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# Ch5 notes - Chapter 5 PROPERTIES OF GASES THE GAS LAWS...

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Chapter 5: PROPERTIES OF GASES; THE GAS LAWS GASES: molecules very far apart. LIQUIDS, SOLIDS (CONDENSED PHASES): molecules touch: gas density << liquid / solid density Properties of Gases -Expand to fit available Volume (create a force on ALL sides of vessel : a pressure measurement ) -Compressible. -Volume & Pressure vary greatly with Temperature. Pressure. Definition: force per unit area. SI Unit: N/m 2 = 1kg.m -1 .s -2 = 1 Pascal (Pa) Standard atmospheric pressure is : 76 cm Hg = 760 mm Hg = 760 torr = 1 atmosphere = 101.325 kPa = 1.01325 bar (GIVEN ON EXAMS) Measuring Presure directly - use a barometer indirectly - use a manometer Units are the same in each case How a Barometer Works Gravity pulls down on Hg in column; atmospheric pressure pushes on pool of Hg to force it up the tube; forces balance; supports column of Hg P = ρ g h If atmospheric pressure drops , column will shorten . How a Manometer Works U tube with Hg. Height Difference gives pressure difference . Use barometer reading to get actual pressure THE GAS LAWS Use the following parameters: Pressure, P ; Temperature, T; Volume, V; Number of moles of Gas, n P,V do not always have to be in SI units AS LONG AS you are careful to read the question. T MUST ALWAYS BE IN KELVIN. Boyles’ Law: P 1 V 1 = constant (for constant n, T) PRESSURE is INVERSELY RELATED to VOLUME: Example: At 25 o C a sample of He has a volume of 4.00 x 10 2 mL under a pressure of 7.60 x 10 2 torr. What volume would it occupy under a pressure of 2.00 atm at the same T? Charles’ Law and The Absolute Temperature Scale: V 1 = constant (for constant n, P) T 1 VOLUME is PROPORTIONAL to TEMPERATURE in KELVIN EXTRAPOLATION of V-T plot backwards gives ABSOLUTE ZERO: ZERO KELVIN. 0 ° C is where water freezes - not absolute zero. 0K = absolute zero, all molecular motion ceases and ideal gas* volume tends to zero 0° C = 273.15 K *real gases liquefy before reaching zero K Example: A sample of hydrogen, H 2 , occupies 1.00 x 10 2 mL at 25.0 o C and 1.00 atm. What volume would it occupy at 50.0 o C under the same pressure? It is CRITICAL to convert ANY temperature to KELVIN!!

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Standard Temperature and Pressure A convenient reference point. Symbol: STP. Standard P 1.00000 atm or 101.3 kPa Standard T 273.15 K or 0.00 o C LEARN THIS!! Combined Gas Law: Combining Boyle’s and Charles’ Laws gives : P 1 V 1 = constant (for n constant) It is critical that T is in K!!! T 1 Example: If we have a 2.5L of a gas at 25°C and 1.5atm, and we increase the temperature by 100 °C and find the pressure is now 800 torr, what is the new volume? Example: A sample of nitrogen gas, N 2 , occupies 7.50 x 10 2 mL at 75.00C under a pressure of 8.10 x 10 2 torr. What volume would it occupy at STP? Example: A sample of methane, CH 4 , occupies 2.60 x 10 2 mL at 32oC under a pressure of 0.500 atm. At what temperature would it occupy 5.00 x 10 2 mL under a pressure of 1.20 x 10 3 torr?
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Ch5 notes - Chapter 5 PROPERTIES OF GASES THE GAS LAWS...

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