CHM 1470 Exp #2

pdf

School

Oakland University *

*We aren’t endorsed by this school

Course

1470

Subject

Chemistry

Date

Feb 20, 2024

Type

pdf

Pages

Uploaded by ProfessorAnt19542

CHM 1470 Oakland University Department of Chemistry Accuracy and Precision Introduction Every experimental measurement should include a number and a unit. The digits reported in a measurement are called significant figures; all digits known with certainty and one estimated digit. The measuring devices used in the laboratory determine the number of significant figures and the unit that is reported. In chemistry, the glassware used contributes to the number of significant figures reported for the raw data and the calculated values that are derived from the experimentally determined measurements. Most errors in data collection are due to human error, not necessarily from the instrument. Therefore, Knowledge of how to properly use and read the different glassware available is key to good experimental results. With every measurement made there are two questions that should also be considered, “How close is the measurement to the “true” value?” and “Will a similar valued be produced if the measurement is repeated?” These two questions are directly related to the accuracy and precision of scientific measurements. Accuracy describes how a measurement differs from its “true” value and it is often characterized by the percent er ror: ??𝑎????? ?𝑎??? − ???? ?𝑎??? 𝑃?????? 𝐸???? = | ???? ?𝑎??? | × 100% The smaller the percent error, the more accurate the measurement. Precision describes the reproducibility of the measurements; that is, how similar repeated measurements are to each other . Highly precise measurements produce a similar result each time the measurement is repeated. In contrast, a low precision measurement produces highly scattered results. The most common way to evaluate the precision of a measurement is to compute the standard deviation, S. The standard deviation estimates how much each single measurement fluctuates above and below the mean. The standard deviation can then be calculated using Equation 1: 𝑆 = [ ∑ (𝑥 𝑖 − 𝑥̅) 2 𝑁 𝑖=1 𝑁−1 ] 1 2 (1) The accuracy and precision of a measurement must be independently considered. Just because a measurement is accurate does not mean that it is precise. Likewise, just because a measurement is precise does not mean it will always be accurate. The accuracy and precision of the

measurements are influenced by the type of experimental errors that occur. An experimental error can be systematic or it can be random. A systematic error causes a measurement to have a consistent deviation from the true value. Thus, when systematic error dominates a measurement, the results can have high precision (i.e., it is very repeatable), but the measurement will always have low accuracy. An example of a systematic error is a balance that always generates masses that are 5 g lower than the true values. Random errors do not produce a consistent deviation, instead the results will fluctuate above and below the true value. The size of the fluctuations produced by random error determines the precision of the measurement. The fluctuations produced by random error can also decrease the accuracy of a measurement; however, we expect that repeating a measurement many times dominated by random error and then calculating the mean result will increase the accuracy. This is because the positive and negative deviations caused by random error will tend to cancel each other out when the mean is calculated. The standard error of the mean, S m , is a statistic that is used to calculate the uncertainty (i.e., the size of the fluctuations) in a result that is the mean of N individual measurements. The Equation 2 is used to calculate S m , where S is the standard deviation of the individual measurements calculated using Equation 1, and N is the number of measurements used in the calculation. 1 𝑆 ? = × 𝑆 (2) √𝑁 The uncertainty of the mean, S m , is dependent on the number of measurements, and Equation 2 tells us that the uncertainty of the mean decreases as we increase the number of measurements. For this experiment we will focus the mean value and accuracy by calculating the mean average value and percent error. Part 1 will investigate the various properties of the individual pieces of glassware commonly used in the lab. Part 2 will examine the accuracy and precision of select glassware used in the lab. Part 3 of the lab will involve measurement of both volume and mass of water and graphical analysis of the data to determine the density of the sample. Materials & Equipment 100 mL or 150 mL beaker Pipet bulb Milligram balance 50 mL graduated cylinder Buret reader 10 mL graduated cylinder 25 mL volumetric flask Buret clamp DI water 50 mL buret Ring stand 25 mL volumetric pipet Funnel

