chemistry lab mini report

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Concentration, M/L 0 1,13E-06 2,25E-06 4,50E-06 9,00E-06 1,80E-05 Absorbance at 630 nm 0 0,11 0,329 0,384 0,434 0,286 0,6 0,5 Molarity Sample Standard 1 (blank) Standard 2 Standard 3 Standard 4 Standard 5 Standard 6 0,4 0,3 0,2 0,1 0 0 0,0000010,000002 Beer's Law y = 44556x + 0,101 R² = 0,7171 0,0000020,0000030,0000040,0000050,0000060,0000070,0000080,0000090,00001 Absorbance Appendix Appendix-1 Appendix 1: Routine Laboratory Techniques 1 Balances In this course the top-loader digital balances, used for routine weighing, yield results to the nearest 0.01g. The digital analytical balances are used for higher-precision weighing and yield results to the nearest 0.0001g. a) Never weigh a chemical straight onto the balance pan. b) Always check that the balance is zeroed prior to weighing. c) Any spillages on or near the balances should be cleaned up immediately. 2 Volumetric Flasks Volumetric flasks (right) are designed to contain (TC) a particular volume when the bottom of the meniscus hits the mark on the neck of the flask. Flasks sized 100 mL or less measure to two decimal places (e.g. 100.00 mL); those greater tend to measure to one decimal place (e.g. 500.0 mL). To use: dissolve the desired mass of reagent or mix the solutions in the volumetric flask by swirling prior to filling the flask completely. For the last few drops of liquid, use a Pasteur pipet or medicine dropper for good control; squirt bottles are not recommend. Once the meniscus is at the right mark, firmly stopper the flask, and invert repeatedly (do not shake) to assure uniform mixing. 3 500 mL mark Pipets Pipets are used to deliver a known volume of a solution into a vessel. Volumetric pipets are constructed to deliver a single volume with great reproducibility when filled to the calibration mark, while measuring pipets have a graduated scale and can be used to deliver, with lower accuracy, any volume within the scale range. Grade A 500 mL TC 20oC Calibration mark Volumetric pipet Graduated pipet Fall 2017 Student and Instructor Version Appendix Appendix-2 The delivered volume can be expected to be accurately known only if the following instructions for the pipet are adhered to: a) The pipet must be clean (draining distilled water should leave no droplets on the pipet wall). For cleaning, use warm (not hot!) soapy water. b) Use a dry rubber suction bulb to draw liquid into the pipet. Do not mouth pipet. c) Draw into the pipet a little of the solution to be pipetted, and rinse the pipet by turning it while holding it nearly horizontal. Drain the liquid into a waste vessel. d) Draw the desired liquid to above the graduation mark(s), then quickly place your forefinger over the top to hold the liquid in place. e) Wipe the exterior of the pipet free of adhering droplets, and then drain the pipet into a waste vessel until the bottom of the meniscus touches the graduation mark. 'Touch off' against the side of the waste vessel. f) Place the tip of the pipet against the wall of the intended receiving vessel and deliver the required volume while holding the pipet vertically (this will require tilting the vessel slightly). For volumetric pipets, allow the pipet to 'drain' by holding it in place for an additional 10 seconds, and do not attempt to deliver the last drop — the pipet is calibrated with this drop in it! g) For repeated pipetting of the same solution, continue with step d). h) Immediately after use, rinse the pipet clean with water and finally with distilled water. Do not attempt to dry the inside of the pipet! 4 Buret A buret is a glass tube with multiple graduations so that the volume of liquid delivered through the valve at the bottom (stopcock) can be calculated. While it can depend on the quality of the buret, most 50 mL or smaller burets allow for two decimal places to be read (50.00 mL). 0 10 Read a buret as follows: a) Hold a black-line card behind and just below the meniscus to make it visible. b) Hold your eye level with the bottom of the meniscus to avoid parallax error. c) Take the reading at the bottom of the meniscus estimating to the nearest 0.01 mL. Always read your buret to two decimal places. 