Pressure

• Pressure is force per unit area– In the English system,

pounds per square inch or psi

– Atmospheric pressure is about 14.7 psi

• Pressure = Force

Area

• 1 atm = 14.7 psi• 1 atm = 760 mmHg

– The mmHg is also called the Torr after Torricelli, inventor of the barometer

• SI unit of measurement, the pascal (Pa)– 1 Pa is the pressure exerted by

a 0.1 mm high film of water on the surface beneath it

– The bar = 105 Pa– 1.013 bar = 1 atm = 760 mmHg

= 14.7 psi = 100 kPa

Gas Pressure Measurement

• The barometer measures pressure in terms of the height of a column on liquid mercury– The atmosphere exerts a

force on a pool of mercury, causing it to rise

– One standard atmosphere of pressure is a column of mercury 760 mm high

– Mercury is used to keep the column a manageable height

Gas Pressure Measurement cont.

• The manometer measures gas pressure by differential– The height of the column

of liquid is proportional to the pressure

– Gas pressure can be more or less than atmospheric pressure

Dalton’s Law of Partial Pressure

• Dalton’s law of partial pressures states that the sum of the partial pressures of gases sum to the total pressure of the gases when combined.

• Ptot = P1 + P2 + P3 + …

Boyles’s Law

• The product of the pressure and volume for a trapped sample of gas = a constant (k)

• PV = k • P1V1 = P2V2 for analysis

of a system before and after

Sample Boyle’s Law problem

• A quantity of gas under a pressure of 106.6 kPa has a volume of 380 dm3. What is the volume of the gas at 103.3 kPa, if the temperature is held constant?

• P1 x V1 = P2 x V2

• (106.6 kPa) x (380 dm3) = (103.3 kPa) x (V2)

• V2 = 400 dm3

Absolute Zero and the Kelvin Scale

• Absolute Zero is the temperature where all motion stops (-273C)

• For gases, the SI unit uses the Kelvin (K) scale.– Kelvins = (273 + C)

Charles’ Law

• The volume of each gas is directly proportional to temperature

• V = bT– B = a constant– V1/T1 = V2/T2

– Temperature is in Kelvins (273 + C)

Charles’ Law Calculation

• At constant pressure, the volume of a gas is increased from 150 dm3 to 300 dm3 by heating it. If the original temperature of the gas was 20 oC, what will its final temperature be (oC)?

• T1 = 20 oC + 273 = 293 K

• T2 = X K

• V1 = 150 dm3

• V2 = 300 dm3

Avogadro’s Law

• Volume is directly proportional to the number of moles of gas

• V = an– V1/n1 = V2/n2

The Combined Gas Law

1 1 2 2

1 2

PV PV

T T

Sample Problem

• A sample of helium gas has a volume of 0.180 L, a pressure of 0.800 atm and a temperature of 29°C. What is the new temperature(°C) of the gas at a volume of 90.0 mL and a pressure of 3.20 atm?