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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

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

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

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 pressure1

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

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

Conversions

  • All of these units measure pressure, so we can convert between all of them:

    • 2.59 atm  torr

    • 657 mmHg  kPa


Atmospheric pressure

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

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

Manometer

  • Measures the pressure of a gas.

  • Compares Pgas to Patm

Pgas < Patm

Pgas > Patm


Partial pressure

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

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 problems1

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 problems2

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

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 water1

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

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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