Procedure: Part 1 In Part 1, various pieces of commonly used glassware will be analyzed for the properties below using quantities specific to each piece. In addition, glassware can be classified as “to deliver” (TD) or a “to contain (TC) based on the manner in which it is intended to be used. A “to contain” piece of glassware is calibrated to hol d specified volumes of liquid. A “to deliver” piece of glassware is calibrated to dispense specified volumes of liquid. The international system of units uses “IN” for “to contain” and “EX” for “to deliver”. For each piece of glassware analyzed, identify whether it is TD or TC. Record your observations of the different pieces of glassware used in this experiment in Table 4. Identify the incremental marks and the minimum and maximum volumes that can be measured with each piece of glassware in columns A – C. Record how many decimal place values should be reported and how many significant figures should be reported if a specified volume is measured using each piece of glassware in column D – E. Finally, indicate whether the piece of glassware is classified as a TC or TD device. If an entry does not apply to a specific piece of glassware, enter NA for not applicable. Analyze each piece of glassware for the following: A. Frequency of Incremental marks B. Minimum volume that can be measured C. Maximum volume that can be measured D. The number of decimal places reported in a measurement. E. Number of significant figures that should be reported when measuring 25 mL of liquid. F. Classification of glassware type as TD or TC. Part 2 In Part 2, the accuracy of select pieces of laboratory glassware will be analyzed. Depending on whether the glassware is classified as a TC or TD device, the procedure used to analyze for the accuracy will differ. The following glassware will be analyzed: 1. 100 mL or 150 mL beaker 2. 50 mL graduated cylinder 3. 25 mL volumetric flask 4. 50 mL buret 5. 25 mL pipet Twenty-five mL of deionized water will be measured using each piece of glassware. Twenty-five mL of water will be added to the “to contain” glassware (items 1 – 3), and 25 mL of water dispensed from the “to deliver” glassware (items 4 and 5). The actual volumes of water contained or delivered will be determined by measuring the masses of the 25 mL of liquid, and then using the density of water at the measured water temperature, the masses will be convert to volumes. Two mass measurements of water will be completed for each piece of glassware in the lab. In the normal data collection process, entire experiments are completed and then repeated for the accuracy and precision of experimental results. The data collected by each group in the lab will be

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version

Access to all documents
Unlimited textbook solutions
24/7 expert homework help

shared to represent repeating the experiment. To share the data, transfer the calculated volumes for each piece of glassware to Part 2 Individual Data Submission page, and then turned this page into your instructor before leaving the lab. The class data will be compiled by your instructor and then shared in Moodle. To investigate the accuracy of the common glassware used in the lab, the mean and % error for each piece of glassware will be calculated using the class data. A copy of the class data needs to be submitted with your final lab report. Procedure: Each group should obtain a kit that will contain all necessary items that are not found in your lab drawer. A milligram balance should be used for all mass measurements. Before any measurements are taken, make sure that each piece of glassware is dry. The beaker and graduated cylinder can be dried with a paper towel, but the volumetric flask cannot be dried using paper towel. The volumetric flasks should be dry when you receive it in your kit. If they are not dry, you should exchange them for dry flasks. Once all glassware is dry, initial mass measurements can be made. Make sure to use the same balance for all measurements throughout Part 2. Since there are a limited number of balances you should weigh your glassware, and then let others use the balance. Return to your bench and measure 25 mL of deionized water in each piece of glassware. Reweigh ALL glassware with the measured water using the SAME balance. Make sure to label your beakers, so you do not mix them up. “To contain” devices: 1. Record the empty container masses for items #1 – 3. 2. As accurately as you can, add 25 mL of water to each piece of glassware. • For the final additions of water, use an eye-dropper to add or remove water to get the bottom of the meniscus to the desired calibration mark. • Volumetric Flask – To properly add liquid to a volumetric flask, a funnel should be used. The funnel should be removed as the liquid level approached the calibration mark. Use an eye-dropper to add liquid to get the bottom of the meniscus aligned with the calibration mark. • Water droplets – Careful to not leave water droplets on the inside of the neck of the volumetric flask, graduated cylinder or beaker above the water surface. This liquid is not part of the measured liquid, and should be removed. Absorbent paper towel will work. 3. Record the masses of the “container plus 25 mL of water” for each piece of glassware. • Before massing the water-filled containers, make sure the outside of the glassware is dry and there are no drops of water on the inside of the container above the water. Water can be removed using absorbent paper towel. 4. Repeat the process (steps 1 – 3) for the second trial. “To Deliver” devices Since the liquid is dispensed out of the glassware, beakers will be used to mass the measured volumes of water. 1. Dry 2 beakers. Label one with “pipet” and the other with “buret”. 2. Record the initial mass of the beakers 3. Deliver as accurately as possible 25 mL of water from the pipet to the beaker labelled “pipet”. Helpful reminders for pipets – a. The pipet is calibrated to retain the last bit of liquid in the tip. Do not blow out the liquid. b. Water Droplets - Before transferring any liquid, make sure the outside of the pipet does not have

