Gases and Gas Laws

5.1 – Atmospheric Pressure

Atmospheric pressure is the force exerted by the weight of the air in the atmosphere on the Earth's surface. It is a fundamental concept in understanding the behavior of gases under typical conditions.

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

• Pressure: The force exerted by gas molecules as they strike surfaces around them.

• Atmospheric Pressure: The pressure exerted by the mixture of gases (mainly N2, O2, Ar, CO2, Ne, He, CH4) in the atmosphere.

• Barometer: A device used to measure atmospheric pressure, invented by Evangelista Torricelli. At sea level, standard atmospheric pressure pushes mercury up a barometer tube to a height of 760 mm.

Standard Pressure: 760 mmHg

Factors Affecting Barometric Pressure

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

• Weather: Changes in weather patterns can increase or decrease 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)

• Conversion Factors:

These conversion factors are frequently used to convert between different pressure units in calculations.

Example: Converting Pressure Units

• Convert 49 torr to atmospheres, mm Hg, and pascals:

5.3 – Boyle’s Law

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

• Statement: The volume of a fixed amount of gas is inversely proportional to its pressure at constant temperature.

• Mathematical Form:

• As pressure increases, volume decreases, and vice versa (for a given amount of gas at constant temperature).

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

5.4 – Charles’ Law

Charles’ Law relates the volume of a gas to its temperature at constant pressure.

• Statement: The volume of a fixed amount of gas is directly proportional to its absolute temperature (in Kelvin) at constant pressure.

• Mathematical Form:

• As temperature increases, volume increases (at constant pressure).

Example: Heating a balloon causes it to expand as the gas molecules move faster and occupy more space.

5.5 – Avogadro’s Law

Avogadro’s Law connects the volume of a gas to the number of moles present, at constant temperature and pressure.

• Statement: The volume of a gas is directly proportional to the number of moles of gas at constant temperature and pressure.

• Mathematical Form:

• Adding more gas (increasing moles) increases the volume, provided temperature and pressure are constant.

Example: If 0.50 mol of O2 at 1 atm and 25°C occupies 12.2 L, converting all O2 to O3 (ozone) and keeping T and P constant, the volume of O3 can be found using Avogadro’s Law.

5.6 – The Combined Gas Law and the Ideal Gas Law

The Combined Gas Law merges Boyle’s, Charles’, and Avogadro’s Laws to relate pressure, volume, and temperature for a fixed amount of gas.

• Combined Gas Law:

• Used when the amount of gas is constant but pressure, volume, and temperature change.

Ideal Gas Law: The most general equation relating pressure, volume, temperature, and moles of a gas.

• P: Pressure (atm)

• V: Volume (L)

• n: Moles of gas

• R: Universal gas constant ()

• T: Temperature (K)

Note: Always use Kelvin for temperature in gas law calculations.

Example: Using the Ideal Gas Law

• A sample of H2 gas occupies 8.56 L at 0°C and 1.5 atm. How many moles of hydrogen are present?

Example: Finding Volume at New Conditions

• 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.

Summary Table: Gas Laws

Additional info: For more advanced topics such as Dalton's Law of Partial Pressures, Kinetic Molecular Theory, and Real Gases, refer to subsequent sections or chapters. The above notes provide a comprehensive overview of the fundamental gas laws and their applications in general chemistry.