DO NOW! – As soon as you enter the room, start answering this question!

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# DO NOW! – As soon as you enter the room, start answering this question! - PowerPoint PPT Presentation

DO NOW! – As soon as you enter the room, start answering this question!. What is pressure? How does a gas exert pressure? Be sure to include details about particle motion. DO NOW!. Pressure is a force exerted over an area , a force over an area of a surface

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DO NOW! – As soon as you enter the room, start answering this question!
• What is pressure?
• How does a gas exert pressure?
• Be sure to include details about particle motion.
DO NOW!
• Pressure is a force exerted over an area
• , a force over an area of a surface
• When gas molecules collide with the walls of their container, they exert a force.
Units of Pressure
• All of these are equal:
• 1.00 atm
• 101.325 kPa
• 760. mmHg
• 760. torr

You need to memorize these conversions!

Units of Pressure
• Standard Pressure – the pressure exerted at sea level by dry air
• 1.00 atm (atmosphere)
• 101.325 kPa (kilopascals) = 1.00 atm
• pascal is the SI unit for pressure (1 Pa = 1 )
• 760. mmHg (millimeters of mercury) = 1.00 atm
• 760. torr (Torr [named after Evangelista Torricelli, inventor of the barometer]) = 1.00 atm

1.00 atm = 101.325 kPa = 760. mmHg = 760. torr

Other units of pressure
• 14.7 psi (or lb./in2) (pounds per square inch)
• 29.9 inHg (inches of mercury)
• 1.01325 bar (1 bar = 100,000 Pa)

Conversions
• All of these units measure pressure, so we can convert between all of them:
• 2.59 atm  torr
• 657 mmHg  kPa
Atmospheric Pressure
• At sea level, every surface has 1.00 atm (14.7 lb./in2 for reference) of pressure pushing on it at all times.
• How does the pressure mat work?
• There is no air underneath the mat, so there is no atmospheric pressure underneath it pushing up.
• This means there are (theoretically) more than 2,000 lbs. of force pushing down on the mat!
Barometer
• Measures the pressure of the atmosphere
• Atmospheric pressure pushing down on the pool of liquid forces liquid to stay in the tube.
• The height of the liquid in the tube measures pressure, regardless of the shape or size of the tube.
Manometer
• Measures the pressure of a gas.
• Compares Pgas to Patm

Pgas < Patm

Pgas > Patm

Partial Pressure
• Dalton’s Law of partial pressure
• the total pressure of a gas mixture is the sum of the partial pressures of the gases in the mixture.
• Total pressure = sum of partial pressures
• PT = P1 + P2 + P3 + …

+

=

+

=

Partial Pressure Problems
• A sample of gas contains N2 at 185 torr, Ar at 90. torr, and O2 at 35 torr. What is the total pressure of the gas?
Partial Pressure Problems
• A sample of gas contains N2 at 185 torr, Ar at 90. torr, and O2 at 35 torr. What percent of the gas is Ar?
Partial Pressure Problems
• A sample of gas mixture is 35% N2. If the total pressure of the gas mixture is 724 kPa, what is the partial pressure of N2?
Collection over Water
• Gases are often collected over water
• In the test tube where gas is collected, water vapor is also present.
• Vapor pressure varies with temperature.
Collection over Water
• A 30.0 mL sample of gas is collected over water at 20 °C and 101.325 kPa of pressure. What is the pressure of the dry gas alone? Vapor pressure at 20 °C is 2.3 kPa.
Exit Slip – You’re not allowed to leave unless you’ve filled this out!
• Why was the pressure mat so difficult to lift off the lab bench?