Physical Characteristics of Gases

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$Holt Math Minnesota Test Prep Workbook for Grade 9$

$Math Skills: More Practice$

You can understand Dalton’s law in terms of the kinetic-molecular theory. The rapidly moving particles of each gas in a mixture have an equal chance to collide with the container walls. Therefore, each gas exerts a pressure independent of that exerted by the other gases present. The total pressure is the result of the total number of collisions per unit of wall area in a given time. (Note that because gas particles move independently, the other gas laws, as well as Dalton’s law, can be applied to unreacting gas mixtures.) PO2 = 0.12 atm 1 L container at 0 o C P N2 = 0.12 atm 1 L container at 0 o C P Total = 0.24 atm 1 L container at 0 o C Oxygen molecule, O 2 FIGURE 10-14 Samples of oxygen gas and nitrogen gas are mixed. The total pressure of the mixture is the sum of the pressures of the gases. Copyright © by Holt, Rinehart and Winston. All rights reserved. Nitrogen molecule, N 2 Oxygen molecule, O 2 Nitrogen molecule, N 2 PHYSICAL CHARACTERISTICS OF GASES 323 SECTION 10-3 Visual Strategy FIGURE 10-14 Point out that equal amounts of gas at the same temperature and volume have equal pressure. Application Dalton’s law of partial pressures has important applications for people who work at high altitudes, such as mountain climbers and pilots. For example, at an altitude of about 10 000 m (higher than Mount Everest), air pressure is about one-third of an atmosphere. The partial pressure of oxygen at this altitude is 0.3 atm × 0.21 (percent of O 2 in air) × 760 mm Hg, or less than 50 mm Hg. By comparison, the partial pressure of oxygen in human alveolar blood needs to be about 100 mm Hg. Thus, respiration cannot occur normally at this altitude, and an outside source of oxygen is needed in order to survive. Additional Example Problem 1. Calculate the partial pressure in millimeters of mercury exerted by the four main gases in air at 760 mm Hg: nitrogen, oxygen, argon, and carbon dioxide. Their abundance by volume is 78.08%, 20.95%, 0.934%, and 0.035%, respectively. Ans. N2 : 593.4 mm Hg O2 : 159.2 mm Hg Ar: 7.10 mm Hg CO2 : 0.27 mm Hg 323
SECTION 10-3 DEMONSTRATION Amount of Oxygen in Air Wet the inside of a test tube, and add iron filings until the tube is about one-tenth full. Gently shake the tube to disperse the filings onto the wet tube walls. Place a finger over the test tube mouth, invert the tube, and place it in a 250 mL beaker of water so that the mouth is 1 cm below the water’s surface. Clamp the tube in place. As the iron filings are oxidized, oxygen will be removed from the air. By the next day, the water level will have risen about one-fifth of the tube length, an amount equal to the volume occupied by the oxygen. Safety: Wear safety goggles during setup. Disposal: Throw rusty iron into the trash. Class Discussion Ask students what kinds of gases could not be collected by water displacement (gases that are soluble in or that react with water). Discuss other methods by which such gases can be collected. HANDBOOK CONNECTION The effect of gas pressures in a mixture is further discussed using the exchange of O 2 and CO 2 in the blood as an example. See Group 14 of the Elements Handbook. FIGURE 10-15 Hydrogen can be collected by water displacement by reacting zinc with sulfuric acid. The hydrogen gas produced displaces the water in the gas collecting bottle. It now contains some water vapor. SAMPLE PROBLEM 10-6 324 324 CHAPTER 10 Gases Collected by Water Displacement Gases produced in the laboratory are often collected over water, as shown in Figure 10-15. The gas produced by the reaction displaces the water, which is more dense, in the collection bottle. You can apply Dalton’s law of partial pressures in calculating the pressures of gases collected in this way. A gas collected by water displacement is not pure but is always mixed with water vapor. That is because water molecules at the liquid surface evaporate and mix with the gas molecules. Water vapor, like other gases, exerts a pressure, known as water-vapor pressure. Suppose you wished to determine the total pressure of the gas and water vapor inside a collection bottle. You would raise the bottle until the water levels inside and outside the bottle were the same. At that point, the total pressure inside the bottle would be the same as the atmospheric pressure, P atm . According to Dalton’s law of partial pressures, the following is true. P atm = P gas + P H2 O Water vapor, molecule, H 2 O Hydrogen gas molecule, H 2 Suppose you then needed to calculate the partial pressure of the dry gas collected. You would read the atmospheric pressure, Patm , from a barometer in the laboratory.To make the calculation, subtract the vapor pressure of the water at the given temperature from the total pressure. The vapor pressure of water varies with temperature. You need to look up the value of PH2O at the temperature of the experiment in a standard reference table like that in Table A-8 of this book. Oxygen gas from the decomposition of potassium chlorate, KClO 3 , was collected by water displacement. The barometric pressure and the temperature during the experiment were 731.0 torr and 20.0°C, respectively. What was the partial pressure of the oxygen collected? Copyright © by Holt, Rinehart and Winston. All rights reserved.
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Physical Characteristics of Gases

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