Unformatted text preview: Experiment 2 – Scientific Measurement and Error Fall 2009 Name TA Type name Type TA's name Lab Day and Time Section Type day & time Type section number Documentation List references here (Lab Partners, Lab Manual, Textbook, etc.) Grading Rubric
Possible Points Materials and Methods • Reference lab manual • Only describe deviations Results and Discussion • Insert titles for tables • Showed complete sample calculations • Summarized all data in tables • Answered questions correctly • Uses experimental data to clearly expresses differences between accuracy and precision • Error analysis Laboratory Technique 10 Points Received 70 20 TOTAL (100) TA Comments/Suggestions: CHEMISTRY 101L REPORT TEMPLATE
Scientific Measurements and Error EXPT. 2 Introduction The purpose of this report is to determine the accuracy and precision of common scientific equipment. The purpose of the experiment is to learn how scientific measurements are carried out properly using an electronic balance and various pieces of volumetric glassware, and to acquire an appreciation of the significance of measurements. Materials and Methods The procedure for this experiment was taken from reference the lab manual. Describe the procedure for calibrating the Pasteur pipet. This should be written in past tense using clear paragraph organization following the guidelines for writing experimental procedures. Results and Discussion The goal of the first part of the experiment is to determine the choose one: accuracy or precision of the electronic balances in the lab. The data used for analysis are summarized in Table 1. Table 1. Insert title for the table. Sample Weighing bottle with lid Weighing bottle without lid Weighing bottle lid only Mass (g) Mass (g) Mass (g) Mass (g) Trial 1 Trial 2 Trial 3 Trial 4 Weighing bottle with lid on different balance One way to express the choose one: precision or accuracy of a measurement is through standard deviation. The standard deviation is a statistic that tells you how tightly the measurements are clustered around the mean in a set of data. In other words, it is a measure of the spread of the data. The mean is calculated as follows:
_ x= sum of results = number of results (sum all acceptable measured values) = insert number of measured values calculated value The deviations from the mean are
_ deviation = x i − x calculate deviation 1 calculate deviation 2 calculate deviation 3 calculate deviation 4 (if required) Standard deviation is calculated using the formula sum of the squares of the deviations from the mean s= = number of observations - 1 show sample calculation of standard deviation All of the calculated values are summarized in Table 2. Table 2. Insert title for the table. Sample Weighing bottle with lid Weighing bottle without lid Weighing bottle lid only Weighing bottle with lid on different balance ∑i x i − x N −1
_2 Average Mass (g) Standard Deviation Answer the following question in present tense using clear paragraph organization. How do the data demonstrate the accuracy or precision of the electronic balances? Explain why the Q-test is not used in this experiment and when it would be appropriate to use the Q-test. Because every measurement made in the laboratory is really an approximation, it is important to learn how to choose the appropriate equipment for a given task and to record values that reflect the accuracy and precision of the device that is used to make the measurement. An important distinction with respect to volumetric glassware is whether the glassware is calibrated to contain (TC) a certain volume or to deliver (TD) a certain volume. Glassware that is marked “TC” is used describe a laboratory technique that would require the use of "TC" glassware. On the other hand, glassware marked “TD” is used describe a laboratory procedure that would require the use of "TD" glassware. Glassware that has no marking is used describe a task that could be done with glassware not marked "TC" or "TD". A summary of the volumetric glassware used in the General Chemistry laboratory is given in Table 3. Table 3. Insert title of table 3. Glassware 100-mL beaker 400-mL beaker 10-mL graduated cylinder TC or TD Calibration Temperature (oC) Volume Graduation (mL) 100-mL graduated cylinder 100-mL volumetric flask 250-mL volumetric flask 5-mL graduated pipet 15-mL volumetric pipet 50-mL buret To determine the accuracy and precision of graduated cylinders, water measured into a 10-mL graduated was emptied into a 100-mL graduated cylinder ten times. The volume in the 100-mL graduated cylinder was determined and compared to results obtained from three other students. These data and associated calculations are summarized in Table 4. Table 4. Insert title for describing data.
Volume Trial 1 (mL) Volume Trial 2 (mL) Volume Trial 3 (mL) Volume Trial 4 (mL) Average Volume (mL) Standard Deviation Using paragraph organization, answer the following questions. How do the data and your observations demonstrate the accuracy and/or precision of graduated cylinders? How should a graduated cylinder be used in the lab? The third of the experiment is to compare and contrast two pieces of glassware that are commonly used to deliver a known volume of liquid: a 5-mL graduated pipet and a 50-mL buret. This analysis consisted of comparing the accuracy and precision of each of the devices to deliver 3.0 mL aliquots of water, as illustrated in Table 5. Table 5. Insert title describing data. Volume Delivered by 5-mL Graduated Pipet (mL)*
* As measured with a 10-mL Graduated Cylinder Initial Buret Reading (mL) N/A N/A Final Buret Reading (mL) N/A N/A Volume Delivered by 50-mL Buret (mL)*
* As measured with a 10-mL Graduated Cylinder Trial 1 Trial 2 Trial 3 Trial 4 Mean Standard Deviation Answer the following question in present tense using clear paragraph organization. Which piece of glassware is more accurate? More precise? What are the advantages/disadvantages of each piece of glassware? How should these pieces be used in the lab? The final part of the experiment is to calibrate a Pasteur pipet to deliver 1.000 mL of water. Define calibration and relate to the procedure you developed to calibrate the Pasteur pipet. Describe how the accuracy and precision of the pipet was determined and indicate how it is related to data presented in Table 6. Show sample calculation for the determination of the volume of water delivered by the Pasteur pipet Table 6. Insert title describing data Vial I Mass of Empty Vial (g) Mass of Vial + Water (g) Mass of Water (g) Volume of Water (mL) Average Volume (mL) Standard Deviation Percent Error Vial II Vial III Vial IV ...
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- Fall '08
- Standard Deviation, tc, INSERT TITLE, Pasteur pipet, Pasteur pipet Table