Using the Ideal Gas Law

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Dec 6, 2023

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Using the Ideal Gas Law Experiment 2 Site: Hayden-McNeil Online Course Course: Temple University - CHEM I (1033), Spring 2022 Book: Using the Ideal Gas Law Printed by:Rayna George Date: Monday, April 4, 2022, 9:59 AM
Table of contents Additional Reading and Abstract Background Safety/Hygiene/Waste Disposal Procedure Suggested Setup for Lab Notebook Post Lab Report Questions
Using the Ideal Gas Law Additional Reading The concepts in this experiment are also discussed in sections 8.1–8.3 of your lecture textbook OpenStax Chemistry: Atoms First 2e. Abstract The purpose of the experiment is to verify the stoichiometry of a chemical reaction between a metal and an acid using the Ideal Gas Law. The metal sample will be reacted with an excess of aqueous 6.0 M solution of hydrochloric acid, which will result in the production of hydrogen gas. The gas sample is collected over water, and then the moles of gas calculated by applying Dalton’s Law of Partial Pressures and the Ideal Gas Law. The mole ratio of metal to hydrogen gas will be compared with the ratio expected from the balanced chemical reaction. In addition, an unknown metal sample will be supplied and its identity will be determined using the moles of hydrogen gas, the mass of the metal and information regarding the reaction stoichiometry.
Background Equal volumes of all gases, measured at the same temperature and pressure, contain equal numbers of particles. Amadeo Avogadro proposed this hypothesis in 1811. Stanislao Cannizzaro came upon Avogadro’s hypothesis nearly 50 years after it had been proposed. He realized that this hypothesis could be used to determine the molar masses of gaseous elements and compounds. If equal volumes of gases contain equal numbers of particles, then the masses of those gas volumes should be in the same ratio as the molar masses of their constituent particles. The volume of gas chosen for comparison was the volume occupied by one mole of a substance. However, the volume occupied by a mole of gas depends on the temperature and pressure of the gas. Therefore a standard temperature and pressure were chosen. Standard temperature and pressure (STP) are 273 K and 101.3 kPa (1.00 atm). At STP the volume occupied by one mole of a gas is 22.4 L, the standard molar volume. Experiments on gases by chemists led to the ideal gas law, PV = nRT, where P is the pressure in atm, V is volume in liters, n is the moles of gas, and T the temperature in Kelvin. R is the gas constant which can be calculated as follows: The ideal gas law can be used to calculate the number of moles of a gas if we are given the pressure, volume, and temperature of the gas. The gas constant, R = 0.0821 (L atm)(mol K ) is used for any pressure, volume, temperature and moles of gas. For example, a sample of CO (g) has a pressure of 735 mmHg and occupies a volume of 250.0 mL at a temperature of 20.0 C. We can calculate the moles of the gas but we must convert pressure to units of atm (1.00 atm = 760 mmHg), volume to liters, and tempertaure to Kelvin: T = 20.0 °C = 20.0 + 273 = 293 K R = = = ) PV nT (1.00 atm)(22.4 L) (1.00 mol)(273 K) 0.0821 (L atm)(mol - 1 K - 1 –1 –1 2 o P = 735 mmHg × = 0.967 atm 1.00 atm 760 mmHg V = 250.0 mL × = 0.2500 L 1 L 1000 mL
In this experiment you will react a known mass of a metal with an excess of hydrochloric acid and collect the generated hydrogen gas over water in a buret that is plugged by a stopper at the top. The evolved gas will rise to the top of the water-filled tube, displacing an equal volume of water. Since the collected hydrogen gas will be saturated with water vapor and at conditions other than at a pressure of 1.00 atm, adjustments must be made to the observed pressure. Dalton’s Law of Partial Pressures states that the total pressure of a gas mixture is equal to the sum of the partial pressures of each gas. In this experiment the pressure of the gas collected in the buret, P , will be: P = P P or P = P P The quantity, P , is related to the atmospheric pressure, P (which varies daily and will be given to you by your instructor), and the difference in levels of water inside and outside the buret. This difference in levels, Δ h, can be measured using a ruler. We need to convert the difference in water levels to an equivalent difference in mercury levels. The difference is divided by 13.6, because mercury’s density is 13.6 times greater than water’s. Since the level of water inside the buret is higher than the level outside the tube, Δ h is divided by 13.6 and must also be subtracted from the atmospheric pressure: to give: The vapor pressure of water, P , varies with temperature as seen in this table. Note the temperatures can be read to 1 decimal place in the table. For example, the vapor pressure of water at 15.0 ºC is 12.8 mm, at 15.2 ºC is 12.9 mm, etc. Vapor Pressure for Water, mmHg Temperature (ºC) 0.0 0.2 0.4 0.6 0.8 15.0 12.8 12.9 13.1 13.3 13.5 n = = = 1.00 × (g) PV RT (0.967 atm)(0.2500 L) )(293 K) (0.0821 atm.L.mol -1 .K -1 10 -2 mol CO 2 Total Total H2 + Water H2 Total Water Total Air = - P Total P Air h 13.6 = - - P H2 P Air h 13.6 P Water Water
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