any water droplets. To remove water droplets from the outside of the pipet, touch the tip of the pipet to the stock container wall. If after transferring the liquid to the beaker a water droplet remains, this droplet is part of the liquid that was dispensed from the pipet. Collect this droplet by touching the tip of the pipet to the inside of the beaker. 4. Deliver as accurately as possible 25 mL of water from the buret into the b eaker labelled “buret”. Helpful reminders for burets – a. Remove the buret from the buret clamp before filling. A funnel should be used to fill the buret. Don’t forget to close the stopcock. b. Once the buret is filled, drain the liquid through the tip into a waste container (not the labelled beaker) to bring the liquid level below the zero calibration mark and to fill the tip of the buret with water. Careful to not get an air bubble in the tip. c. Water droplets – follow the same guidelines outline in pipets. d. Record the initial volume of the buret to the nearest hundredths place. e. Pre-determine the final volume measurement needed on the buret to deliver 25 mL. (Add 25.00 mL to the initial volume.) f. Place the labelled beaker under the buret tip and carefully deliver the 25 mL as accurately as possible. 5. Record the masses of the “labelled beaker plus 25 mL of water” for the pipet and buret. Make sure the outside of the beakers are dry before massing the containers. 6. Repeat for second trial. Calculations for the 100mL beaker need to be completed before leaving the lab : 1. The mass data needs to be converted to volumes using the density of water at the given temperature. The temperatures for which the density of water values is provided and you should use the density which corresponds with nearest measured temperature values recorded. 2. Record the calculated volumes for each piece of glassware on the Individual data submission page. 3. Show a sample volume calculation using Beaker #1 data on the Individual Data Submission page. 4. Submit your completed group submission page with calculated volumes of water for the various glassware to the folder provided in Moodle within 24 hours of completing the lab . Part 3 For Part 3 of the lab you will investigate graphical analysis of DI water samples in order to determine the density of the DI water and the mass of the graduated cylinder. Procedure: Note: you will use the same 10 mL grad cylinder for Part 3 of the lab. 1. To a dry 10 mL grad cylinder, add between 2 and 3 mL of DI water to the cylinder. Be sure to dry any droplets on the inside of the cylinder that remain above the water level. 2. Read and record the volume of water in the grad cylinder. Follow the measurement technique used in parts 1 and 2 of this lab. Significant figures should be preserved.

3. Mass the grad cylinder and water on the milligram balance and record. 4. Add 1-2 additional mL to the same grad cylinder (total volume is now 4-5 mL) record the total volume. 5. Mass the grad cylinder and water on the milligram balance and record. 6. Repeat steps 4-5 two additional times. Total volume after the 4 trials should be between 8 and 10 mL. Data Analysis Part 1 Produce an Excel spreadsheet. Use the appropriate function to determine the mean and formula to calculate the % error. You will not use a calculator to calculate the mean or % error. All calculations will be done using Excel. Use the Excel functions – “Average” for the mean and a formula for % error. Print out the Excel spreadsheet and include it in your lab report after you have completed all calculations outlined below using Excel. Constructing an Excel spreadsheet Use the data from the class data generate an Excel spreadsheet similar to the spread sheet in Figure 2. A brief explanation of several Excel techniques associated with the Data Analysis section of this experiment are listed below: Figure 2. Below is a list of common Excel formatting and formula entry techniques, most of which will be used in this experiment. 1. Column widths are adjusted by moving the cursor to the right side dividing line between columns and when cursor has the shape ↔ with a vertical line between the arrows holding down mouse and dragging column boundary to desired position. 2. If you want to report a specific number of digits in your values, the column needs to be converted to a “Number” format using the dropdown menu and select “Number”. Then using the tabs with the zeros and decimals in the number format section you can increase or decrease the number of digits reported after the decimal. 3. To enter a simple mathematical function, type the equals sign and then the numbers or cells in which you intend to use in the calculation. • To divide a cell by value, type the equal sign, the cell number, forward slash, and then the value. e.g. To divide a cell B2 by 4, then type “=B2/4”. • To divide a cell value by another cell value, then type the equal sign, cell#1/cell#2. e.g. To divide cell B2 by B3, then type “=B2/B3”. To use Excel mathematical functions, click the cursor in the cell in which the function is going to be calculated and type an equals sign. All functions start with an equals sign followed by the name of the function, and then include the range of variables used in the calculation within parenthesis. • i.e. To calculate the average, type into B14 “=AVERAGE(B2:B13)” for the average of the values for the cells 2 – 13 in column B. 4. Once you have entered the function in a cell, the same function can be easily copied and pasted into other adjacent cells. Position the cursor at the bottom right corner of the cell until it has a cross

