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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

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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)

    (Don’t worry about these ones.)


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?


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