry

Writing and Predicting Chemical Formulas

Formulas of Binary Ionic Compounds

Binary ionic compounds are formed from the combination of metal cations and nonmetal anions. The chemical formula represents the simplest ratio of ions that results in a neutral compound.

• Cation: A positively charged ion, usually a metal.

• Anion: A negatively charged ion, usually a nonmetal.

• Formulas are written by balancing the total positive and negative charges.

• Example: Magnesium chloride is formed from Mg2+ and Cl- ions. The formula is MgCl2.

Common Ion Charges by Group

The charge of ions can often be predicted by their group in the periodic table:

Group	Common Ion	Charge
IA	Na+, K+	+1
IIA	Mg2+, Ca2+	+2
IIIA	Al3+	+3
VA	N3-	-3
VIA	O2-, S2-	-2
VIIA	F-, Cl-	-1

Additional info: Transition metals may have variable charges, indicated by Roman numerals in their names (e.g., Fe2+ is iron(II)).

Formulas of Compounds with Polyatomic Ions

Polyatomic ions are groups of atoms that carry a charge and act as a single unit in chemical reactions. Compounds containing polyatomic ions are written by combining the cation and the polyatomic anion in ratios that balance the overall charge.

• Examples of polyatomic ions: NO3- (nitrate), SO42- (sulfate), NH4+ (ammonium).

• Example: Calcium nitrate is Ca(NO3)2.

Ion	Formula	Charge
Nitrate	NO3-	-1
Sulfate	SO42-	-2
Phosphate	PO43-	-3
Ammonium	NH4+	+1

Nomenclature of Ionic and Covalent Compounds

Naming Ionic Compounds

Ionic compounds are named by stating the cation first, followed by the anion. For transition metals, the charge is indicated in parentheses using Roman numerals.

• NaCl: Sodium chloride

• FeCl2: Iron(II) chloride

• FeCl3: Iron(III) chloride

Naming Covalent Compounds

Covalent (molecular) compounds are named using prefixes to indicate the number of atoms of each element.

• CO: Carbon monoxide

• CO2: Carbon dioxide

• N2O4: Dinitrogen tetroxide

Prefix	Number
Mono-	1
Di-	2
Tri-	3
Tetra-	4
Penta-	5
Hexa-	6

Stoichiometry and Chemical Calculations

The Mole and Avogadro's Number

The mole is a fundamental unit in chemistry that represents $6.022 \times 10^{23}$ particles (atoms, molecules, or ions). It allows chemists to count entities by weighing them.

• Avogadro's Number: $6.022 \times 10^{23}$ particles/mol

• Molar Mass (MM): The mass of one mole of a substance, expressed in grams per mole (g/mol).

Calculating Moles, Mass, and Number of Particles

• To convert between mass and moles: $\text{moles} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}}$

• To convert between moles and number of particles: $\text{number of particles} = \text{moles} \times 6.022 \times 10^{23}$

• To convert between mass and number of particles: $\text{number of particles} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}} \times 6.022 \times 10^{23}$

Formula Mass and Molecular Mass

The formula mass is the sum of the atomic masses of all atoms in a formula unit of an ionic compound. The molecular mass is the sum for a molecule.

• Example: For NaCl, $\text{formula mass} = 22.99 + 35.45 = 58.44$ g/mol

• Example: For H2O, $\text{molecular mass} = 2 \times 1.01 + 16.00 = 18.02$ g/mol

Empirical and Molecular Formulas

Empirical Formula Determination

The empirical formula shows the simplest whole-number ratio of atoms in a compound. It can be determined from percent composition data.

• Convert percent composition to grams (assuming 100 g sample).

• Convert grams to moles for each element.

• Divide by the smallest number of moles to get the simplest ratio.

Molecular Formula Determination

The molecular formula shows the actual number of atoms of each element in a molecule. It is a whole-number multiple of the empirical formula.

• Calculate the empirical formula mass.

• Divide the molar mass by the empirical formula mass to find the multiple.

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

Percent Composition and Chemical Analysis

Percent Composition

Percent composition expresses the mass percentage of each element in a compound.

• $\%\,\text{element} = \frac{\text{mass of element in 1 mol of compound}}{\text{molar mass of compound}} \times 100$

• Example: In H2O, $\%\,\text{H} = \frac{2.02}{18.02} \times 100 = 11.2\%$

Polyatomic Ions and Acid Nomenclature

Common Polyatomic Ions

Name	Formula	Charge
Nitrate	NO3-	-1
Sulfate	SO42-	-2
Phosphate	PO43-	-3
Hydroxide	OH-	-1
Carbonate	CO32-	-2

Naming Acids

• Acids containing anions ending in -ide use the prefix hydro- and the suffix -ic (e.g., HCl: hydrochloric acid).

• Acids containing anions ending in -ate use the suffix -ic (e.g., HNO3: nitric acid).

• Acids containing anions ending in -ite use the suffix -ous (e.g., HNO2: nitrous acid).

Stoichiometric Calculations

Law of Conservation of Mass

In chemical reactions, mass is conserved. The total mass of reactants equals the total mass of products.

• Example: $\text{mass of reactants} = \text{mass of products}$

Stoichiometric Relationships

• Use balanced chemical equations to relate moles of reactants and products.

• Example: $2H_2 + O_2 \rightarrow 2H_2O$

• From the equation, 2 moles of H2 react with 1 mole of O2 to produce 2 moles of H2O.

Limiting Reactant and Percent Yield

• Limiting Reactant: The reactant that is completely consumed first, limiting the amount of product formed.

• Theoretical Yield: The maximum amount of product that can be formed from the limiting reactant.

• Percent Yield: $\%\,\text{yield} = \frac{\text{actual yield}}{\text{theoretical yield}} \times 100$

Summary Table: Key Polyatomic Ions

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

Practice Problems and Worked Examples

• Write the formula for a compound formed from Ca2+ and NO3-: Ca(NO3)2

• Calculate the number of moles in 18 g of H2O: $\text{moles} = \frac{18}{18.02} = 1.00$ mol

• Determine the percent composition of C in C2H6: $\%\,C = \frac{2 \times 12.01}{30.08} \times 100 = 79.8\%$

Additional info: These notes cover essential topics for introductory chemistry, including chemical formulas, nomenclature, stoichiometry, and percent composition, with tables and examples for clarity.