20 30 40 50 Stopcock Fall 2017 Student and Instructor Version Appendix Appendix-3 Appendix 2: Data Analysis 5 Precision and Accuracy Precision is the term used to refer to the consistency and reproducibility of repeated measurements and is often reported as the standard deviation or relative standard deviation. Accuracy is the term used to refer to the level of agreement between an experimental result and the true or accepted value and is often reported as a percent error. In the absence of a comparative standard, good precision is taken as an 'indicator' of good accuracy. 6 POOR accuracy POOR precision GOOD accuracy POOR precision Sources of Error Systematic (or Determinate) errors affect accuracy and are attributable to equipment malfunction or to the biases inherent in the experimental procedure. Systematic errors cause results to be 'high' or 'low'. A good experimenter will POOR accuracy GOOD accuracy GOOD precision GOOD precision seek out and eliminate systematic errors. The extent of systematic errors can be expressed as a percent error and are usually expressed with an indication of direction of the error (to represent a positive or negative bias). Random (or Indeterminate) errors contribute to the uncertainty of an experimental result due to a lack of precision. This may come from the 'estimation of a reading' or the inherent ‘noise’ of a particular sensor or device. The level of uncertainty is expressed as '+/- standard deviation' or by the convention of using 'significant figures' (see below). 7 Statistical Analysis of Data 7.1 Median, Average, Standard Deviation and Percent Difference The meaning of the statistical terms median, average and standard deviation are described below along with the equations used to calculate these values. In this course, we will use Microsoft Excel to do the calculations. Your calculator may also be able to these calculations. The median is the middle value of a set of values, i.e. half the values in the set are larger than the median, the other half smaller. For an even number of results, the median lies halfway between the middle two values. This is used when there are a small number of values or when there are outlier values that can distort the average. ", is the value obtained by dividing the sum of a set of values by the number of the The average (or mean), 𝒙 values in the set. x= 1 × å xi n i Fall 2017 Student and Instructor Version Appendix Appendix-4 The sample standard deviation, s, tells you how far from the mean the measurements range. It therefore reflects the precision of the data and the amount of random error in the experiment. It includes units and the same number of decimal places as the mean. Standard deviation is calculated using the following formula, ∑.)(𝑥) − 𝑥̅ )% 𝑠= 𝑛−1 where 𝑥̅ is the mean, 𝑥) is the value of the individual measurement, n is the number of values in the data set. In this course, you will use the Excel function: stdev.s to calculate the standard deviation. Means that have calculated standard deviations (s) are reported with their units as: mean ± s. The calculated mean value should be rounded off to include only the first uncertain digit (thereby indicating the precision or the reproducibility). The first uncertain digit is determined from the first uncertain digit in the standard deviation rather than significant figure rules. Example: For the repeated set of measured lengths 83.54 cm, 83.99cm, 83.05 cm, for which the calculated average is 83.53 cm and the standard deviation is 0.5 cm. To report the value as mean ± s, we round the mean to match the uncertainty in the standard deviation. In this case, the first uncertain digit in the standard deviation is in the first decimal place so we round 83.53 to 83.5 and report the mean as 83.5 ± 0.5 cm. Notice that this gives fewer digits in the mean than sig fig rules. The relative standard deviation (RSD) is calculated by dividing the standard deviation by the mean or median and multiplying by 100%. By relating the standard deviation to the size of the measurement, it is easier to compare the quality of the results. There is no universally accepted target for RSD that can be used to classify an analysis as ‘good’ or ‘bad’ because acceptable RSD values depends on the method and samples used. However, for the analysis performed in first year chemistry lab, an RSD less than 1% would be considered ‘good’, less than 2% is ‘acceptable’, and greater than 2% suggests some operator error. RSD = 100 × standard