Atmospheric Pressure

The Atmosphere and Atmospheric Pressure

The atmosphere is the layer of gases surrounding Earth, essential for supporting life, acting as a waste receptacle for exhaust gases, and shielding us from harmful radiation. All gases exert pressure on their surroundings, which is the force exerted by gas molecules as they strike the surfaces around them.

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

• Atmospheric pressure is measured using a barometer, invented by Italian physicist Torricelli.

• At sea level, standard atmospheric pressure is defined as 760 mm Hg.

Factors Affecting Atmospheric Pressure:

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

• Weather: Changes in weather can alter atmospheric pressure.

Units of Pressure

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

• 1 atm (atmosphere) = 760 mm Hg (millimeters of mercury) = 760 torr = 101,325 Pa (pascals) = 14.7 psi (pounds per square inch)

• These units are commonly used as conversion factors in calculations.

Example: Convert 49 torr to other pressure units:

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

Gas Laws

Boyle's Law

Boyle's Law describes the relationship between the pressure and volume of a gas at constant temperature. It states that pressure and volume are inversely proportional.

• Mathematical form: (at constant T)

• If the volume of a gas decreases, its pressure increases, and vice versa.

• Example: Squeezing a balloon decreases its volume, so the pressure inside increases.

• For two situations:

Charles' Law

Charles' Law describes the relationship between the volume and temperature of a gas at constant pressure. Volume and temperature are directly proportional, provided temperature is measured in kelvin.

• Mathematical form: (at constant P)

• As temperature increases, volume increases.

• Example: Heating a balloon causes it to expand.

• For two situations:

Example Calculation: 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: ,

• Apply Charles' Law:

• Solve for .

Avogadro's Law

Avogadro's Law states that the volume of a gas is directly proportional to the number of moles of gas at constant temperature and pressure.

• Mathematical form: (at constant T, P)

• As the amount of gas increases, the volume increases.

• For two situations:

Example Calculation: Suppose you have 12.2 L of O2 gas containing 0.50 mol at 1 atm and 25°C. If all this O2 were converted to ozone (O3), what would be the volume of the ozone at the same temperature and pressure?

• Calculate moles of O3 produced:

• Apply Avogadro's Law:

• Solve for .

The Combined Gas Law

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

• Mathematical form:

• For two situations:

The Ideal Gas Law

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

• Mathematical form:

• is the universal gas constant,

• Units must be consistent: pressure in atm, volume in L, temperature in K, amount in mol.

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

• Convert temperature:

• Apply Ideal Gas Law:

Solving Gas Law Problems

Gas law problems may involve one or more variables changing. The correct law to use depends on which variables are held constant and which are changing.

• Boyle's Law: Pressure and volume change, temperature constant.

• Charles' Law: Volume and temperature change, pressure constant.

• Avogadro's Law: Volume and moles change, temperature and pressure constant.

• Combined Gas Law: Multiple variables change.

• Ideal Gas Law: Used when amount (moles) is involved and only one situation is described.

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 always necessary to convert mL to L if the units are consistent throughout the calculation.

Summary Table: Gas Laws

Key Tips:

• Always convert temperatures to kelvin for gas law calculations.

• Check units for consistency, especially when using the ideal gas constant.

• Identify which variables are changing and which are constant to select the correct law.

Additional info: These notes cover the foundational gas laws and concepts in general chemistry, including atmospheric pressure, units of pressure, and the relationships between pressure, volume, temperature, and amount of gas. For more advanced topics (e.g., Dalton's Law, Kinetic Molecular Theory, Real Gases), refer to subsequent chapters or study guides.