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version

Access to all documents
Unlimited textbook solutions
24/7 expert homework help

shape, and then drag the cursor over cells you want to paste the function into. The same function will calculate the value for the same range of cells in relation to the cell. • i.e. If the above function in B14 is pasted into C14, then C14 cell will have the average for the values in cells C2 through C13. 5. The Functions used for this experiment are a. Mean = “AVERAGE” Part 2 Enter the shared data from Part into an Excel spreadsheet. Use the appropriate Excel functions to determine the mean. You will not use a calculator to calculate the mean or % error. All calculations will be done using Excel. Use the Excel functions – “AVERAGE” for the mean and a formula for the % error. Print out the Excel spreadsheet and include it in your lab report after you have completed all calculations outlined below using Excel. 6. Estimate the accuracy of each piece of glassware by calculating the percent error in measurements of 25 mL samples. The “true volume” is 25 mL ( the number of significant figures is dependent on the piece of glassware ) and the mean volume of water is the average value calculated using the compiled class data. 5. Constructing an Excel spreadsheet 6. Use the class data posted in Moodle, model your spreadsheet as close as possible to the example in Figure 3 (your Excel sheet will have more entries than shown in Figure 3). Enter Excel calculated results in Table 5. 7. Figure 1. Example Excel spreadsheet for finding actual volumes of 25 ml samples of water.

CHM 1470 Oakland University Department of Chemistry Pre-Laboratory Exercise 1. Prepare a procedural outline for Part 1 - 3 of this experiment on a separate 8.5” x 11” piece of paper to be used in the laboratory while you conduct this experiment. This outline should list the detailed steps you will follow to collect the data without using this document. 2. Review the common laboratory glassware and the techniques associated with the glassware used in this experiment found in the Laboratory Skills document in Moodle. Review How to use of pipet, buret, volumetric flask, reading glassware, and use of the balances in this document. 3. View four glassware videos (Use of Buret, Use of Pipet, Use of Volumetric Flask, Use of Top Loading Balances).

CHM 1470 Oakland University Department of Chemistry Part 1 – Data Sheet Name __ Lab Partner’s Name (s): ________________________________________ Table 1. Various properties of the different types of glassware. A B C D E F Incremental marks Minimum volume that can be measured Maximum volume that can be measured Number of decimal places in reported value Number of sig. figures for “X” mL sample “TC” or “TD” Beaker 25 mL 100 or 150 mL Beaker Erlenmeyer Flask 75 mL 125 or 250 mL Erlenmeyer Flask Graduated Cylinder 25 mL 50 mL Grad. Cylinder Buret 25 mL Buret 50 mL Volumetric Flasks 25 mL 25 mL Vol. Flask Volumetric Pipets 25 mL 25 mL Vol. Pipet In general, you should estimate one digit beyond the smallest incremental mark. Use “NA” for minimum volumes that do not apply.

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version

Access to all documents
Unlimited textbook solutions
24/7 expert homework help

CHM 1470 Oakland University Department of Chemistry Part 2 – Data Sheet Name (Individual Group Data) Lab Partner’s Name(s): ____________________ Show correct significant figures for masses and units for all data to receive full credit. 1 100 or 150 mL beaker Trial #1 Trial #2 Water temperature Water Mass beaker + water Mass beaker 2 50 mL graduated cylinder Trial #1 Trial #2 Water temperature Mass grad. cyl + water Mass grad cyl 3 25 mL volumetric flask Trial #1 Trial #2 Water temperature Mass flask + water Mass flask 4 50 mL buret Trial #1 Trial #2 Water temperature Mass beaker + water Mass of beaker 5 25 mL pipet Trial #1 Trial #2 Water temperature Mass beaker + water Mass of beaker Density of water at various temperatures 20°C = 0.99823 g/mL 25°C = 0.99707 g/mL 30°C = 0.99567 g/mL

CHM 1470 Oakland University Department of Chemistry 11 Rev 8-28-21 Part 3 – Data Table Trial Mass volume 1 2 3 4 Calculations (Part 2): 100 or 150 mL beaker Trial #1 Trial #2 Mass of water Volume of water 50 mL graduated cylinder Mass of water Volume of water 25 mL volumetric flask Mass of water Volume of water 50 mL buret Mass of water Volume of water 25 mL pipet Mass of water Volume of water