deviation mean or median value The percent difference is applied when comparing two experimentally measured values while percent error is used when comparing an experimental value to an actual (known, ‘true’, or literature) value. In this way, percent error reflects the accuracy of the experimental value and is a measure of the systematic error or bias in an experiment. A negative % error indicates that the experimental result is less than the actual or true result. Like RSD, acceptable %error values depend on the method and sample used. However, for the analysis performed in first year chemistry lab, less than 2% would be considered ‘good’, less than 4% is ‘acceptable’ and greater than 4% suggests some operator error. Percent difference = experimental1 -experimental2 ×100 average of experimental 1 and 2 Percent error= experimental-actual ×100 actual Fall 2017 Student and Instructor Version Appendix Appendix-5 7.2 Q-Test The Q-test is used for identification and rejection of an outlier. It can be used only if four or more data points are obtained. In general you should have a reason for questioning the validity of one data point (e.g. in trial #1, too much titrant volume was added as suggested by the indicator colour at the endpoint). Nonetheless, either the maximum or minimum value in a data set (whichever shows the largest gap with its closest neighbour) can be regarded as a potential outlier. After it has been applied to a set of data, the mean and the standard deviation can be determined. The Q-test is designed to reject only one data point from each data set by comparing Qcalculated and Qtable values. If Qcalculated > Qtable, then suspect data point can be discarded. Q calculated = gap range Table 1: Values of Qtable (with 95 % confidence). Number of Pieces of Data Qtable 4 0.829 5 0.710 6 0.625 7 0.568 Example: The following five concentrations of NaOH were determined by titration: 0.1093 M, 0.1098 M, 0.1088 M, 0.1100 M and 0.1078 M. Consider this data and answer the following questions. a) Use the Q-test to determine if either the largest or smallest value be discarded as an outlier. b) What is the mean molarity? Step 1: Rank the values and find the larger gap to identify the possible outlier. smallest largest 0.1078 M 0.1088 M 0.1093 M 0.1098 M 0.1100 M gap = 0.1088-0.1078 = 0.0010 gap = 0.1100-0.1098 = 0.0002 A larger gap is observed between the smallest value and the rest of the data set so the possible outlier value is 0.1078 M. Step 2: Calculate Qcalculated and use the Q-test to determine if this value should be discarded. The range is the total spread of the data calculated as the absolute difference between the largest value and the smallest value. Q calculated = gap 0.0010 𝑀 = = 0.45 range 0.1100 − 0.1078 Comparing Qcalculated (0.45) with the Qtable value for five pieces of data from Table 1, (0.710) we see that Qcalculated is NOT greater than Qtable; therefore, the data point cannot be discarded. The mean molarity is calculated with all the data as 0.1091 M. Fall 2017 Student and Instructor Version Appendix Appendix-6 7.3 R2 Value The R-squared (R2) or coefficient of determination is often determined when fitting data to a linear graph. It essentially reflects how well data on one axis (x) can be used to predict a point on the other (y) through the use of the formula for the best fit line (trendline). R2 values vary between 0.0000 and 1.0000; the closer to 1, the stronger the predictive power of the graph. If all data points fall perfectly on the line of best fit, R2 will be 1 or 0.9999; if more variation is seen in the data, the value will be lower. In chemistry, we often use the phrase “how many nines?” this refers to the number of nines there are after the decimal in the R2 value. For example, an R2 value with three nines could be 0.9992 (the variation is in the fourth decimal place) or two nines could be 0.9956. Graphs which have R2 values with three or four nines are considered very predictive and those with R2 values with two nines are acceptable. For the methods and samples used in first year chemistry, you should expect R2 with at least two nines, if you have less than two nines then there may have made a mistake and you should consult your instructor. 