Lecture_2_082808 - Lecture 2 August 28, 2008 Chapter 1 The...

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Page 1 of 11 Lecture 2 – August 28, 2008 Chapter 1 – The Properties of Gases gas - a fluid state of matter with large separations between atoms fluid - takes the shape of its container pressure - Force/unit area = Force/length 2 = Pa = N•m -2 = kg•m•s -2 = mass/(length x time 2 ) How is it measured? Sealed Manometer F app = m L g p app A = m L g p app A = V ρ g p app A = Ah ρ g (cylinder) p app = ρ gh For the case where p app is actually a vacuum pump (suction), one could achieve a negative height. You will see this in later organic chemistry laboratory experiments. Open Manometer F net = m L g (p app - p ext )A = m L g (p app - p ext )A = V ρ g (p app - p ext )A = Ah ρ g (cylinder) p app = p ext + ρ gh right left
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Page 2 of 11 Size Does Not Matter!!! - This is a question I frequently receive, "Does A in the derivation of the manometers matter?" The answer is no!!! Area = "A" will correspond to the tube with the smallest cross-sectional area (or some fraction thereof). Why??? To understand this we need to think back to physics class. As the liquid throughout the manometers is stationary, there is no net force on any volume element. There are also no lateral forces. Hence the only force that needs to be considered is gravity as we did in the derivation. As gravity acts downward, we need to ask ourselves, what is holding up a given volume element. Consider the picture below where I have highlighted the downward forces in either blue or red. Note how all of the blue lines terminate at the wall of the manometer. In contrast, I have drawn the red lines from each side to cancel in the middle of the bottom of the manometer. In essence, we only need to consider the smallest cross-sectional area that is continuous all the way through the container as any other force line will hit a wall. Temperature
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This note was uploaded on 09/29/2008 for the course CHEM 3615 taught by Professor Aresker during the Fall '07 term at Virginia Tech.

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Lecture_2_082808 - Lecture 2 August 28, 2008 Chapter 1 The...

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