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CHAPTER 4 – THE PROPERTIES OF GASES THE NATURE OF GASES 4.1 – Observing Gases bulk matter – forms of matter consisting of large numbers of molecules compressible – can be confined into a smaller volume o because gases are more compressible than solids and liquids, it can be assumed that there is a lot of space between the molecules of gases rapidly moving molecules (balloon deflation example) The fact that gases are readily compressible and immediately fill the space available to them suggests that molecules of gases are widely separated and in ceaseless chaotic motion. 4.2 – Pressure formula for pressure P = F/A P = pressure F = force A = area Units (SI) o 1 Pascal (Pa) = 1 kg . m -1 . s -2 pressure that a gas exerts on the walls of its container results from the collisions of its molecules with the container’s surface o more vigorous the storm of molecules on a surface, the stronger the force = higher pressure Measurement of pressure o Pressure gauge Measurement of the “gauge pressure” the difference between the pressure inside the tire and the atmospheric pressure o Barometer Measures the pressure of the atmosphere Derivation of how to find pressure… P = F/A (formula for pressure) = mg/A (force = mass times acceleration due to gravity) = dhAg/A ( density formula and volume formula) = dhg
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o manometer – u-shaped tube that can either be closed or open open tube – the pressure in the system is equal to that of the atmosphere when the levels of the liquid in each arm of the U-tube are the same closed tube – one side is connected to a closed flask (the system) and the other side is vacuum. The difference in heights of the two columns is proportional to the pressure in the system. The pressure of a gas, the force that it exerts divided by the area subjected to the force, arises from the impacts of its molecules. 4.3 – Alternative Units of Pressure 1 Pa = 1 kg m -1 s -2 = 1 N m -2 1 bar = 10 5 Pa = 100 kPa 1 atm = 1.01325 x 10 5 Pa = 101.325 kPa 1 atm = 760 torr 1 atm = 14.7 lbs. in -2 The units for reporting pressure are torr, atmosphere, and (the SI unit) pascal. Pressure units can be interconverted by using the information above. THE GAS LAWS 4.4 – Boyle’s Law Boyle’s Experiment (J-tube) o Poured mercury into tube, trapping air in the short end of the J o More mercury he added, more air was compressed o Conclusion? = volume of a fixed amount of gas (O 2 in experiment) decreases as the pressure increases o Isotherm – curve of Boyle’s law For a fixed amount of gas at constant temperature, volume is inversely proportional to pressure V ~ 1/P Volume ~ 1/Pressure Pressure exerted by a given amount of gas at constant temperature is inversely proportional to the volume to which it is confined P ~ 1/V Pressure ~ 1/Volume Boyle’s Law can also be written as… PV = constant Visualizing with the molecular model… o As a gas is compressed, molecules are confined to a smaller volume
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This note was uploaded on 02/25/2012 for the course CHEM 2142 taught by Professor Roberts during the Fall '10 term at Texas State.

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