8 Significant Figures The precision of a measured number can be indicated by writing the number with the correct number of significant figures (digits), i.e. the number should include all digits known with certainty and the first 'uncertain' digit. The uncertain digit is the last digit recorded – when reading from a scaled object (such as a ruler, thermometer, buret, or graph) – it is estimated by looking between the measured markings. Because it is an estimated value, this is where one should expect variation to occur if the measurement is repeated again; hence why it reflects the precision of measurement. Therefore recording data with the correct number of significant figures is absolutely essential, and marks will be deducted for mistakes. Significant figures are often abbreviated as sig. figs. Rules for multiplication/division The answer must contain the same number of sig. figs. as the least precise measurement used in the calculation. 72.5674 mm six sig. figs x 3.34 mm three sig. figs. (limiting term) 242.3751160 mm2 initial answer must be rounded off to three sig. figs. FINAL ANSWER = 2.42 x 102 mm2 (three sig. figs.) Rules for addition/subtraction The answer must contain the same number of decimal places as the least precise measurement used in the calculation. 456.367963 kg nine sig. figs. – 452.1 kg four sig. figs.; least number of decimal places limiting term 4.267963 kg initial answer must be rounded off to one decimal place FINAL ANSWER = 4.3 kg (2 sig. figs.) Rules for logarithms In logarithmic values, only those numbers to the right of the decimal place count as significant. For example, pH = 10.26 has only two significant figures and corresponds to a [H+] = 5.5 x 10-11 M pKa = 4.730 has three significant figures and corresponds to Ka = 1.86 x 10-5 Fall 2017 Student and Instructor Version Appendix 9 Appendix-7 Using Excel 2013 (Windows version) to Plot a Calibration Curve There are videos on VIULearn that demonstrates this skill. Follow the steps below to plot a calibration curve in Excel. 1. TO ENTER THE DATA, type the standard name in column A, the corresponding concentration in column B, and corresponding emission or absorbance in column C. Your numbers will be different than the ones below. Your spreadsheet should look something like the figure below. To enter data in scientific notation, use E to mean 10, for example 6.626 × 10‒34 would be entered as 6.626E-34 and 6.022 × 1023 would be 6.022E23. 2. TO INSERT A GRAPH into your spreadsheet select the block of data in columns B and C and click on the INSERT tab (1). Under the CHARTS menu, select SCATTER (2). This will open a drop down menu, choose SCATTER WITH ONLY MARKERS (3). Refer to the figure below for additional help. Fall 2017 Student and Instructor Version Appendix Appendix-8 3. TO INSERT TITLES TO THE GRAPH, select the chart by clicking any whitespace inside the chart. If the chart is selected, a double line appears around the chart, the data is highlighted in different colours on the spreadsheet, and a new tab for CHART TOOLS appears at the top of your screen (arrows in the figure below). Click on ADD CHART ELEMENT (4) to open a drop down menu. Select AXIS TITLES (5) > PRIMARY HORIZONTAL (6). Type in your title for the x-axis. Remember to include units and press ENTER when you are finished. The title will appear in the formula bar as you are typing. Repeat the same process to add a title for the y-axis (vertical axis). To add a graph title choose CHART TITLE (7). The graph title should describe what the graph shows and/or what the graph is used to determine. It should also include your initials. Y vs. X titles are insufficient. Fall 2017 Student and Instructor Version Appendix Appendix-9 4. TO ADD A TRENDLINE, make sure that the chart is selected. Go to ADD CHART ELEMENT (4 in the above figure) > TRENDLINE (8 in the above figure). Click on MORE TRENDLINE OPTIONS. This will open a FORMAT TRENDLINE menu to the right of the spreadsheet (see figure to the right). Choose LINEAR, DISPLAY EQUATION ON CHART and DISPLAY R-SQUARED VALUE ON CHART. The equation for the line will appear on your graph in the form of y = mx + b. 5. TO PRINT THE GRAPH, make sure that the chart is selected then click on the print icon at the top left hand corner. In general, the data is not needed, and the graph should fill the entire page. Your final product should resemble the example below. 