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Gas Laws

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**Characteristics of Gases. . .**

Substances that exist as solids and liquids can exist as gases. Called vapors. Properties: Highly compressible and occupy the full volume of the container. Subjected to increased pressure, the volume decreases. Always form homogenous mixtures with other gases.

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**Characteristics of Gases. . .**

Behavior: Even though gases fill the container, they occupy a small fraction of the containers volume, therefore each gas molecule behaves largely as though other molecules were absent.

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**Pressure. . . Is the force acting on an object per unit area.**

A column of air 1m2 in cross section extended to the top of the atmosphere exerts a force of 105N. Thus, the pressure of a 1m2 column of air extending to the top of the atmosphere is 101kPa. Animations/AtmosphericPressure.html SI unit is the pascal. 1Pa = 1N/m2 1N = 1kgm/s2

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**Atmospheric Pressure. . . Atmospheric pressure:**

gravity exerts force on the earth’s atmosphere. Measured using a barometer. Standard atmospheric pressure is the pressure needed to support 760mmHg in a column. 1atm = 760mmHg = 760torr = kPa = Pa 1bar = 105Pa

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**Torricelli’s Barometer**

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**Pressure of Enclosed Gases. .**

Pressures of gases not exposed to the atmosphere are measured using manometers. If the U tube is closed, the pressure of the gas is the difference in height of the liquid (usually Hg).

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**Pressure of Enclosed Gases. .**

A closed manometer is filled with mercury and connected to a container of nitrogen. The difference in the height of the mercury in the two arms is 691mm. What is the pressure of the nitrogen in kilopascals?

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**Pressure of Enclosed Gases. .**

A closed manometer is filled with mercury and connected to a container of helium. The difference in the height of the mercury in the two arms is 86.0mm. What is the pressure in kilopascals of the helium? 86.0mm 1kPa = 11.5kPa 7.50mm

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**Pressure of Enclosed Gases. .**

If the U tube is open to the atmosphere, a correction term needs to be added. If Pgas <Patm, then Pgas + Ph = Patm If Pgas >Patm, then Pgas = Patm - Ph

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Hint: The greater height of the liquid in one arm of a manometer is a measure of the greater pressure of the gas in the opposite arm.

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**Pressure of Enclosed Gases. .**

The open manometer is filled with mercury. The difference between the mercury in the two arms is 6mm. What is the total pressure in kPa of the gas in the container if the atmospheric pressure is 101.3kPa? 101.3kPa – 0.8kPa = 100.5kPa

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A vessel connected to an open end mercury manometer is filled with gas to a pressure of 0.835atm. The atmospheric pressure is 755 torr. In which arm of the manometer will the level of the mercury be higher? What is the height difference between the two arms of the manometer?

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Practice. . . On a certain day the barometer in a lab indicates that the atmospheric pressure is 764.7torr. A sample of gas is placed in a vessel attached to an open end mercury manometer. A meter stick is used to measure the height of the mercury above the bottom of the manometer. The level of mercury in the open end arm of the manometer has a measured height of 136.4mm. The arm of the manometer that is in contact with the gas has a height of 103.8mm. What is the pressure of the gas in atmospheres?

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**Pgas = Patm + (difference in height of arms)**

= torr + (136.4torr – 103.8torr) = 797.3torr 797.3torr 1atm 760torr = 1.049atm

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Boyle’s Law The volume of a gas will vary inversely with the pressure exerted on the gas, if the temperature and mass of the gas are held constant. P1V1 = P2V2 Show vacuum pump with balloon/ do problems Use Boyle’s Law Lab/ Chemistry with Computers

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CHARLES’ LAW At constant pressure, the volume of a fixed amount of gas is directly proportional to its absolute temperature. Therefore: V1T2 = V2T1 Use soda can demo for Charles’ Law Egg in bottle

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Avogadro’s Law Equal volumes of gases at the same temperature and pressure contain an equal number of particles. Therefore: at STP, the volume of one mole of any gas is 22.4L.

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**Dalton’s Law of Partial Pressure**

States that the sum of the partial pressures of all the components in a gas mixture is equal to the total pressure of the gas mixture. PT = pa +pb +pc . . .

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Ideal Gas Law An ideal gas is a gas as described by the postulates of the kinetic molecular theory. No ideal gas exists. The equation can be used to describe the behavior of a real gas. All gases are real gases. At low temperatures and high pressures real gases show non ideal behavior.

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**Ideal Gas Law PV = nRT P = pressure V = volume n = moles**

R = a constant 8.314L kPa/mol K 0.0821L atm/mol K 62.4L mmHg/mol K T = temperature

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