Ø Several Units Are Used When Describing Pressure

3.8 Gas Laws

Pressure Units

Ø  Several units are used when describing pressure

Unit / Symbol
Atmospheres
Pascals, kiloPascals
Millimeters of mercury
Pounds per square inch

Important Conversion Factor

1 atm = 101300 Pa = 101.3 kPa = 760 mm Hg = 760 torr

1. Convert 654 mm Hg to atm

2. Convert 879 mm Hg to kPa

3. Convert 15.6 atm to kPa

Kelvin

Ø  Temperature scale with an absolute zero used in gas laws

Ø  Represented as K

Ø  To convert Celsius temperature to Kelvin, use °C + 273 = K

Examples

1.Convert 15.6 °C into K

2. Convert 234 K into °C

Standard Temperature & Pressure

Ø  Also known as STP

Ø  1 atm of pressure or any equivalent and 0 °C (273 K)

The Gas Laws: Before & After

Boyles Law

v  Volume and pressure have an ______relationship

when temperature and moles are constant

V1P1 = V2P2

Tripling pressure reduces volume by one-third

Example:

A gas sample is 1.05 atm when at 2.5 L. What volume is it if the pressure is changed to .980 atm?

Charles Law

v  Volume and temperature have a ______

proportional relationship

when pressure and moles

are constant.

ü  Temperature must be in Kelvin units!

V1/T1 = V2/T2

Doubling volume doubles the Kelvin temperature of gas

Example:

ü  What is the final volume if a 10.5 L sample of gas is changed from 25.0° C to 50.0° C?

Gay-Lussac Law

v  Pressure and temperature have a ______

proportional relationship when volume and moles are constant.

ü  Temperature must be in Kelvin units!

P1/T1 = P2/T2

Halving Kelvin temperature will cut the pressure of the gas in half

Example:

A sample of hydrogen gas at 47.0 °C exerts a pressure of .329 atm. The gas is heated to 77.0° C at constant volume and moles. What will the new pressure be?

Avogadro’s Law

v  Moles & Volume have a ______

proportional relationship when temperature and pressure are constant.

ü  Temperature must be in Kelvin units!

V1/n1 = V2/n2

Example

A sample with 0.15 moles of gas has a volume of 2.5 L. What is the volume if the sample is increased to 0.55 moles?

Combined Gas Law

v  Expresses the relationship between volume , pressure, and temperature when moles is held constant. Temperature must be in Kelvin units!

V1P1 = V2P2

n1T1 n2T2

Example:

What is the final volume if a 15.5 L sample of gas at 755 mm Hg and 298K is changed to STP?

You only need the combined gas law equation to create the 4 laws discussed above!

V1P1 = V2P2

n1T1 n2T2