Type:
Instructor Manual
Resource:
Chemistry
Edition:
7th Edition
Author(s):
John E. McMurry Robert C. Fay Jordan Fantini
Instructor’s Resource Manual
LABORATORY MANUAL Sandra Chimon Rogers Calumet College of St. Joseph With Contributions by Stephanie Dillon, Florida State University Barbara Mowery, York College of Pennsylvania
CHEMISTRY McMURRY • FAY • ROBINSON SEVENTH EDITION
Experiment 1: Measurement and Expression of the Experimental Data Instructor Notes and Lab Preparation Each fall and spring semester a week after school starts, things are hectic, students are still registering for classes, and some sort of holiday interrupts the first week of labs. This allows the instructors only to check in the labs but then have an off week before the students are able to start performing experiments. Rather than allow the students to waste a full week of lab time, assigning this math and graphing review as the first experiment of the semester is a great way to refresh their minds and get them back into the swing of things. Part A is designed for the first semester, and Part B is designed for the second semester of general chemistry. As the check-in process is generally quick, at the end of the check-in day, I have each of my teaching assistants give a short lecture on the math involved.
Particular Points to Stress Scientific or exponential notation is used so often in scientific literature and calculation; it seems we sometimes forget how confusing it can be to someone dealing with it for the first time. I always stress that students should look at the values they calculate to make sure they make sense. For example, one of the most common mistakes students make when dealing with exponents is when they must change an exponent to add or subtract two exponential values. The process requires students “move the decimal” in the correct direction to “match up” the exponents. The most common mistake is the movement of the decimal in the wrong direction. So ask the students to check and make sure that the number resulting after the move still makes sense, that is, does it have the same value it did before the move? Simply plugging it into a calculator should do the trick. Another sticky point with many students is an overall lack of graphing skills. I have placed a brief Excel® tutorial at the end of the manual to encourage students to use this program or one similar to it to complete the graphing exercises. If you choose to have your students complete the graphs by hand, I understand the reasoning, but I would encourage you to consider asking for the graphs in both styles. The reality of the world today is that computers are everywhere. Skillful use of technical programs like Excel® is a must for scientific analysis. The most common mistakes that should be discussed with respect to graphing are (1) attempts to fit straight lines to data that do not fit y = mx + b and (2) irregular spacing of values on an axis. I often see graphs with axes that are marked at regular intervals with the values given as part of the problem. Markings of this sort always generate perfectly straight lines, which thrill the students, but give me migraines. Last but not least, the discussion we have about logarithms and the proper use of significant figures in logarithms is generally the first exposure students will have to the subject. Special attention should be paid to the introduction of this material.
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Lab Report Key Exercises 1A 1) Indicate how many significant figures are present in each of the following: (a) 01.501 × 103 4 (b) 700.01 5 (c) 193005 6 –4 (d) 6.010 × 10 4 (e) 0.070 2 2) Round the following to three significant figures (put in scientific notation where necessary): (a) 151626 1.52e + 05 (b) 0.00844 8.44e – 03 (c) 0.000010350 1.04e – 05 (d) 1255 1.26e + 03 (e) 33.353 33.4 3) Perform the following mathematical operations: (a) (6.49 × 105/2) – 45.38 = 3e + 05 (b) 5.32 × 10–15 × 2.447 × 1016 = 1.30e + 02 (c) (3.15 × 10–1 – 9.7 × 10–3) × [(4.7 × 10–2)/(2.33 × 10–3)] = 6.2 (d) 1.62 × 10–3 + 8.1 × 10–4 = 2.43e – 03 (e) (0.45 + 167.6)/1.6 × 10–2 = 1.1e + 04 4) Express the following using scientific notation: (a) The Bohr radius: 0.0000000000529189379 m 5.29189379e – 11 m (b) The speed of light: 29979245.8 m/s 2.99792458e + 08 m/s (c) Charge of a proton: 0.0000000000000000001602176487C 1.602176487e – 19 C (d) Rydberg constant: 10973731 m–1 1.0973731 e+ 07 m–1 5) Calculate the following and round to the proper number of significant figures: (a) Antilog 0.485 = 3.05 (b) Ln 60 = 4.09 (c) e–1.41 = 2.4E–01 –2 (d) Log 3.975 × 10 = –1.4007 (e) Antilog 6.86 = 7.2e + 06 (f) Log 8.74 = 0.942 6) Calculate the average and standard deviation for the following set of calibration data: 3.8842, 3.8611, 3.8263, 3.8191, 3.8945, 3.8916, 3.8511, 3.8651 average = 3.8616 STDev = 0.0285
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7) Graph the following data and answer the questions about the resulting line. Temperature (K) 273 283 293 303 313 323 333 343 353 363 373
Pressure (atm) 1.10 1.14 1.18 1.22 1.26 1.30 1.34 1.38 1.42 1.46 1.50
Q1: According to the graph, how would you describe the relationship between temperature and pressure (linear or nonlinear)? linear 8) Create a graph of the absorbance (optical density) versus molar concentration and answer the questions about the resulting line.
