Gas Laws

• The gas laws are simple mathematical relationships between pressure, volume, temperature, and the amount of a gas.

• They were developed by scientists who experimented with a variety of gases and were able to observe the relationships among these variables.

Boyle’s Law

• What happens to the volume of a gas when you increase the pressure?

• What happens to the pressure when you increase the volume?

• Boyle’s Law relates the volume and pressure of a gas. It assumes that the temperature and the number of gas particles remain constant/

• represent initial conditions while represent a different set of conditions.

• If you have any three of the values, you can calculate the fourth.

• The units of pressure and volume MUST be the same on both sides of the equation!

• A balloon filled with helium gas has a volume of 500. mL at a pressure of 1.00 atm. The balloon is released into the air. When the balloon is high enough that the pressure is 0.750 atm, what is the volume of the gas?

• The volume of a sample of gas is 425 mL at a pressure of 1.05 atm. What would be the pressure of the gas, in kPa, if the volume is decreased to 215 mL while the temperature is held constant?

Charles’ Law

• If the temperature of a gas increases, but the pressure stays the same, what will happen to the volume of the gas?

• Charles’ Law is the mathematical relationship between temperature and volume.

• It assumes that the pressure and the number of gas particles remain constant.

• What would happen if one of the temperatures was a negative number?

• To solve the problem of negative temperatures, the Kelvin temperature scale is used for all gas laws.

• The lowest temperature in the Kelvin scale is 0K, also known as absolute zero. Note that no degree symbol is used with the Kelvin scale.

• To convert from oC to K, simply add 273.– This means that the magnitude of a celsius degree

and a Kelvin is exactly the same, the Kelvin scale has just been moved downward by 273 degrees.

– Therefore, absolute zero is 0K or -273oC

• A sample of neon gas has a volume of 752 mL at 25.0oC. What will the volume be at 100.oC if the pressure is constant?

• A sample of nitrogen gas has a volume of 375 mL at a temperature of 0.0oC. If the gas is heated and the volume increases to 500. mL, what is the new temperature of the gas (in Celsius degrees)?

Gay-Lussac’s Law

• Gay-Lussac’s Law relates the pressure and temperature of a gas while the volume is held constant.

• If the temperature of a gas decreases, what will happen to the pressure?

• Don’t forget that the temperature MUST be in Kelvins.

• At 125oC, the pressure of a sample of oxygen is 764 mm Hg. What will be the pressure of the sample if the temperature is increased to 175oC?

• A sample of helium gas has a pressure of 115 kPa when the temperature is 22oC. What will be the temperature, in Celsius, if the pressure is reduced to 103 kPa?

Combined Gas Law

• In the first 3 gas laws, one variable has been held constant.

• Often when a gas sample undergoes changes, pressure, temperature, and volume ALL change.

• Boyle’s, Charles’ and Gay-Lussac’s laws can be combined into a single law called the combined gas law.

• Note that the combined gas law reduces to each of the 3 other laws, if one variable is held constant.

• The volume of a gas is 32.5 mL at 22oC and 0.985 atm. What will be the volume of the gas at STP?

• A sample of fluorine gas occupies 1.5 L at 26.5oC and 1.00 atm. What will be the pressure of the gas if the volume increases to 2.5 L and the temperature increases to 40.0oC?