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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

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!

• 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?