9.1 An example of a calibration curve 1) Meaningful graph title that says what the graph shows and/or what it is used for. It must also include your initials. Y vs. X titles are insufficient. 2) Axes titles with units 3) Equation of the trendline and R2 value 4) Trendline overlaid on the data markers 1 Calibration Curve for Fe by Atomic Emission Spectrophotometry - JJ Emission at 510 nm An example of a calibration curve for iron is shown to the right. The calibration curve should include: 0.80 2 0.00 0.00 0.60 4 0.40 3 0.20 1.00 2.00 y = 0.1804x + 0.0006 R² = 0.999 3.00 4.00 5.00 Concentration of Fe (mg/L) Fall 2017 Student and Instructor Version Appendix Appendix-10 Appendix 3 Locker Equipment Check List Equipment Picture Size Quantity 50 mL 100 mL 150 mL 250 mL 400 mL 600 mL 2 2 2 2 1 1 125 mL 5 250 mL 5 Funnel --- 1 Graduated Cylinder 100 mL 1 Medicine Dropper --- 3 Pipet Bulb --- 1 Scoopula --- 1 Spatula --- 1 Stirring Rod --- 1 Beakers Check Check In Out Erlenmeyer Flask Fall 2017 Student and Instructor Version Appendix Equipment Appendix-11 Picture Size Quantity Check In small 6 medium 6 large 6 Thermometer and case --- 1 Watch Glass 90 mm 1 Water Bottle small 1 Test Tubes and Rack Check Out Fall 2017 Student and Instructor Version Lab D: Spectrophotometric Determination of Dye Content in a Powder D-1 Lab D: Spectrophotometric Determination of Dye Content in a Powder PRE-LAB ASSIGNMENT Read the Lab and watch the videos posted on VIULearn before answering the questions below. Your instructor will check your pre-lab assignment at the start of the lab period. 1. What are the two dyes in lemon-lime Kool-Aid, what colours are they and what are their λmax value? Name of the dye Colour Observed λmax 2. Define λmax. What are the units for λmax? 3. In this lab you will use a blue dye called erioglaucine disodium salt (EDS). What is the concentration of EDS in a solution prepared by diluting 5.00 mL of 0.250 M EDS to 25.00 mL in a volumetric flask? Show your work. (ANSWER: 0.0500 M, 3 sf) 4. If the calibration curve for this experiment follows the equation: 𝑦 = 9.623𝑥 − 0.0004 Calculate the concentration of dye if the sample solution has an absorbance of 0.135. Show your calculations. (ANSWER: 0.0141 M, 3 sf) Fall 2018 Student Version Lab D: Spectrophotometric Determination of Dye Content in a Powder D-2 Lab Overview PART 1 (Experiment): Determination of Dye Content in a Powder General Notes • This lab is done individually. • Review instructions on VIULearn and in Appendix 2 on how to graph a calibration curve in Excel. Learning Objectives • To practice making volumetric solutions. • To gain experience using the Spec-20 spectrophotometers. • To practice using Excel to create a calibration curve. Scientific Purpose • To determine the mass percent of EDS in a jelly or drink powder. Waste • All waste can be disposed of down the sink. PRELAB NOTES (this space is left for you to take notes during the prelab talk). Fall 2018 Student Version Lab D: Spectrophotometric Determination of Dye Content in a Powder D-3 Background Keywords/terms: absorption spectrophotometry; Beer’s law; colorimetry; artificial food colour Artificial food colours (AFC) are added to jelly dessert and drink powders to make them more attractive. AFC absorb light intensely and because this absorption occurs at visible wavelengths in the electromagnetic spectrum we see these dyes as coloured. Our eyes detect the wavelengths of light that are not absorbed by the solution and our brain interprets it as a colour. A spectrophotometer is an instrument that measures the absorbance (which is a measure of light intensity) of a solution at different wavelengths. A scanning spectrophotometer is able to scan through a range of wavelengths and is used to produce an absorbance spectrum with axes of absorbance versus wavelength. Figure 1a and 1b shows the absorbance spectra for a blue dye called erioglaucine disodium salt (EDS) (C37H34Na2N2O9S3; MM = 792.85 g/mole) and a yellow dye called tartrazine (C16H9N4Na3O9S2; MM = 534.36 g/mol), respectively. The wavelength that corresponds to the peak in the spectrum (that is where the absorbance is largest) is called the wavelength of maximum absorbance (λmax). For EDS λmax is about 630 nm. (a) (b) Figure 1: Visible absorption spectrum of (a) erioglaucine disodium salt and (b) tartrazine. In some cases, for example Lemon-lime Kool-Aid, a mixture of two dyes is