CHEM 121 Principles of Chemistry: Practice Midterm II Study Guide

Practice Midterm II: Key General Chemistry Topics

Significant Figures and Measurement

Understanding significant figures is essential for reporting scientific measurements accurately. Significant figures reflect the precision of a measured or calculated quantity.

• Definition: Significant figures are the digits in a number that carry meaning contributing to its measurement resolution.

• Rules:

  • All nonzero digits are significant.

  • Zeros between nonzero digits are significant.

  • Leading zeros are not significant.

  • Trailing zeros in a decimal number are significant.

• Example: 0.00450 has three significant figures.

Density and Unit Conversions

Density is a fundamental property defined as mass per unit volume. Unit conversions are frequently required in chemistry calculations.

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

• Example: To find the volume needed for a given mass and density, rearrange the formula: $\text{Volume} = \frac{\text{Mass}}{\text{Density}}$

• Unit Conversions: Use conversion factors to change units (e.g., grams to kilograms, Celsius to Fahrenheit).

• Example: Convert 25 nm to meters: $25\ \text{nm} \times \frac{1\ \text{m}}{1 \times 10^9\ \text{nm}} = 2.5 \times 10^{-8}\ \text{m}$

Temperature Conversions

Temperature can be measured in Celsius, Fahrenheit, or Kelvin. Converting between these scales is a common task.

• Formulas:

  • Celsius to Fahrenheit: $F = \frac{9}{5}C + 32$

  • Fahrenheit to Celsius: $C = \frac{5}{9}(F - 32)$

  • Celsius to Kelvin: $K = C + 273.15$

• Example: Convert 104°F to Celsius: $C = \frac{5}{9}(104 - 32) = 40^\circ C$

Precision and Accuracy

Precision and accuracy are important concepts in measurement.

• Accuracy: How close a measurement is to the true value.

• Precision: How close repeated measurements are to each other.

• Example: A set of darts clustered together but far from the bullseye is precise but not accurate.

Atomic Structure and the Periodic Table

The periodic table organizes elements by increasing atomic number and similar chemical properties.

• Groups: Vertical columns; elements in the same group have similar properties.

• Periods: Horizontal rows.

• Alkali Metals: Group 1 elements, highly reactive.

• Alkaline Earth Metals: Group 2 elements.

• Transition Metals: Groups 3-12, often form colored compounds.

• Metals, Nonmetals, Semimetals: Classified by physical and chemical properties.

• Example Table:

Type	Properties
Metal	Conducts electricity, malleable, ductile
Nonmetal	Poor conductor, brittle, not ductile
Semimetal	Intermediate properties

Atomic Mass and Isotopes

Atomic mass is the weighted average of all naturally occurring isotopes of an element.

• Isotope: Atoms of the same element with different numbers of neutrons.

• Calculation: $\text{Average Atomic Mass} = \sum (\text{Fractional Abundance} \times \text{Isotope Mass})$

Quantum Numbers and Electron Configuration

Quantum numbers describe the properties of atomic orbitals and the electrons in them.

• Principal Quantum Number (n): Energy level (n = 1, 2, 3, ...)

• Angular Momentum Quantum Number (l): Shape of orbital (l = 0 to n-1)

• Magnetic Quantum Number (ml): Orientation (ml = -l to +l)

• Spin Quantum Number (ms): Electron spin (+1/2 or -1/2)

• Example: The set n = 2, l = 1, ml = 0, ms = +1/2 is valid.

Shorthand Electron Configuration

• Definition: Uses noble gas core to simplify electron configuration notation.

• Example: Fe (atomic number 26): [Ar] 4s2 3d6

Periodic Trends

Periodic trends describe how certain properties of elements change across the periodic table.

• Atomic Radius: Increases down a group, decreases across a period.

• Effective Nuclear Charge (Zeff): The net positive charge experienced by valence electrons.

• Order of Increasing Zeff: Na < O < F < Ne

Electromagnetic Spectrum

The electromagnetic spectrum includes all types of electromagnetic radiation, ordered by wavelength or frequency.

• Order of Increasing Wavelength: Ultraviolet < Visible < Infrared < Microwave

• Example Table:

Type	Wavelength Range
Ultraviolet	10 nm – 400 nm
Visible	400 nm – 700 nm
Infrared	700 nm – 1 mm
Microwave	1 mm – 1 m

Stoichiometry and Chemical Calculations

Stoichiometry involves calculations based on the relationships between reactants and products in chemical reactions.

• Example: Calculating the mass of an element in a compound, or the area covered by a volume of liquid.

• General Steps:

  1. Write the balanced chemical equation.

  2. Convert given quantities to moles.

  3. Use mole ratios to find the desired quantity.

  4. Convert back to required units.

Applications and Problem Solving

Many questions require applying concepts to solve real-world problems, such as converting units, determining the number of significant figures, or calculating the area covered by a liquid spill.

• Example: If a film of gasoline 1.0 mm thick covers 10 barrels (42 gallons each), calculate the area covered.

• Approach: Convert gallons to cubic meters, then use the formula: $\text{Area} = \frac{\text{Volume}}{\text{Thickness}}$

Additional info: These notes are based on a practice midterm exam and cover foundational topics in General Chemistry, including measurement, atomic structure, periodic trends, and problem-solving techniques.