Chemistry 101: Introduction to Chemistry – Study Guide and Practice Exam Notes

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)

Compound	Solubility in Water
LiCl	Soluble
NaBr	Soluble
KF	Soluble
NH4Cl	Soluble
AgCl	Insoluble
CaSO4	Slightly Soluble
Fe(OH)3	Insoluble
KOH	Soluble

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: $\mathrm{NaOH} (aq) + \mathrm{HCl} (aq) \rightarrow \mathrm{NaCl} (aq) + \mathrm{H_2O} (l)$

  • Complete Ionic: $\mathrm{Na^+} (aq) + \mathrm{OH^-} (aq) + \mathrm{H^+} (aq) + \mathrm{Cl^-} (aq) \rightarrow \mathrm{Na^+} (aq) + \mathrm{Cl^-} (aq) + \mathrm{H_2O} (l)$

  • Net Ionic: $\mathrm{OH^-} (aq) + \mathrm{H^+} (aq) \rightarrow \mathrm{H_2O} (l)$

Balancing Chemical Equations

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

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

  • $\mathrm{FeCl_3} (aq) + 3\mathrm{KOH} (aq) \rightarrow \mathrm{Fe(OH)_3} (s) + 3\mathrm{KCl} (aq)$

Mole Calculations and Molar Mass

Mole Concept

The mole is a counting unit in chemistry, representing $6.022 \times 10^{23}$ entities (Avogadro's number).

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

• Example: Water (H2O) molar mass: $2 \times 1.008 + 16.00 = 18.02$ g/mol.

Calculating Moles and Molecules

• Formula: $\text{Moles} = \frac{\text{Mass (g)}}{\text{Molar Mass (g/mol)}}$

• Number of Molecules: $\text{Number of molecules} = \text{Moles} \times 6.022 \times 10^{23}$

• Example: 50.0 g of isopropyl alcohol ($\mathrm{C_3H_8O}$), molar mass = 60.10 g/mol:

  • Moles: $\frac{50.0}{60.10} = 0.832$ mol

  • Molecules: $0.832 \times 6.022 \times 10^{23} = 5.01 \times 10^{23}$ 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: $\frac{40.00}{12.01} = 3.33$ mol

  • H: $\frac{6.71}{1.008} = 6.66$ mol

  • O: $\frac{53.29}{16.00} = 3.33$ mol

  • Empirical formula: C1H2O1

Molecular Formula

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

• Formula: $n = \frac{\text{Molar Mass}}{\text{Empirical Formula Mass}}$

• Example: If empirical formula mass is 30.03 g/mol and molar mass is 180.18 g/mol, $n = 6$, 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: $\text{Percent} = \frac{\text{Mass of component}}{\text{Total mass}} \times 100\%$

• Example: In a 200 g sample with 10.5 g sulfur-rich compounds: $\frac{10.5}{200} \times 100\% = 5.25\%$

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): $12.01 + 4 \times 35.45 = 153.81$ amu

Summary Table: Types of Compounds

Type	Example	Key Features
Molecular Element	O2, N2	Same element, bonded as molecules
Molecular Compound	CO2, H2O	Nonmetals bonded together
Ionic Compound	NaCl, KNO2	Metal and nonmetal ions

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