CSUS Department of Chemistry Experiment 7 Chem.1A

Transcription

1 EXPERIMENT #7 Gas Laws PRE-LAB ASSIGNMENT Name: Lab Section: 1. An expandable container of gas maintained at constant temperature has an initial volume of L at a pressure of 762 torr. On a stormy day, the volume changes to L. What is the pressure at these new conditions? 2. At 60 C, the length of an air column in a Charles law capillary apparatus is 45.3 mm. What will the length be at 30 C? 3. If the barometric pressure is 756 torr, and mercury at points A and B in a Boyle s law apparatus are 54.2 and 12.1 cm, respectively, what is the pressure of the air in the closed tube (between points C and B)? Refer to the diagram of the apparatus in the lab text. Page 1 of 6

2 Experiment 7: Gas Laws I. INTRODUCTION: When the pressure of any gas is sufficiently low or the temperature sufficiently high, the gas will behave ideally. This behavior is governed by the ideal gas equation of state: (1) PV = n RT where: P = pressure (atm) V = volume (L) T = temperature (K) Latm n = moles and R = gas constant = mol K Two of the primary observations, which led to the development of this equation, were the following: a) Boyle s Law: For a fixed number of moles of an ideal gas maintained at a constant temperature, the pressure is inversely proportional to the volume. That is: (2) K B 1 P or P = V V (3) P V = K B Where the value of K B (Boyle s law constant) is a function of the temperature and quantity of gas. b) Charles s Law: For a fixed number of moles of an ideal gas maintained at a constant pressure, the volume is directly proportional to the absolute temperature. In other words: V (4) V T or = KC T Where the value of K C (Charles s law constant) is a function of the pressure and the quantity of gas. II. EXPERIMENTAL: (please work in pairs) Teflon rod Page 2 of 6

3 (a) Boyle s Law: The Boyle s law devices will be located in the hoods. Please do not remove them, as they are loaded with a significant amount of mercury. Please take care when using the apparatus it is somewhat fragile and spills of mercury are difficult to clean up. Prolonged exposure to mercury is hazardous and so please wear one of the gloves provided when touching the Teflon rod that is part of the device. For more information on Hg or any other reagent used in the laboratory, please consult the MSDS terminals located on each floor of Sequoia Hall. First, record the barometric pressure of the laboratory using the barometer in the balance room. Your instructor will give a brief demonstration. Using one of the Boyle s law devices, record levels (in mm) at points A, B, and C (refer to the diagram on the previous page). Carefully lower the Teflon rod by pushing down gently on the mercury column to a point, then hold the rod steady. You will note that levels A and B both rise. Record six sets of A and B levels (remember, C is constant). For the first set, do not press down on the Teflon rod, simply record the levels as is. In this experiment, we relate the distance between points C and B to the volume of air trapped in the corresponding tube. Since the volume of a cylinder is proportional to the length, this correspondence is valid. (b) Charles s Law: The Charles s law devices will be located at the instructor s desk. Please be gentle, as they are loaded with mercury and are somewhat fragile. These devices are not thermometers; please do not attempt to shake the Hg down. Immerse the device in water baths at five different temperatures, in addition to room temperature. Measure the exact temperature using your laboratory thermometer. 1. ~ 0 C (ice water) 2. Room temperature ~20.0 C (no bath needed, use the temperature of your thermometer in your drawer) 3. ~ 40 C 4. ~ 60 C 5. ~ 80 C 6. ~ 100 C (boiling water) At each temperature, record the corresponding ruler reading at points D and E in mm. Note: Immerse as much of the device as possible, but DO NOT submerge completely. Water MUST NOT enter the open end of the capillary. Be sure that you use the same apparatus for all your readings. Page 3 of 6

4 Data and Calculations: (turn in this page and the following only) Name: Lab Section: Data: (a) Boyle s Law: Barometric Pressure (mm Hg): Top of closed tube (point C in mm): Measurement at point A (mm) Measurement at point B (mm) A B Pressure* C B *(A B + barometric pressure) (b) Charles s Law Temperature ( C) Temperature (K) Point D, bottom of Hg (in mm) Point E, end of capillary (in mm) D E Instructor date and initials: Page 4 of 6

5 CALCULATIONS: (a) Boyle s Law Equation (3) states that the product of the pressure in the closed tube and the volume of the air trapped in the tube (C B) should be a constant. Calculate the six P V products from the data you collected and the average (indicate the UNITS of each product): Average P V product: K B = (show units) Equation (2) tells us that a plot of pressure in the closed tube on the y-axis against 1/(C B) on the x-axis should be a straight line with slope = K B. The y-intercept should equal zero. Construct such a plot and find the slope and intercept using Excel or a comparable graphing program. The slope and intercept are found by fitting a trend-line to your data. (we will demonstrate this in lab) (show units for both) Slope from graph = y intercept: Calculate the % difference between your average K B value and the K B determined from the slope of your plot. Does your y intercept come close to zero? Do you feel that your data behaves in accordance with Boyle s law? Explain why or why not. Page 5 of 6

6 CALCULATIONS: (b) Charles s Law In part (b), we once again measured a quantity proportional to volume (D E). Equation (4) tells us that (D E) T should be a constant (K C ), where T is the absolute temperature. Calculate the six V ratios and their T average: Average V T ratio: K C = (show units) Now construct a plot of (D E) on the y-axis against temperature in C. Equation (4) tells us that a plot of (D E) against temperature in C should be a straight line with x-intercept = 273 C. What value of absolute zero do you obtain when you extrapolate the line to a value of (D E) = 0? Also, generate a second plot and determine the temperature at (D E) = 0 by fitting a trend-line to your data using Excel or a comparable graphing program. (show units for both) Slope from graph = y intercept: Calculate the % difference between your average K C value and that obtained from the second plot. How well does your data obey Charles s law? Explain. NOTE: You will turn in a total of 3 graphs for this experiment using Excel or some other graphing program: one graph for Boyles law and two(2) graphs for Charles s law. Page 6 of 6