General Chemistry Exam 1 Study Guide: Key Concepts and Skills

Significant Figures (Sig Figs)

Rules and Applications

Significant figures are the digits in a measurement that are known with certainty plus one estimated digit. They are crucial for expressing the precision of measurements and calculations in chemistry.

• The Rules:

  • All nonzero digits are significant.

  • Zeros between nonzero digits are significant.

  • Leading zeros are not significant.

  • Trailing zeros are significant only if there is a decimal point.

  • Zero Rules: Pay special attention to how zeros are counted in different contexts.

• Sig Figs in Mathematical Functions:

  • For multiplication/division: The result should have as many significant figures as the measurement with the fewest significant figures.

  • For addition/subtraction: The result should have as many decimal places as the measurement with the fewest decimal places.

• Example: $2.50 \times 3.1 = 7.8$ (rounded to two significant figures)

Unit Conversions

Temperature and Metric Prefixes

Unit conversions are essential for expressing measurements in different units and for solving chemical problems.

• Celsius to Kelvin:

  • Formula: $K = ^\circ C + 273.15$

  • Example: $25^\circ C = 25 + 273.15 = 298.15\ K$

• Metric Prefixes:

  • Common prefixes: kilo (k, $10^3$), milli (m, $10^{-3}$), centi (c, $10^{-2}$), nano (n, $10^{-9}$)

  • To convert: Multiply or divide by the appropriate power of ten.

  • Example: $1\ \text{kg} = 1000\ \text{g}$

Chemical and Physical Properties

Classification of Matter

Matter can be classified based on its composition and properties.

• Pure Element vs Compound:

  • Element: A substance made of only one type of atom (e.g., O2).

  • Compound: A substance made of two or more elements chemically combined (e.g., H2O).

• Homogeneous vs Heterogeneous Mixture:

  • Homogeneous: Uniform composition throughout (e.g., salt water).

  • Heterogeneous: Non-uniform composition (e.g., salad, sand in water).

• Physical Change vs Chemical Change/Reaction:

  • Physical Change: Alters the form but not the composition (e.g., melting ice).

  • Chemical Change: Produces new substances (e.g., rusting iron).

Density and Specific Gravity

Calculations and Applications

Density is a fundamental property used to identify substances and relate mass and volume.

• How to Calculate Density:

  • Formula: $\text{Density} = \frac{\text{Mass}}{\text{Volume}}$

  • Example: $\frac{10\ g}{2\ mL} = 5\ g/mL$

• Identifying Substances by Density:

  • Compare calculated density to known values to identify a substance.

• Density and Specific Gravity:

  • Specific gravity is the ratio of the density of a substance to the density of water (at 4°C, $1\ g/mL$).

  • Formula: $\text{Specific Gravity} = \frac{\text{Density of Substance}}{\text{Density of Water}}$

• Effect of Size/Weight:

  • Density and specific gravity are independent of the size or weight of the sample.

Ionic Compounds

Symbols, Naming, and Molar Mass

Ionic compounds are formed from the electrostatic attraction between cations and anions.

• Recognizing Atomic and Ionic Symbols:

  • Atomic symbols: One or two letters (e.g., Na, Cl).

  • Ionic symbols: Show charge (e.g., Na+, Cl-).

• Writing and Naming Ionic Compounds:

  • Combine cation and anion to form a neutral compound (e.g., Na+ + Cl- → NaCl).

  • Name: Cation name + anion name (e.g., sodium chloride).

• Calculating Molar Mass:

  • Add the atomic masses of all atoms in the formula.

  • Example: NaCl: $22.99\ g/mol + 35.45\ g/mol = 58.44\ g/mol$

Identifying Ionic and Covalent Bonds

Bond Types and Identification

Chemical bonds can be classified as ionic or covalent based on the nature of the atoms involved.

• Ionic Compounds: Formed between metals and nonmetals; involve transfer of electrons.

• Covalent Compounds: Formed between nonmetals; involve sharing of electrons.

• Identification:

  • If a compound contains a metal and a nonmetal, it is likely ionic.

  • If a compound contains only nonmetals, it is likely covalent.

Valence Electrons

Counting and Importance

Valence electrons are the outermost electrons of an atom and determine its chemical reactivity.

• How Many Valence Electrons Does Each Element Want?

  • Most elements seek to have 8 valence electrons (octet rule), except hydrogen and helium (2 electrons).

• How to Identify Valence Electrons:

  • Group number in the periodic table indicates the number of valence electrons for main group elements.

  • Example: Oxygen (Group 16) has 6 valence electrons.

Nuclear Chemistry

Particles, Equations, and Dosage

Nuclear chemistry involves changes in the nucleus, including radioactive decay and nuclear reactions.

• Recognizing Particles:

  • Alpha particle: $^4_2He$

  • Beta particle: $^0_{-1}e$

  • Positron: $^0_{+1}e$

• Balancing Nuclear Equations:

  • Sum of atomic numbers and mass numbers must be equal on both sides.

• Determining Protons, Neutrons, and Mass Number:

  • Protons = atomic number

  • Neutrons = mass number - atomic number

• Dosage Unit Conversions:

  • Convert between units such as curies, becquerels, grays, and sieverts as needed.

• Amount Left After Half-Lives:

  • Formula: $\text{Amount remaining} = \text{Initial amount} \times (\frac{1}{2})^{n}$, where $n$ is the number of half-lives.

Covalent Bonding

Lewis Structures, Polarity, and Naming

Covalent bonding involves the sharing of electrons between nonmetal atoms.

• Drawing Lewis Structures:

  • Show all valence electrons as dots; connect atoms with shared pairs (lines).

• Identifying Molecular Shape:

  • Use VSEPR theory to predict shapes (e.g., linear, bent, tetrahedral).

• Polarity of Bonds and Molecules:

  • A bond is polar if there is a significant difference in electronegativity between atoms.

  • A molecule is polar if it has polar bonds arranged asymmetrically.

• Naming Covalent Compounds:

  • Use prefixes (mono-, di-, tri-, etc.) to indicate the number of each atom.

  • Example: CO2 is carbon dioxide.

• Determining Molar Mass:

  • Add atomic masses of all atoms in the molecule.

Molecular Chemistry

Balancing Equations and Mole Calculations

Molecular chemistry focuses on the composition and reactions of molecules.

• Balancing Chemical Equations:

  • Ensure the same number of each atom on both sides of the equation.

• Converting Between Moles and Grams:

  • Use molar mass as a conversion factor.

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

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