CHM 1470 Oakland University Department of Chemistry 12 Rev 8-28-21 Part 2 – Group Data Submission Sheet Name _______________________________________ Name ___ ______________________________ Calculated Volumes for select glassware: Sample volume calculation for beaker trial #1. Show all work (calculation set-up) for full credit. 1. Calculate the mass of water in the beaker. 2. Calculate the volume of water in beaker. Trial #1 Trial #2 Beaker Graduated Cylinder Volumetric flask Buret Volumetric pipet

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version

Access to all documents
Unlimited textbook solutions
24/7 expert homework help

CHM 1470 Oakland University Department of Chemistry 13 Rev 8-28-21 Part 2 – Class Data Name(s) ________ , ________________________ Table 5. Calculated Data – Mean, and percent errors for volumes Fill in the table with the values calculated by Excel using the class data. Calculated Value Beaker Graduated cylinder Volumetric flask Buret Pipet Actual volume Mean Percent Error Show correct significant figures and units for all data that require units for full credit. Attach the Excel spreadsheet used to find volume means and percent error. A sample spreadsheet with fictitious data is shown in Figure 1 of the Data Analysis section. Show below sample calculations for the mean volume, and percent error using the two trial beaker volumes from your data . 1. Calculation of mean volume for 25 mL contained in a 100 or 150 mL beaker using the 2 trials measurements listed in your data. 2. Calculation of percent error in measuring 25 mL using a 100 ml or 150 mL beaker.

CHM 1470 Oakland University Department of Chemistry 14 Rev 8-28-21 Part 3- data analysis 1. Using Excel, construct a graph of the data similar to the one you made in Exp. 1. Be certain to correctly determine the independent and dependent variables for your graph. (Hint: Think about how you collected the data.) Be sure to completely label your graph. There should be a title, and the axes should be labelled including the variable name and units in parentheses. 2. Determine the following information using your graph. a. The slope of the line. (3 pts) (include units) b. The y-intercept _________________ (include units) (1 pt) c. The equation that describes the line on the graph. (1 pt) d. What does the slope tell us? (Hint: look at the units) (2 pts) e. What does the intercept represent? Explain how you got your answer. (3 pts) 3. For the beaker compare the sample calculated values of the mean and percent error of your 2 trials on page 13 to the Excel calculated values (tabled values on p. 13) for the class beaker data. Did including more trials in the calculations change the accuracy, percent error? Explain. (4 pts)

CHM 1470 Oakland University Department of Chemistry 15 Rev 8-28-21 Name __________________________________ Partner ___________________________________ Post-Laboratory Exercise (Page 1 of 3) 1. Which of the following sets of measurements whose true values are 18.5 mL, 20.0 g and 55.25 mmHg, respectively, is most accurate and which is most precise? Show any work that supports your answer. (4 pts.) A. 17.2 mL B. 20.2 g C. 50.04 mmHg 19.5 mL 19.8 g 49.98 mmHg 18.8 mL 20.3 g 50.08 mmHg Most Accurate Most Precise 2. For the questions below (a – d), identify which general piece of glassware analyzed in Part 1 & 2 of this experiment is the most appropriate for the scenario described. a. If you were asked to add 50 mL of a stock solution to a reaction vessel, which piece of glassware would you use to obtain this volume of solution? (2 pt.) b. If you are told to get approximately 100 mL of a stock solution that will be use to prepare smaller size samples of that solution for an experiment, which piece of glassware would you use? (2 pt.) c. If you need to add 27.50 mL of a solution, which piece of glassware would you use to deliver this volume and explain how you would determine if the 27.50 mL was measured? (3 pts.)

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version

Access to all documents
Unlimited textbook solutions
24/7 expert homework help

CHM 1470 Oakland University Department of Chemistry 16 Rev 8-28-21 d. Which pieces of glassware can you obtain hundredths digit accuracy? What volume or range of volumes can you measure with each piece of glassware? (4 pts.) 3. Which piece of glassware should you use to make a solution of a specific volume and concentration? Explain. (3 pt) 4. Which piece of glassware is the least accurate using the collected data? Explain. (3 pt) 5. What does each value (3) represent in the writing found on this volumetric pipet? (2 pts.)

CHM 1470 Exp #2

Related Documents