Gas Laws and Properties of Gases

5-1 Atmospheric Pressure

Atmospheric pressure is the force exerted by gas molecules in the Earth's atmosphere as they collide with surfaces. It is a crucial concept in understanding how gases behave under typical conditions.

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

• Pressure: The force exerted by gas molecules per unit area on surfaces.

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

• Standard Pressure: 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)

• Conversion Factors:

Example: Convert 49 torr to other units:

• Atmospheres:

• mm Hg:

• Pascals:

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

5-3 Boyle's Law

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

• Statement: For a fixed amount of gas at constant temperature, the pressure and volume are inversely proportional.

• Formula:

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

5-4 Charles' Law

Charles' Law explains how the volume of a gas changes with temperature at constant pressure.

• Statement: For a fixed amount of gas at constant pressure, the volume and temperature (in Kelvin) are directly proportional.

• Formula:

• Example: Heating a balloon causes it to expand as the gas volume increases with temperature.

Sample Problem: A 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: ,

• Apply Charles' Law:

• Solve for :

5-5 Avogadro's Law

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

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

• Formula:

• Example: More moles of gas occupy more volume under the same conditions.

Sample Problem: If 0.50 mol of O2 is converted to O3 at constant temperature and pressure, calculate the volume of O3 produced.

• Convert moles:

• Apply Avogadro's Law:

• Solve for using and ,

5-6 The Combined Gas Law and the Ideal Gas Law

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

• Combined Gas Law:

• Ideal Gas Law:

• Universal Gas Constant (R):

Units of R: Ensure pressure is in atm, volume in liters, temperature in Kelvin, and moles in mol.

Sample Problem: A sample of H2 gas occupies 8.56 L at 0°C and 1.58 atm. How many moles are present?

• Convert temperature:

• Apply Ideal Gas Law:

5-7 Gas Law Problem Solving Tips

Gas law problems may involve one or more variables changing. Identify which law applies and ensure all units are consistent.

• Convert temperatures to Kelvin.

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

• For PV=nRT problems, usually only one situation is given.

Example: Calculate the volume of 0.845 mol N2 at 315 K and 0.920 atm.

5-8 Charles' Law Application

Charles' Law can be used to calculate the change in volume when temperature changes at constant pressure.

• 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: ,

• Apply Charles' Law:

• Solve for :

Note: It is not necessary to convert mL to L if both volumes are in the same units.

Summary Table: Gas Laws

Additional info: These notes cover the foundational gas laws and properties of gases, which are essential for understanding more advanced topics such as Dalton's Law of Partial Pressures, Kinetic Molecular Theory, and Real Gases. For further study, review the effects of intermolecular forces and deviations from ideal behavior.