used produce the colour. Figure 2 is the absorbance spectrum for Lemon-Lime Kool-Aid so shows characteristic peaks at 630 nm and 420 nm for EDS and tartrazine, respectively. Notice that these peaks are at the same wavelengths as the λmax for each of the pure dyes (Figure 1). Each dye absorbs light independently and does not interfere with the other dye, therefore we can analyze each dye separately. This experiment focuses on the EDS in jelly and drink powders. Figure 2: Visible absorption spectrum of Lemon-Lime Kool-Aid showing two maximum peaks of absorbance for two dyes. Fall 2018 Student Version Lab D: Spectrophotometric Determination of Dye Content in a Powder D-4 Absorbance data can be used to determine the concentration of an unknown. To do so, we create a graph called a calibration curve, which correlates a measurable property (in this experiment the absorbance) with concentration. The calibration curve for this experiment is created by measuring and plotting the absorbance of several solutions with known dye concentration (called standard solutions) against their concentrations in mol/L (see Figure 3). Absorbance @ 403 nm 0.25 0.2 0.15 y = 4.63x - 0.0007 R² = 0.9999 0.1 0.05 0 0 -0.05 0.01 0.02 0.03 0.04 0.05 0.06 Concentration of tartrazine (M) Figure 3: Calibration curve for the determination of the concentration of tartrazine in Kool-Aid. The relationship between absorbance and concentration is linear so can be fit to a trendline described by y = mx + b. It is customary to assign absorbance (dependent variable) to the y-axis and concentration (independent variable) to the x-axis. The y-intercept (b) is expected to be zero, or very close to it, because there should be no absorption if there are no dye present. Remember that the more dye present, the greater the absorbance therefore the slope of this line should always be positive. The predictive power of the calibration curve is measured using the coefficient of determination (R2). An R2 of 1 indicates that all of the data points fall directly on the line and the linear model fits well; whereas an R2 close to zero indicates very little correlation (refer to Appendix 2, Section 7.3 for more details). Once the relationship between absorbance and dye concentration is known (i.e. we know the equation of the line) then we can calculate the dye concentration of any unknown solution from the absorbance of that solution. Consider the example in Figure 3. The calibration curve has the equation: y = 4.63x − 0.0007 If absorbance of an unknown sample was 0.150, the we can determine the tartrazine concentration of the unknown by substituting the absorbance value into the equation of the line (y = mx + b) as the “y” value and then solving for x. 0.150 = 4.6384x – 0.0007 (from Figure 3) x = 0.0322 M Fall 2018 Student Version Lab D: Spectrophotometric Determination of Dye Content in a Powder D-5 For absorption experiments, the calibration curve correlates to Beer’s law: A=e bc Where, A = absorbance, unitless e = molar absorptivity, M-1cm-1 b = path length, cm c = concentration, M The absorbance, A, is unitless and usually measured at λmax. The path length, A = ε·b·c + 0 b, is the distance the light travels through the sample and is 1.00 cm for our instruments. The molar absorptivity, e, is characteristic of the compound and is a measure of the colour intensity. The concentration of the analyte (in this experiment the dye), c, is measured in M. Beer’s law and the equation of a line y = m·x + b in the calibration curve are related as shown on the right. Using this relationship, the slope from the calibration curve can be used to calculate the molar absorptivity. In this experiment, you will create a calibration curve by measuring the absorbance of five dye standard solutions and fitting this data with a linear trendline. You will then determine the concentration of dye in a sample solution that is prepare from a powder. Using this information you will calculate the mass % of dye in a powder and compare that value to an accepted value. Fall 2018 Student Version Lab D: Spectrophotometric Determination of Dye Content in a Powder D-6 Procedure & Observations Procedure 1. Preparation of workstation and glassware a) Label a 600 mL beaker for your waste. b) Prepare five 50 mL volumetric flasks and one 100 mL volumetric flask by rinsing each with three aliquots of distilled water. 