Chemistry 130 Pre-lab Titrations: Part II

Transcription

1 Chemistry 10 Pre-lab Titrations: Part II Backgroun We are going to treat the titration that you performe last week as a job interview for the chemistry ivision of a pharmaceutical company. The calculations that you o in this pre-lab exercise will provie you with an analysis of how well you i in the interview an help the company ecie if it shoul hire you. In the experiment/job interview last week your goal was to etermine the concentration of a soium hyroxie solution. To o this, you titrate a M H 2 SO solution with NaOH. You knew both the molarity an volume of the H 2 SO solution that you starte with an you use that information to fin the concentration/molarity of the NaOH solution. Like any goo scientist, you knew one measurement was not enough, so you performe at least three titrations. Ieally, the molarity of NaOH you obtaine for each trial shoul be the same. This woul make sense as all of the NaOH came from the same container. The metho the company is going to use to evaluate your skills uring the job interview is to compare the closeness of your three values to each other an also by comparing your average NaOH concentration to the true NaOH concentration. You are going to have a secon job interview with the same company this week. This job interview will also involve a titration. Whether or not you get hire epens on the titrations you i last week an to a larger extent on the titrations that you o this week. In the sections that follow, you are going to analyze your titrations the same way the company will. Base on your analysis of the titrations from last week, you will know if you nee to change things or if you just nee to keep up the same goo work. Analyzing Your Results When experimental values are obtaine in the laboratory it is important that we know how meaningful or goo our measurements are. If possible, we want to etermine how close our experimental value or the average of our experimental values is to what the value shoul be, i.e. the true value. If the true value is known, we can etermine how accurate our experimentally obtaine value is by calculating the absolute an percent relative errors. As we have alreay one these types of calculations multiple times this semester we won t elaborate any more here. If the true value is not known, we nee some other way of etermining how goo our measurements are. In this case, one metho for eciing the merit of our experimentally obtaine values is by etermining the precision of our measurements. Precision is a measure of the closeness of a set of experimentally obtaine values to each other. The closer the values are the better the precision of the experiment. While goo precision oes not ensure that your results are accurate, in the absence of knowing a true value it provies an acceptable metho for gauging the worth of your experimentally obtaine values. It is assume that if your precision is poor,

2 your results are probably not accurate. On the other han, if your precision is goo, it is assume that your results are probably accurate. In the text that follows, the calculations use to etermine precision will be iscusse. Aitionally example problems will be given so that you get practice calculating precision. At the en of this pre-lab exercise, you will calculate the precision associate with your titrations last week. Calculating Precision When trying to etermine the precision associate with a set of experimentally obtaine values, you must perform a series of calculations. These calculations are outline below an an example problem is given. Absolute Deviation, : is the absolute value of the ifference between either two values, e.g. A an B, or a value an some average (or mean). average (or mean) X A B or X B Percent Relative Average Deviation from the Mean, % ra: This is the calculation/tool that the chemical company will use to best assess your precision. Although the length of this term is long an may seem overwhelming, it you take it a wor at a time, you ll see you alreay know how to calculate it. You will calculate the absolute eviation of each measurement from the mean, average them, making the average eviation relative (iviing by the average) an convert to a percent (multiplying by 100). The average eviation an %ra equations are shown below. The average eviation equation is shown for an example where there are four measurements. If there are more or less measurements, your average eviation equation will have more or less terms in the numerator. average eviation ( ) X - Measurement 1 X - Measurement 2 X - Measurement X - Measurement average eviation % ra x100% % ra x100% average X In the example below, you will calculate the average, average eviation, an the percent relative average eviation for a sample experiment.

