General Chemistry Principles I: Gases, Stoichiometry, and Energetics (Fall 2025 Study Notes)

Gas Laws and Partial Pressure

Partial Pressure in Gas Mixtures

When a volatile substance evaporates in a closed container, it displaces some of the air, affecting the partial pressures of the remaining gases. The total pressure is the sum of the partial pressures of all gases present.

• Partial Pressure: The pressure exerted by a single gas in a mixture, calculated as its mole fraction times the total pressure.

• Mole Fraction: The ratio of moles of a component to the total moles in the mixture.

• Dalton's Law of Partial Pressures: $P_{total} = P_1 + P_2 + ... + P_n$

• Example Calculation: If a liquid fully evaporates in a closet of volume 2.2 m3, displacing air, and the closet is held at 1.2 atm, with air originally 20% O2 and 80% N2:

  • Let $P_{evap}$ be the pressure of the evaporated vapor.

  • Remaining air pressure: $P_{air} = (1 - x)P_{total}$, where $x$ is the fraction displaced.

  • Partial pressures: $P_{O_2} = 0.2 \times P_{air}$, $P_{N_2} = 0.8 \times P_{air}$

  • Check: $P_{O_2} + P_{N_2} + P_{evap} = P_{total}$

Additional info: This topic is foundational for understanding gas mixtures, vapor pressure, and applications in laboratory and atmospheric chemistry.

Humidity and Air Density

Humidity affects the density of air because water vapor is less dense than dry air. When water vapor replaces dry air, the overall mass per unit volume decreases.

• Dry Air Composition: Typically 75% N2, 19% O2 by mass.

• Water Vapor: Molar mass = 18 g/mol, compared to N2 (28 g/mol) and O2 (32 g/mol).

• Effect on Density: Adding water vapor (lower molar mass) to air reduces the mass density.

• Example: If mass density decreases by 6% when air is humidified, this is due to the replacement of heavier molecules with lighter water vapor.

Additional info: Understanding air density changes is important in meteorology and environmental science.

Stoichiometry and Precipitation Reactions

Stoichiometry in Precipitation

Stoichiometry allows us to calculate the amounts of reactants and products in chemical reactions. In precipitation reactions, insoluble products form and can be measured.

• Precipitation Reaction: When two solutions are mixed, an insoluble solid (precipitate) may form.

• Example Reaction:

  • $Hg^{2+}(aq) + 2NaI(aq) \rightarrow HgI_2(s) + 2Na^+(aq)$

  • $Ag^+(aq) + NaI(aq) \rightarrow AgI(s) + Na^+(aq)$

• Calculating Moles:

  • Volume of NaI solution: 0.100 L

  • Concentration: 1.22 M

  • Moles NaI added: $0.100 \times 1.22 = 0.122$ moles

• Mass Relationships: The total mass of precipitate (AgI and HgI2) is given; use stoichiometry to find individual masses.

Additional info: Stoichiometry is essential for quantitative analysis in chemistry, including titrations and gravimetric analysis.

Gas Reactions and Atmospheric Chemistry

Gas Phase Reactions in the Stratosphere

Gas reactions are important in atmospheric chemistry, such as the breakdown of ozone by chlorine-containing compounds.

• Example Reaction:

  • $CF_3Cl(g) + 2O_3(g) \rightarrow Cl(g) + 3O_2(g) + CF_3(g)$

• Calculating Moles and Pressures:

  • Given: 5 L container, 25.0 mm Hg, 15.0 g CF3Cl, temperature constant.

  • Moles CF3Cl: $\frac{15.0}{104.46} = 0.14359$ moles

  • Use Ideal Gas Law: $P = \frac{nRT}{V}$

  • Find initial and final moles of O3 and O2 after reaction.

• Application: These calculations help understand ozone depletion and the environmental impact of CFCs.

Additional info: Atmospheric gas reactions are central to environmental chemistry and climate science.

Empirical Formulas and Metal Oxides

Determining the Identity of a Metal in an Oxide

Empirical formulas represent the simplest whole-number ratio of elements in a compound. Metal oxides are commonly analyzed to determine the metal present.

• Empirical Formula: The simplest ratio of atoms in a compound, e.g., MO.

• Analysis: If the oxide contains a known mass of oxygen, use stoichiometry to find the molar mass and identify the metal.

• Example: If an oxide MO contains 2 g of oxygen, calculate the mass of the metal and compare to known atomic masses to identify it.

Additional info: This method is widely used in analytical chemistry to determine unknown compounds.

Tables

Stoichiometry Table: Precipitation Reaction

This table summarizes the relationships between reactants and products in the precipitation reaction involving NaI, Ag+, and Hg2+.

Reactant/Product	Moles Added	Mass (g)
NaI	0.122	Calculated from molar mass
AgI	To be determined	Part of 28.1 g total precipitate
HgI2	To be determined	Part of 28.1 g total precipitate

Additional info: The table helps organize stoichiometric calculations for quantitative analysis.