Gas Laws and Atmospheric Pressure

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 chemistry, affecting many physical and chemical processes.

• Atmosphere: The layer of gases surrounding Earth, supporting life, acting as a waste receptacle for exhaust gases, 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 atmospheric pressure pushes mercury up to 760 mm in a barometer tube.

• Standard Pressure: 760 mm Hg (also called 1 atm).

• Factors Affecting Atmospheric Pressure: Altitude and weather conditions can change atmospheric pressure.

Units of Pressure

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

• Common Units: mm Hg, torr, atm, Pa, psi

• Conversion Factors:

• Example: To convert 49 torr to other units:

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

Gas Laws En Route to the Ideal Gas Law

Several fundamental gas laws describe the relationships between pressure, volume, temperature, and amount of gas. These laws are the foundation for the Ideal Gas Law.

Boyle's Law

Boyle's Law describes the inverse relationship between the pressure and volume of a gas at constant temperature.

• Formula:

• Key Point: As pressure increases, volume decreases (and vice versa), provided temperature is constant.

• Example: Squeezing a balloon decreases its volume, so pressure and volume are inversely proportional.

Charles' Law

Charles' Law states that the volume of a gas is directly proportional to its temperature (in Kelvin) at constant pressure.

• Formula:

• Key Point: As temperature increases, volume increases (directly proportional).

• Example: Heating a balloon causes it to expand.

Avogadro's Law

Avogadro's Law relates the volume of a gas to the number of moles at constant temperature and pressure.

• Formula:

• Key Point: More moles of gas occupy more volume (directly proportional).

• Example: If you double the amount of gas in a container, the volume doubles (at constant T and P).

Combined Gas Law

The Combined Gas Law combines Boyle's, Charles', and Avogadro's Laws to relate pressure, volume, temperature, and moles.

• Formula:

• Key Point: Useful when more than one variable changes.

Ideal Gas Law

The Ideal Gas Law is a general equation that relates pressure, volume, temperature, and moles of a gas using the universal gas constant.

• Formula:

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

• Example: Calculate moles of H2 gas:

Worked Examples

• Boyle's Law Example: If Pa, L, Pa, find : L

• Charles' Law Example: A sample of gas at 15°C and 1 atm has a volume of 2.58 L. What volume will it occupy at 38°C and 1 atm? Convert temperatures to Kelvin: K, K , so L

• Avogadro's Law Example: Suppose you have 12.2 L of O2 gas containing 0.50 mol at 1 atm and 25°C. If all O2 is converted to O3, what is the volume of O3 at the same conditions? First, calculate moles of O3 produced: Apply Avogadro's Law: , so L

• Ideal Gas Law Example: A sample of H2 gas occupies 8.56 L at 0°C and 1.58 atm. How many moles are present? mol

• Combined Gas Law Example: A sample of methane gas with a volume of 38 mL at 5°C is heated to 86°C at constant pressure. Calculate its new volume. Convert temperatures: K, K , so mL

Tips for Solving Gas Law Problems

• Always convert temperatures to Kelvin ().

• Use the correct units for pressure, volume, and temperature as required by the gas constant R.

• Identify which law applies based on which variables are held constant.

• Combined Gas Law is useful when more than one variable changes between two situations.

• Ideal Gas Law is typically used for single-situation problems.

Summary Table: Gas Laws

Additional info: These notes cover the foundational gas laws and pressure concepts essential for General Chemistry. For more advanced topics, such as Dalton's Law of Partial Pressures, Kinetic Molecular Theory, and Real Gases, further study is recommended.