# Chapter5 - 1 Chapter Five Gases 2 Gases What Are They Like...

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1 Gases Chapter Five

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2 Gases … What Are They Like? Gases are composed of widely separated particles in constant, random motion. Gases flow readily and occupy the entire volume of their container. Vapor – a gas that is a liquid at room temperature and pressure (water vapor and methanol vapor , but gaseous oxygen and gaseous hydrogen ). Many low molar mass molecular compounds are either gases or easily vaporizable liquids.
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4 An Introduction to Kinetic-Molecular Theory Provides a model for gases at the microscopic level. Pressure : collision of gas molecules with wall of container. Temperature : related to average speed of gas molecules. Molecules are in rapid, random motion. Movement of gases through three-dimensional space is called translational motion .
5 Gas Pressure Pressure is the force per unit area. In SI, force is expressed in newtons ( N ) and area in square meters (m 2 ). The unit of pressure in SI is the pascal (Pa) with the units N/m 2 . Kilopascals (kPa) are often used instead since the pascal is such a small unit. The atmosphere and mmHg ( Torr ) are the most common scientific units for pressure. Converting from one unit to another simply requires the appropriate conversion factor(s).

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7 Barometers Used to measure atmospheric pressure. One atmosphere ( atm ) : pressure exerted by a column of mercury exactly 760 mm high. One millimeter of mercury is called a Torr . 1 atm = 760 mmHg = 760 Torr = 101.325 kPa

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8 A Mercury Barometer The pressure exerted by the column of mercury … … must be the same as that exerted by the atmosphere.
9 Properties of Gases Four properties off a gas: pressure, volume, temperature, amount in moles They are related through several simple gas laws.

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10 Boyle’s Law: Pressure-Volume Relationship For a fixed amount of a gas at constant temperature, the volume of the gas varies inversely with its pressure. For a fixed amount of a gas at constant temperature, the product of pressure and volume is a constant. PV = constant or P initial V initial = P final V final
11 Graphical Representation of Boyle’s Law When volume is increased there is more area for the molecules to “hit”; less force per area. Estimate the pressure at 3 V and at 5 V . Which plot is easier to use for making this estimation?

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12 Example A 4.50-L cylinder contains He (g) at an unknown pressure. It is now connected to a 92.5 L evacuated cylinder. When the connecting value between the two cylinders is opened, the pressure falls to 1.40 atm. What was the pressure in the 4.50-L cylinder?
13 Charles’s Law: Temperature-Volume Relationship The volume of a fixed amount of a gas at constant pressure is directly proportional to its Kelvin (absolute) temperature. V initial V final T initial V final Absolute zero is the temperature obtained by extrapolation to zero volume.

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