General Chemistry 115: Exam 1B Study Notes

General Chemistry Fundamentals

Unit Conversions and Useful Constants

Understanding unit conversions and key constants is essential for solving a wide range of chemistry problems, from stoichiometry to solution preparation.

• Density (D): The mass per unit volume of a substance. Formula: $D = \frac{\text{mass}}{\text{volume}}$

• Volume of a Sphere: The space occupied by a sphere. Formula: $\text{Vol (sphere)} = \frac{4}{3} \pi r^3$, where $\pi = 3.141592$

• Common Conversion Factors:

  • 1 mile = 5280 feet

  • 1 foot = 12 inches

  • 1 inch = 2.54 cm

  • 1 liter = 1.05669 quarts

  • 1 mile = 1.60934 km

  • 1000 mg = 1 g

  • 1 mL = 1 cm3

Nomenclature and Chemical Formulas

Elements, Ions, and Compounds: Naming and Symbolism

Chemists use systematic rules to name elements, ions, and compounds. Mastery of nomenclature is crucial for clear communication and understanding of chemical reactions.

• Element Symbols: Each element is represented by a unique one- or two-letter symbol (e.g., Na for sodium, Fe for iron).

• Common Polyatomic Ions: Groups of atoms with a net charge, such as SO42− (sulfate), PO43− (phosphate), and NO3− (nitrate).

• Compound Naming:

  • Ionic Compounds: Name the cation (positive ion) first, then the anion (negative ion). For transition metals, indicate the charge with Roman numerals (e.g., iron(III) chloride).

  • Covalent Compounds: Use prefixes (mono-, di-, tri-, etc.) to indicate the number of each atom (e.g., carbon dioxide, CO2).

  • Acids: Binary acids use the prefix 'hydro-' and the suffix '-ic' (e.g., HCl is hydrochloric acid). Oxyacids are named based on the polyatomic ion (e.g., H2SO4 is sulfuric acid).

• Examples:

  • Na2SO4: Sodium sulfate

  • HCO3−: Hydrogen carbonate (bicarbonate)

  • SrO: Strontium oxide

  • Ca(NO2)2: Calcium nitrite

Significant Figures

Counting Significant Figures

Significant figures reflect the precision of a measured or calculated quantity. Correctly identifying significant figures is essential for reporting scientific data.

• Rules for Counting Significant Figures:

  • All nonzero digits are significant.

  • Zeros between nonzero digits are significant.

  • Leading zeros (zeros before the first nonzero digit) are not significant.

  • Trailing zeros in a number with a decimal point are significant.

  • Trailing zeros in a whole number without a decimal point may or may not be significant (context-dependent).

• Examples:

  • 1.057 × 104 has 4 significant figures.

  • 0.00230122 has 6 significant figures.

  • 13.3 × 104 has 3 significant figures.

  • 0.00048 has 2 significant figures.

  • 499.100 has 6 significant figures.

Writing and Interpreting Chemical Formulas

Formulas for Compounds

Writing correct chemical formulas requires knowledge of ion charges and the rules of chemical nomenclature.

• Steps to Write a Formula:

  1. Identify the cation and anion (including their charges).

  2. Balance the total positive and negative charges to make a neutral compound.

  3. Use subscripts to indicate the number of each ion needed.

• Examples:

  • Tetraphosphorous decoxide: P4O10

  • Potassium sulfate: K2SO4

  • Barium chlorate: Ba(ClO3)2

  • Sulfur dioxide: SO2

  • Magnesium chloride: MgCl2

Solution Concentration: Molarity

Calculating Molarity

Molarity (M) is a measure of the concentration of a solute in a solution, defined as moles of solute per liter of solution.

• Formula: $M = \frac{\text{moles of solute}}{\text{liters of solution}}$

• Steps to Calculate Molarity:

  1. Convert the mass of solute to moles using the molar mass.

  2. Convert the volume of solution to liters if necessary.

  3. Divide the number of moles by the volume in liters.

• Example: To find the molarity of a solution with 26.0 g of BaCl2 (molar mass = 208.2 g/mol) dissolved in 450.0 mL of solution:

  • Calculate moles: $\frac{26.0\ \text{g}}{208.2\ \text{g/mol}} = 0.125\ \text{mol}$

  • Convert volume: $450.0\ \text{mL} = 0.4500\ \text{L}$

  • Calculate molarity: $M = \frac{0.125}{0.4500} = 0.278\ \text{M}$