Gas Laws and Properties

5-1 Atmospheric Pressure

Atmospheric pressure is the force exerted by gas molecules in the Earth's atmosphere as they strike surfaces. It is a crucial concept in understanding gas behavior under typical conditions.

• Atmosphere: The layer of gases surrounding Earth, supporting life and shielding from harmful radiation.

• Pressure: The force exerted by gas molecules on their surroundings.

• Barometer: A device invented by Evangelista Torricelli to measure atmospheric pressure. At sea level, standard pressure is 760 mm Hg.

Factors Affecting Barometric Pressure:

• Altitude: Higher altitudes have lower atmospheric pressure due to fewer air molecules.

• Weather: Changes in weather can alter atmospheric pressure.

5-2 Units of Pressure

Pressure can be measured in several units, which are often used interchangeably in chemistry problems.

• Common Units: mm Hg (millimeters of mercury), torr, atm (atmospheres), Pa (pascals), psi (pounds per square inch).

• Standard Pressure: 1 atm = 760 mm Hg = 760 torr = 101,325 Pa = 14.7 psi

Example: Convert 49 torr to other units:

Note: All these values represent the same pressure in different units.

5-3 Gas Laws En Route to the Ideal Gas Law

Three fundamental gas laws describe the relationships between pressure, volume, temperature, and amount of gas. These laws are combined to form the Ideal Gas Law.

Boyle's Law

Describes the relationship between pressure and volume at constant temperature.

• Formula:

• Relationship: Pressure and volume are inversely proportional.

• Example: Squeezing a balloon decreases its volume, increasing pressure.

Charles' Law

Describes the relationship between volume and temperature at constant pressure.

• Formula:

• Relationship: Volume and temperature (in Kelvin) are directly proportional.

• Example: Heating a balloon causes it to expand.

Avogadro's Law

Describes the relationship between volume and amount of gas (in moles) at constant temperature and pressure.

• Formula:

• Relationship: Volume and moles are directly proportional.

• Example: More moles of gas occupy more volume.

Combined Gas Law

Combines Boyle's, Charles', and Avogadro's laws to relate pressure, volume, temperature, and moles.

• Formula:

• Useful for problems involving changes in multiple variables.

5-4 The Ideal Gas Law

The Ideal Gas Law provides a single equation relating pressure, volume, temperature, and amount of gas.

• Formula:

• Variables: P = pressure (atm), V = volume (L), n = moles, R = universal gas constant, T = temperature (K)

• Value of R:

Example: Calculate moles of H2 gas in 8.56 L at 0°C and 1.58 atm:

• Convert temperature: 0°C + 273 = 273 K

• Use

5-5 Gas Stoichiometry

Gas stoichiometry involves using the Ideal Gas Law and molar relationships to solve chemical reaction problems involving gases.

• Standard Temperature and Pressure (STP): 0°C (273 K) and 1 atm; 1 mole of gas occupies 22.4 L at STP.

• Example: If 0.50 mol of O2 is converted to O3 at 1 atm and 25°C, calculate the volume of O3 produced.

5-6 Solving Gas Law Problems

Gas law problems may involve one or more variables changing. Identify which law applies and use appropriate units.

• Convert all temperatures to Kelvin.

• Use correct units for pressure, volume, and R.

• For combined gas law problems, set up ratios for initial and final conditions.

Example: A sample of methane gas (CH4) with a volume of 38 mL at 5°C is heated to 86°C at constant pressure. Calculate its new volume.

• Convert temperatures to Kelvin: 5°C = 278 K, 86°C = 359 K

• Use Charles' Law:

• Calculate:

Summary Table: Gas Laws

Tip: PV = nRT problems usually involve only one situation, not a before/after scenario.

Additional info: The notes also reference Dalton's Law of Partial Pressures, Kinetic Molecular Theory, and Real Gases, which are important extensions of the gas laws but not covered in detail in the provided material.