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class7 - a gas “ideal?” ± “Think Molecules”...

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Class #9 Introduction to Gases CHEM 107 A. Bengali Texas A&M University

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Gases ) All gases have several properties in common ) Relatively low density ) Compressible ) Expand to fill any container
Ideal Gases ) Gas Law: PV = nRT ) Originally based on empirical observations ) Can also be derived mathematically from the kinetic theory of gases

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Pressure ) Dimensions of force/area ) SI unit is N/m 2 , or Pascals ) Other more common units: atmospheres: 1 atm = 101,325 Pa torr (mm Hg): 760 torr = 1 atm lbs/in 2 : 14.7 psi = 1 atm
Gas Constant - R ) Proportionality constant, found by experiment ) In SI units: R = 8.314 J mol –1 K –1 ) Other units: R= 0.08206 L atm mol –1 K –1 = 62.36 L torr mol –1 K –1

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Units ) Consistent units are ALWAYS required! ) Use any convenient units for P, V ) Then choose R so that units all match up
Temperature ) If T 0, then V 0 ) MUST use absolute T (Kelvin, NOT ° C) 0 ° C = 273.15 K V = nRT P

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) I’ll calculate moles in a balloon of N 2 . ) Use r=4”, or 10 cm ) V=4/3 π r 3 = 4 x 10 3 cm 3 =4 L n= PV RT = ( 1atm )( 4L ) ( 0 . 0821 L atm mol K )( 297K ) = 0 . 16 mol
What makes

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Unformatted text preview: a gas “ideal?” ± “Think Molecules” Properties of an Ideal Gas ) Huge # of molecules ) Point masses, no volume ) No forces between molecules ) Constant, chaotic motion ) Elastic collisions Gas Law = Limiting Case ) Any gas will behave ideally in the limits of LOW PRESSURE HIGH TEMPERATURE ) Explain this in terms of previous properties? Applicability of Ideal Gas Law ) OK for most gases at ordinary T, P ) MUST fail at some point, since solids and liquids exist. ) Deviations at high P, low T. Example ) A plastic bulb is filled with 760 torr of CH 4 at 25 ° C ) The bulb may burst if the temperature exceeds 2 atm ) If the bulb is heated, at what temperature would the pressure reach 2 atm? CH 4 Example ) What is constant? n, V, (R) ) Isolate these terms: P/T = nR/V = constant ) So: P i /T i = P f /T f or T f = P f T i /P i P f = (2 atm)(298 K)/(1 atm) = 596 K = 323 ° C...
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