Chemical Laws and Composition

Law of Constant Composition

The Law of Constant Composition states that all samples of a given compound have the same proportions of their constituent elements, regardless of the source or method of preparation.

• Key Point: The ratio of elements in a compound is always fixed.

• Example: Water (H2O) always contains hydrogen and oxygen in a 2:1 ratio by atoms.

Naming and Identifying Compounds

Naming Ionic and Molecular Compounds

Chemical compounds are named according to systematic rules that reflect their composition and structure.

• Ionic Compounds: Consist of cations (positive ions) and anions (negative ions). The cation is named first, followed by the anion.

• Molecular Compounds: Consist of nonmetals bonded together. Prefixes indicate the number of each atom.

• Examples:

  • Ca(NO3)2: Calcium Nitrate

  • Zn3N2: Zinc Nitride

  • KF: Potassium Fluoride

  • CrO: Chromium(II) Oxide

  • KNO2: Potassium Nitrite

  • Fe(OH)2: Iron(II) Hydroxide

  • HCl(aq): Hydrochloric Acid

  • H2SO4(aq): Sulfuric Acid

  • HC2H3O2: Acetate is the oxyanion

Types of Substances

• Molecular Element: An element that exists as molecules in nature (e.g., O2, N2).

• Molecular Compound: A compound formed from nonmetals (e.g., CO2).

• Ionic Compound: A compound formed from metals and nonmetals (e.g., NaCl, KNO2).

Chemical Formulas and Counting Atoms

Determining Number of Atoms in a Formula

The total number of atoms in a chemical formula is found by summing the subscripts of each element.

• Example: Al(C2H3O2)3 contains 1 Al, 6 C, 9 H, and 6 O atoms, totaling 22 atoms.

Physical States and Solubility

States of Matter

Substances can exist in different physical states: solid, liquid, gas, or aqueous (dissolved in water).

• Key Point: Chemical reactions may involve changes in state.

Solubility

Solubility describes the ability of a substance to dissolve in a solvent, such as water.

• Soluble: Dissolves in the solvent.

• Insoluble: Does not dissolve.

• Example: KOH is soluble in water; AgCl is insoluble.

Solubility Table (Inferred)

Acids and Bases

Strong and Weak Acids/Bases

Acids and bases are classified by their ability to dissociate in water.

• Strong Acids: Completely dissociate (e.g., HCl, HBr, HI).

• Weak Acids: Partially dissociate (e.g., HF).

• Strong Bases: Completely dissociate (e.g., NaOH).

• Weak Bases: Partially dissociate (e.g., NH3).

Chemical Reactions and Equations

Evidence of Chemical Reactions

Chemical reactions can be identified by observable changes.

• Precipitation: Formation of a solid from solution.

• Gas Evolution: Formation of a gas.

• Temperature Change: Exothermic or endothermic reactions.

• Color Change: Indicates new substances formed.

Writing Chemical Equations

Chemical equations represent the reactants and products in a reaction.

• Molecular Equation: Shows all reactants and products as compounds.

• Complete Ionic Equation: Shows all strong electrolytes as ions.

• Net Ionic Equation: Shows only the species that change during the reaction.

• Example: Neutralization of NaOH and HCl:

  • Molecular:

  • Complete Ionic:

  • Net Ionic:

Balancing Chemical Equations

Balanced equations have equal numbers of each atom on both sides.

• Example: Iron(III) chloride reacts with potassium hydroxide:

Mole Calculations and Molar Mass

Mole Concept

The mole is a counting unit in chemistry, representing entities (Avogadro's number).

• Molar Mass: The mass of one mole of a substance, in g/mol.

• Example: Water (H2O) molar mass: g/mol.

Calculating Moles and Molecules

• Formula:

• Number of Molecules:

• Example: 50.0 g of isopropyl alcohol (), molar mass = 60.10 g/mol:

  • Moles: mol

  • Molecules: molecules

Empirical and Molecular Formulas

Empirical Formula

The empirical formula shows the simplest whole-number ratio of atoms in a compound.

• Calculation Steps:

  1. Convert mass percentages to grams (assume 100 g sample).

  2. Convert grams to moles for each element.

  3. Divide by the smallest number of moles to get ratios.

• Example: For a compound with 40.00% C, 6.71% H, 53.29% O:

  • C: mol

  • H: mol

  • O: mol

  • Empirical formula: C1H2O1

Molecular Formula

The molecular formula is a multiple of the empirical formula, determined by the molar mass.

• Formula:

• Example: If empirical formula mass is 30.03 g/mol and molar mass is 180.18 g/mol, , so molecular formula is C6H12O6 (glucose).

Percent Composition

Calculating Percent Composition

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

• Formula:

• Example: In a 200 g sample with 10.5 g sulfur-rich compounds:

Atomic and Formula Mass

Calculating Formula Mass

The formula mass is the sum of the atomic masses of all atoms in a chemical formula.

• Example: Carbon tetrachloride (CCl4): amu

Summary Table: Types of Compounds

Additional info: Some context and examples have been expanded for clarity and completeness, including inferred solubility rules and stepwise calculations for empirical/molecular formulas.