Properties and Behavior of Gases: Pressure, Volume, and Units

Gases: Properties and Measurement

Shape and Volume of Gases

Gases are a state of matter characterized by their ability to fill any container, taking both its shape and volume. Unlike solids and liquids, gases do not have a fixed shape or volume.

• Shape: Gases assume the shape of their container.

• Volume: Gases expand to fill the entire volume of their container.

• Example: Argon gas in a container will have a variable shape and volume, depending on the container's dimensions.

Gas Pressure and Units of Measurement

Pressure is a measure of the force exerted by gas particles as they collide with the walls of their container. It can be expressed in several units, each commonly used in chemistry and physics.

• Common Pressure Units:

  • Atmospheres (atm)

  • Millimeters of mercury (mm Hg)

  • Torr

  • Centimeters of mercury (cm Hg)

  • Inches of mercury (in. Hg)

  • Pounds per square inch (psi)

  • Kilopascals (kPa)

• Conversion Example: To convert 655 mm Hg to atm:

  • Use the conversion factor: $1\ \text{atm} = 760\ \text{mm Hg}$

  • Calculation: $655\ \text{mm Hg} \times \frac{1\ \text{atm}}{760\ \text{mm Hg}} = 0.862\ \text{atm}$

Variables Affecting Gas Pressure

The pressure of a gas can change depending on its volume, temperature, and the number of molecules present. Understanding these relationships is essential for predicting gas behavior.

• Volume: Increasing the volume of a container decreases the pressure (if temperature and number of molecules are constant).

• Temperature: Increasing temperature increases pressure (if volume and number of molecules are constant).

• Number of Molecules: Increasing the number of gas molecules increases pressure (if volume and temperature are constant).

• Example Question: Which variable, when increased, causes a decrease in pressure? Answer: Volume.

Calculating Pressure Changes: Boyle's Law

Boyle's Law describes the inverse relationship between the pressure and volume of a gas at constant temperature. When the volume of a gas decreases, its pressure increases proportionally.

• Boyle's Law Equation:

  • $P_1 V_1 = P_2 V_2$

• Example Calculation:

  • If a gas is at $1.00\ \text{atm}$ and the volume decreases from $5.00\ \text{L}$ to $2.00\ \text{L}$, what is the new pressure?

  • Set up the equation: $1.00\ \text{atm} \times 5.00\ \text{L} = P_2 \times 2.00\ \text{L}$

  • Solve for $P_2$:

  • $P_2 = \frac{1.00\ \text{atm} \times 5.00\ \text{L}}{2.00\ \text{L}} = 2.50\ \text{atm}$

Summary Table: Gas Pressure Units and Relationships

Unit	Symbol	Conversion to atm
Atmosphere	atm	1 atm
Millimeter of mercury	mm Hg	760 mm Hg = 1 atm
Torr	torr	760 torr = 1 atm
Kilopascal	kPa	101.325 kPa = 1 atm
Pounds per square inch	psi	14.7 psi = 1 atm
Centimeter of mercury	cm Hg	76 cm Hg = 1 atm
Inch of mercury	in. Hg	29.92 in. Hg = 1 atm

Additional info: Boyle's Law is one of several gas laws describing the behavior of gases. Other important laws include Charles's Law (relationship between volume and temperature) and Avogadro's Law (relationship between volume and number of moles).