Gases_part1

# Gases_part1 - GASES AND THE KINETIC-MOLECULAR THEORY The...

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G ASES AND THE K INETIC -M OLECULAR T HEORY The Three Physical states: Solids, Liquids, and Gases Matter is arbitrarily classified into three physical states: solids, liquids, and gases. The volumes of gases change greatly as temperature and pressure change. Volumes of solids and liquids change very little with changes in temperature and pressure. Gases are less dense than solids and liquids. At room temperature, the following elements are gases: H 2 , O 2 , N 2 , F 2 , Cl 2 , He, Ne, Ar, Kr, Xe, and Rn. Some Observations on Gases 1. Gases exert pressure on their surroundings. 2. Gases can be compressed. 3. Gases can expand without limit. 4. Gases diffuse into each other, i.e., they are miscible (mix with each other) in all proportions unless they react chemically. These observations suggest that molecules of gases are widely separated and are in random motion. These observations are valid for all gases, regardless of identities! A sample of any gas can be described in terms of four parameters: its volume, temperature, pressure, and the number of molecules (moles) of gas present. 1

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Pressure Pressure is defined as force per unit area. It may be expressed in many units. The mercury barometer is a convenient device for measuring atmospheric pressure. The average pressure at sea level is referred to as standard pressure. Standard pressure is 76 cm of mercury, 760 mm of mercury or 760 torr, and is referred to as one atmosphere of pressure. 1 atmosphere = 76 cm Hg = 760 mm Hg 1 atmosphere = 1.013 x 10 5 Pascals (Pa) = 101.3 kilopascals (kPa). Boyle’s Law: The Pressure-Volume Relationship Boyle’s Law states: At constant temperature, the volume occupied by a definite mass of a gas is inversely proportional to the pressure applied to the gas . V α 1/P or V = k/P or PV = k P 1 V 1 = P 2 V 2 (n, T constant) Example : At 25 o C a sample of helium occupies 400 mL under a pressure of 760 torr. What volume would it occupy under a pressure of 2.00 atmospheres at the same temperature?
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• Fall '09
• MCCLELLAND

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