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CH 301 Chapter 4 notesv2

CH 301 Chapter 4 notesv2 - Chapter 4 PROPERTIES OF GASES...

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Chapter 4: 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 -Expands to fit available Volume (creates a force on ALL sides of vessel described by 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) Other Units: lb/in 2 = 1 pound per square inch = 1 PSI, cm or mm Hg, torr, atmospheres Atmospheric Pressure: Pressure exerted by Earth’s atmosphere on a given day in a given location. Measured directly. Standard atmospheric pressure is : 76 cm Hg = 760 mm Hg = 760 torr = 1 atmosphere = 101.325 kPa = 1 bar 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 Q. T MUST ALWAYS BE IN KELVIN. Boyles’ Law: P 1 V 1 =P 2 V 2 (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 = V 2 (for constant n, P) T 1 T 2 VOLUME is PROPORTIONAL to TEMPERATURE in KELVIN
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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!! 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 = P 2 V 2 (for n constant) T 1 T 2 REMEMBER: it is critical to make sure that T is in K, and V and P are given in the same units before and after the change. 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?
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CH 301 Chapter 4 notesv2 - Chapter 4 PROPERTIES OF GASES...

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