2. Preparation of Standard Solution 1−5 a) Collect the Stock Solution in two labelled, clean, dry 50 mL beakers. Use one beaker for rinsing your pipets and the other for preparing Standard Solution 1−5. b) Record the concentration and physical appearance of the Stock Solution. c) Rinse 2, 4, 6, and 8 mL pipets three times with distilled water followed by three times with Stock Solution. d) Prepare Standard Solution 1-5 by pipetting the appropriate volumes of stock (0, 2, 4, 6 and 8 mL) into five 50 mL volumetric flasks, using one flask for each standard. Dilute each to the mark with distilled water. e) Cap and invert the solution seven times to ensure that the solutions are mixed thoroughly. f) Label the solutions Standard Solution 1-5 in order of increasing concentration. Notes and Observations Concentration of Stock Solution: Physical observations of the Stock Solution: 3. Preparation of the Sample Solution a) Record the identity and the mass % of EDS for Identity of the powder: the powder provided. b) Weigh out the assigned powder. The acceptable range is given on the sample container. Record Mass % of EDS in the powder: the mass of the powder to four decimal places. Remember to include units. Make physical observations of the powder. Mass of the powder used: c) Quantitatively transfer the powder from the weighing boat to the 100 mL volumetric flask. d) Fill the volumetric about ⅓ full with distilled water and swirl to dissolve the solid. e) Carefully dilute to the mark. Cap and invert the solution seven times to ensure that the solutions are mixed thoroughly. Make physical observations of the solution. Label this as the Sample Solution. Fall 2018 Student Version Lab D: Spectrophotometric Determination of Dye Content in a Powder Procedure D-7 Notes and Observations 4. Measuring the Absorbance Using a Spec 20: a) Prepare each cuvette by rinsing it three times with a small amount of solution to be observed. Discard each rinsing. After the final rinse, fill the cuvette until it is about ⅔ full. b) Consider the spectrum of EDS provided and determine lmax for the dye. Record the value. c) Zero the Spec-20 by following the instructions provided with the instrument. Use the lmax for the wavelength and Standard Solution #1 as the blank. d) Measure and record the absorbance of each of the remaining standards and the Sample Solution. (Hint: Keep your data organized by making a table in the observations space with the headings Solution and Absorbance.) 5. Clean-up a) Dispose of all leftover solutions down the drain. b) Rinse the cuvettes, pipets, and volumetric flasks at least three times with distilled water and leave them to dry. c) Add your data to the instructor’s spreadsheet. d) In the remaining time, work on the Data Analysis & Calculations questions. Fall 2018 Student Version Lab D: Spectrophotometric Determination of Dye Content in a Powder D-8 Results & Calculations The space below is intentionally blank, and is included for you to work on your draft results and calculations during the laboratory. Details on results and calculations are on the next page in the Report Instructions. Fall 2018 Student Version Lab D: Spectrophotometric Determination of Dye Content in a Powder D-9 LAB D MINI-REPORT INSTRUCTIONS Your report must be typed using a word processor and handed in to your instructor at the beginning of the next lab period. Calculations must be typed using your word processor’s equation editor. It must contain the following header information and sections: Header Information: Title of expt; Your full name, Date submitted, Page number. Follow these instructions for each section. Data Analysis & Calculations 1) Calculate the concentration of the standard solutions. Show a sample calculation for the concentration of Standard Solution #2 only. For full marks you must show all your work, and include units. 2) Prepare a calibration curve. Prepare a calibration curve using Excel. Directions on how to graph using Excel and an example are included in Appendix 2, Section 9. Your final product should resemble Figure 3 of the introduction. This graph should be attached at the end of the report. For full marks your graph should include: a descriptive title with your initials (Y vs X titles are not descriptive); the equation of the line and the R2 value; labeled axes with units. 