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Concentration (M)
Optical Density (AU)
0.13
0.19
0.28
0.38
0.43
0.56
0.58
0.72
0.73
0.85
0.88
0.96
1.03
1.06
1.18
1.13
1.33
1.18
1.48
1.21
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NOTE: The absorbance, or optical density, of a solution for a particular wavelength of light is given by log(Io/I), where Io is the incident light on a sample and I is the transmitted light. Beer’s law states that the optical density of a substance in solution is directly proportional to its concentration (i.e., it has a linear relationship). Not all substances follow Beer’s law, but a calibration graph of optical density versus concentration can still be used to measure concentrations of unknown solutions. Because optical density is a ratio, it does not have any units; the expression absorbance units is sometimes used. Q1: Does the solution in your graph follow Beer’s law? no Q2: What is the concentration of a solution that has an optical density of 0.28? 0.20 M Q3: What is the concentration of a solution that has an optical density of 1.01? 0.95 M 9) Using the same data as above, calculate the logarithm of the volume. Graph the pressure versus the log10 volume and answer the questions about the resulting line.
Q1: If this graph is a straight line, then it is of the form P = mLog10V + b, in which m is the slope, and b is the y-intercept. Is the graph a straight line or a curved line (nonlinear variation)? straight line with some slight deviation; see fit below Q2: If possible, calculate the equation of the line for the graph. Show your work. P = –2507.3LogV + 3361.7 R² = 0.9903 Q3: Using the resulting equation from question 2, what would be the volume of a system that had a pressure of 5.4 atm? 0.51 mL
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10) Robert Boyle used an open-end barometer (similar to that in the figure) for the experiments, which led to the discovery of Boyle’s law, which describes mathematically the effect of pressure on the volume of a gas. He determined the nature of the law by graphing his data. The law states that PV = constant, where P is the total pressure, which is the sum of Patm + PHg. The pressure can be measured in torr, which corresponds to the height of the mercury column in millimeters. The following table contains some data in an experiment similar to that of Boyle’s, though the open-end manometer is considerably larger. The volume of the gas in the closed end of the tube is measured in milliliters, and the pressure recorded is PHg, the height of the column, not the total pressure.
Pressure (torr) 45 149 226 303 372 455 523 607 720
Volume (mL) 21.7 19.1 17.6 16.3 15.3 14.2 13.5 12.6 11.7
You are to analyze this data by graphing it, discovering that a direct graph (V vs. P) gives a curved line. Scientists often try manipulating data to give a straight line, and when the data is regraphed as P vs 1/V, you will see that a straight line is produced, revealing the nature of Boyle’s law (that pressure and volume are reciprocally related). Further analysis of this straight line will also reveal the atmospheric pressure at which the measurement is made. Since PtotalV = constant Ptotal = constant(1/V) Patm + PHg = constant(1/V) PHg = constant(1/V) – Patm the graph is of the form y = mx + b, where the slope, m, gives the value of the constant, and the P-intercept is the negative of the atmospheric pressure.
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