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

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- What is pressure?
- How does a gas exert pressure?
- Be sure to include details about particle motion.

- 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.

- All of these are equal:
- 1.00 atm
- 101.325 kPa
- 760. mmHg
- 760. torr

You need to memorize these conversions!

- 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

- 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.)

- All of these units measure pressure, so we can convert between all of them:
- 2.59 atm torr
- 657 mmHg kPa

- 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!

- 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.

- Measures the pressure of a gas.
- Compares Pgas to Patm

Pgas < Patm

Pgas > Patm

- 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 + …

+

=

+

=

- 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?

- 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?

- 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?

- Gases are often collected over water
- In the test tube where gas is collected, water vapor is also present.
- Vapor pressure varies with temperature.

- 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.

- Why was the pressure mat so difficult to lift off the lab bench?