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Chapter 5

# Chapter 5 - Chapter 5 Gases The State of a gas is defined...

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The State of a gas is defined by Pressure(P), Volume(V) and Temperature(T ) Volume is expressed in L, mL or cc Pressure : Molecules in a given container are in constant motion and collide with the walls of the container and among themselves. Force exerted during these collisions is related to pressure. vacuum 760 mm Hg Liquid Hg Torricelli’s experiment (1608-1647): fill a capillary tube with Hg, close the open end with thumb, invert it into a beaker of Hg and release the thumb. Pressure exerted by outside air Pressure exerted by Hg column Pressure exerted by Hg column in the tube = pressure exerted by outside air vacuum Chapter 5: Gases

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Force(F) = mass (m) x accelaration due to gravity(g); F = mg So, P= F/A= mg/A But, Area (A) x Height(h) = Volume (V); Ah = V or A = V/h Then, P=mg/(V/h) = mgh/V But m/V=d, density; Therefore, P = dgh Pressure exerted by Hg column of height h If some other liquid is used, in place of liquid Hg, then density of Hg will be replaced by density of that liquid and height of Hg column will be replaced by the height of that liquid column. Pressure exerted by Hg column of height h= density of Hg x accln due to gravity x height of mercury column For a given pressure, d 1 gh 1 =d 2 gh 2 , subscripts 1 and 2 represent two different liquids Pressure(P) = Force (F) Area (A)
At 0 o C, d Hg = 13.5951 g/cc At sea level,under normal conditions, h = 760 mm Hg = 0.760 m Hg P = (13.5951 g/cc)(9.80665 m/s 2 )(0.760 m) = 101.325 g.m 2 /cc.s 2 1 g= 1g x (1kg/1000g)=10 -3 kg P=(101.325)(10 -3 kg) m 2 / (10 -6 m 3 .s 2 )= 1.01325 x10 5 kg. m -1 .s -2 Therefore, 760 mm of Hg column corresponds to a pressure of 1.01325 x10 5 Pa = 1.01325 bar (1 bar = 10 5 Pa) A pressure of 1 atm supports 760 mm Hg at 0 o C. 1 atm = 760 mm Hg = 1.01325 x10 5 Pa = 1.01325 bar 1 torr = (1/760) atm 1 torr = 1 mm Hg at 0 o C Units for Pressure: atm, pascal, torr or bar 1 atm =760 torr

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Measurement of Atmospheric pressure : Barometer, a device used to measure atmospheric pressure Open U-tube : Pressure exerted by air on both sides of open U-tube is the same. So the mercury level on both sides of open U-tube will be the same air air mercury air Remove air in this column J-tube with one end evacuated : Mercury height in the right column decreases and that in the left column increases until the height difference between the two exerts a pressure equal to that of atmospheric pressure. Weather report provides barometric pressure in, normally, inches. 760 mm Hg air mercury Note : The pressure exerted by a column of liquid is given as P=dgh. The height difference for two different liquids is related as d 1 gh 1 =d 2 gh 2 . If water was used in place of mercury, then the Hg Hg H2O H2O H2O Hg Hg H2O -1 evacuate left side Closed end
To pump water out of a shallow well, insert a open ended tube in to the well and attach a pump to the top end to create vacuum. Atmospheric pressure Suck air or create vacuum Pressure in action: Blood pressure When you get your blood pressure reading in the hospital, a inflatable cuff is wrapped around your hand and is pressurized. The pressure in the cuff stops the blood flow in your hand. As the pressure in the cuff is released,

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Chapter 5 - Chapter 5 Gases The State of a gas is defined...

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