3) Calculation of the amount of EDS in the powder. a. Use the equation of the line from your calibration curve to determine the concentration of EDS in the Sample Solution. For full marks you must show all your work, and include units. b. Calculate the mass of EDS in the powder. The molar mass of the EDS is in the background. For full marks you must show all your work, and include units. 4) Calculate the mass percent of EDS in the powder, as defined in the equation below. For full marks you must show all your work and present your calculation logically. Mass % of EDS = 100 × mass of EDS in the powder mass of the powder 5) Calculate the percent error. Calculate the percent error using the mass% of EDS in the powder that was recorded in the lab as the actual value. (See Appendix 2, Section 7.1) Summary of Data and Results Use the Table function in your word processor to format the following tables. 1) Format a table with the column headings to summarize the data used to create the calibration curve: Standard Solution #; Volume of Stock Solution (mL); Concentration of standard solution (M); Absorbance at ____ nm. The table title should be Table 1: Summary of data used for the calibration curve. A partially completed table is included as an example below. Table 1: Summary of data used for the calibration curve Standard Solution 1 Volume of Stock Solution (mL) 0 Concentration of Standard Solution (____) Absorbance at ______ nm Fall 2018 Student Version Lab D: Spectrophotometric Determination of Dye Content in a Powder D-10 2) Format a table with the following column headings to summarize your findings for the powder: Absorbance of the Sample Solution, Concentration of EDS in the Sample Solution, Mass of Powder, Mass % of EDS in the Powder, and % Error. Remember to include units in the table headings and an appropriate table title. A partially completed table is included as an example below. Table 2: ________________________ Absorbance of sample solution at _____nm Concentration of EDS in the sample solution (___) Mass of powder (___) Mass % of EDS in powder (%) % Error (%) Comments Give your answer to the following questions in a total of 3-6 sentences. Use complete sentences, and proper spelling, punctuation and grammar. 1) Comment on the success of your experiment based on your % error. Suggest one change to the procedure that would improve the accuracy of your results. 2) If applicable, identify any mistakes or deviations from the procedure that may have contributed to a poor % error. Explain how the mistake or deviation impacts your results. Conclusion Write a 1-3 sentence conclusion that addresses the scientific purpose. Include any key results and statistical measures that qualify your findings. Use complete sentences, and proper spelling, punctuation and grammar. Appendix Additional materials, and figures that are not formatted within the text of the main body, are attached to the end of the document in the Appendix section. For this report, your Appendix will include: • Your calibration curve. Fall 2018 Student Version Lab D: Spectrophotometric Determination of Dye Content in a Powder D-11 Post-Lab Questions Answer the following questions. Solutions are not provided but you may work with other students and ask your instructor for help. These questions may be on the lab midterm and/or final lab exam. 1) Consider the following calibration curve prepared by a student for Lab D when answering the following questions. Absorbance vs. Concentration of Dye 0.8 Emission Intensity 0.7 0.6 0.5 0.4 y = 9.69x + 0.2131 R² = 0.9996 0.3 0.2 0.1 0 0 0.01 0.02 0.03 0.04 0.05 0.06 Concentration a) Identify three mistakes in the presentation of this calibration curve. b) How is this calibration curve different than what you expect? Suggest a reason for this result. 2) Suppose that a student doing this experiment weighed out 0.314 g of powder but wrote down 0.134 g in her notebook and didn’t catch the mistake when she was doing her calculations. a) How does this error affect the mass % of EDS she calculates compared to the correct value? b) Would the percent error she calculates for mass % EDS be more positive or more negative compared to the % error calculated using the correct mass of the powder? Explain your answer. Fall 2018 Student Version
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