CHAPTER 11

GASES

11.1GASES AND PRESSURE

11.1Why are gases so unique?

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Gases are unique because their physical properties are determined by the pressure, volume, temperature, and the amount of gas present.

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What is pressure?

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Pressure = Force / Area

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Pressure = Force / Area

If the force increases, how does it affect the pressure?

Who exerts more pressure? Why?

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Pressure = Force / Area

As the force increases, the pressure increases.

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Pressure = Force / Area

How does area affect pressure?

Assume the same person is wearing these, which of these exerts more pressure? Why?

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Pressure = Force / Area

As the force increases, the pressure increases.As the area decreases, the pressure increases.

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Pressure = Force / Area

The newton, N, is the SI unit for force.

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How strong is the atmospheric pressure?

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Atmospheric pressure is formed by all the gas particles being pulled toward the Earth.

What gases make up the atmosphere?

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Atmospheric pressure is formed by all the gas particles being pulled toward the Earth.

Atmosphere = 78% N2

21% O2

1% (Ar, CO2, H2O, etc.)

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How does atmospheric pressure vary with altitude?

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What instrument is used to measure pressure?

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A barometer measures atmospheric pressure.

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These can also be used to measure atmospheric pressure.

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A barometer measures atmospheric pressure.

Mercury was/is used because itsvolume changes very littlewith changing temperature.

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A barometer measures atmospheric pressure.

A manometer can be used to measure gases in a container. It can be an open or a closed manometer.

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This is an open manometer.

These are all closed manometers.

These are open manometers at room temperature and standard pressure (101.3 kPa).

1.Which one contains a gas at 90 kPa?2.Which manometer has the lowest

pressure?3.In which one is the pressure the

greatest?4.Which manometer could have a leak?

11.1What are the units of pressure?

11.1Pressure units:

The units we will use are the following: Atmosphere (atm) Millimeters of mercury (mm Hg) Kilopascals or pascals (kPa, Pa) Torr

11.1Pressure units:

The units we will use are the following: Atmosphere (atm) Millimeters of mercury (mm Hg) Kilopascals or pascals (kPa, Pa) Torr

The units commonly used in the US are the following: Inches of mercury Millibars Pounds/in2

p. 364

11.1Pressure units:

STP = Standard temperature and pressure1 atm and 0ºC (273 Kelvin)

11.1Pressure units:

1 atm = 760 mmHg = 101.3 kPa = 760 torr

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Example 1: Convert 745 mmHg to atm and kPa.

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Example 2: Convert 88.86 kPa to atm and mmHg.

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Gas Laws are used to predict the pressure, volume, or amount of gas in a container when the conditions change.

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Dalton’s Law of Partial Pressure

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Dalton’s Law of Partial Pressure

When gases are mixed, the total pressure is calculated by adding up the pressures of all the gases in the mixture.Ptotal = P1 + P2 + P3 . . .

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Dalton’s Law of Partial Pressure

The mole fraction of the gases can also be determined by using the pressure of each gas.

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Example 3: What is the atmospheric pressure if the partial pressures of nitrogen, oxygen, and argon are 604.5 mmHg, 162.8 mmHg, and 0.5 mmHg, respectively?

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If a gas is collected over water (as we will do in lab), there is a small amount of water vapor present. This has to be taken into consideration when calculating the total pressure above the water.

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Example 4: A student generates O2 in a set up similar to p. 366. The gas is collected at 25ºC and the level of water inside and outside the collection jar is equal. If the atmospheric pressure is 758 mmHg, what is the pressure of just the O2? (H2O vapor pressure at 25ºC is 23.8 mmHg)

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Example 5: A tank contains 5 moles of O2, 6 moles of H2S, and 4 moles of Ar at a total pressure of 1620 mmHg. What is the pressure of each gas?