3 Example The mass % of oxygen in calcium oxie was measure four times uring an experiment. The four experimentally obtaine values are shown in the table below. Trial # Mass % % % 55.95% 56.2% The precision of the experiment is calculate below. First the average ( X ) is calculate, then the average absolute eviation ( ) is calculate an finally the percent relative average eviation (%ra) is calculate. a) Calculation of the average X % % % % 22.0 % % % b) Calculation of the average eviation ( ) % % % % % % % % % % % % % 0.08 % c) Calculation of the percent relative average eviation (% ra). % ra x100% X % % ra x 100% 0.1 2% 0.1% %

4 Analysis of Results You may be asking yourself, What oes the magnitue of the %ra tell the pharmaceutical company about my results? In all experiments, the lower the percent relative average eviation the better. You can use the table below to analyze the percent relative average eviations calculate in the example problems an for your titrations. % ra Quality of Precision Will you be hire? 0.5% Excellent If your % relative error is also low, you will be hire with a very large signing bonus. 1.0% Very Goo If your % relative error is also low, you will be hire with a moerate signing bonus 1.5% Goo If your % relative error is also low, you will be hire. 2.0% Acceptable If your % relative error is low, you may be hire but without a signing bonus an only if no-one is available who got a lower %ra.0% Not Great Even if your % relative error is also low, you probably won t be hire..0% Not Acceptable You probably won t be hire. As you can see from the table above, having goo precision is not the only important factor. In aition to having goo precision, you must also be accurate. Precision is only half the story. If you are precise but not accurate, that means you are oing something wrong in your titrations an you are oing the same thing each time. On the other han it is possible to be accurate but not precise. If that is the case, your potential employer might attribute your accuracy to luck rather than to skill in the laboratory. The percent relative error calculation is shown below. A goo percent relative error is generally less than 2.5%. Any % r.e. higher than.5% woul be below average. The tolerance for the percent error is a little higher than the %ra because it is easier to be consistent than it is to be accurate. true value - your measurement % r.e. x100% true value

5 Practice Problem (The solutions are on the next page of this ocument) A stuent prepare an stanarize a solution of soium hyroxie in CHEM 180. The values she obtaine were: M NaOH, M NaOH, an M NaOH. 1. Calculate the average concentration in the space below. 2. Calculate the average eviation of the measurements in the space below.. Calculate the percent relative average eviation in the space below.. Assuming the true concentration of the soium hyroxie solution is M, what is the %r.e. of her average NaOH concentration? 5. Base on the answers to questions an, if you were in the position to hire a chemist, woul you hire this stuent?

6 Solutions to the practice problem A stuent prepare an stanarize a solution of soium hyroxie in CHEM 180. The values she obtaine were: : M NaOH, M NaOH, an M NaOH. 1. Calculate the average concentration in the space below M M M M X M M 2. Calculate the average eviation of the measurements in the space below M M M M M M M M M M M M. Calculate the percent relative average eviation in the space below M % ra x 100% 0. 05% 0.% M. Assuming the true concentration of the soium hyroxie solution is M, what is the %r.e. of their average NaOH concentration? M M M % r.e. x 100% x100% % 0.1% M M 5. Base on the answers to questions an, if you were in the position to hire a chemist, woul you hire this stuent? Yes, I woul hire her in a secon. I woul probably throw all sorts of money her way to ensure that she woul join my company. She not only ha an excellent % ra, but her % r.e. was also awesome!

7 Pre-Lab: Aci-Base Titrations Part II Name 1. In the space below, list the three calculate NaOH concentrations you obtaine from your work last week. Successful Trial #1 Successful Trial #2 Successful Trial # 2. Calculate the average NaOH concentration for your three goo titrations in the space below.. Calculate the average eviation of the three concentrations in the space below.. Calculate the percent relative average eviation in the space below. 5. The true concentration of the soium hyroxie solution you use was M, what is the %r.e. of your average NaOH concentration? 6. Base on the answers to questions an, if you were in the position to hire a chemist, woul you hire yourself? If your % ra is high, but you ha a goo faint pink color as your en point, it probably means you were not reaing the volumes accurately. Have me or your neighbor ouble check your first few measurements this week. If your % ra is low, but your % r.e. is high, it probably means you were reaing the volumes incorrectly but you were consistent in your errors.