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MH expt2 new 1 Sum 16 - Professor - Henary
General and Organic Chemistry Laboratory I (Chem 14BL)
University of California Los Angeles
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2 (REVISED) - SOLUTIONS, CONCENTRATIONS, BEER’S LAW
Topics for Study: Techniques Videos (“Use of a Pipet” and “Solutions Preparation”), concentrations, Beer’s Law and Chapter III in the lab manual. Glossary: molarity, ppm, ppb, colorimeter, cuvette, absorbance, transmittance
TECHNIQUES
In this assignment you will use the following procedures: Manipulative skills use an analytical balance make a volumetric solution use a volumetric pipet perform serial dilutions take measurements on a colorimeter Theoretical skills calculate the molar concentration of a solution calculate serial dilution concentrations convert between the units of molarity, weight/volume percent, ppm, ppb. use a calibration curve to determine a concentration study the spectrum of a compound
SAFETY
Always wear safety glasses or goggles and a protective lab coat or apron. Brilliant Blue FCF*, the food coloring dye, that you will use in this experiment, is intensely colored. Wear gloves and handle the dye carefully avoiding spilling on the balance or bench top. Wipe any spills immediately and wash any residue with large quantities of water.
BACKGROUND
Preparing solutions with specific concentrations or within a range of concentrations constitutes a fundamental skill for any lab scientist. Whether these solutions are prepared by dissolving a solid in the solvent, or by diluting other solutions whose concentrations are known, depends on the circumstances of the experiment, the concentration needed, and the precision required. This last factor, the precision needed, often dictates what equipment and what technique must be used.
There are many concentration units. Tradition in a discipline and ease of use for a specific purpose frequently determine which unit is used at a given context. Chemists tend to use molarity because it facilitates understanding the stoichiometric relationships that are being studied; biochemists use millimoles and microliters to simplify the numbers when they are using very small amounts and very dilute solutions; oceanographers and geologists use parts per million to describe trace concentrations of materials in large water bodies and in ores; analysts often use weight percent because it simplifies the instructions for preparing standard stock reagents. Converting among units should be a routine task.
BEER’S LAW
Electromagnetic radiation, energy propagated through space by electrical and magnetic disturbances, is oftern described in terms of the wave parameters of wavelength and frequency.
- Disodium salt of ethyl [4-[p-[ethyl (m-sulfobenzyl) amino]-α-(o-sulfophenyl) benzylidene]-2,5-cyclohexadien-1-
ylidene] (m-sulfobenzyl) ammonium hydroxide. It is obvious why the common names, Brilliant Blue FCF or Food Blue 2 are used.
The total spectrum or continuum of electromagnetic radiation includes the familiar phenomena of radio waves, radar, visible light and X-rays. Each represents electromagnetic radiation of different energy and wavelength. Visible light, which represents only a small portion of the total spectrum, ranges from wavelengths about 380 nm (nanometer = 10-9 m) for blue light to about 750 nm for red light m). The different colors of visible light, which may be seen in a rainbow, each correspond to a different wavelength.
Common to all electromagnetic radiation is the velocity with which it moves. When the radiation is moving through a vacuum, its speed is represented by the symbol c and is equal to
2 x 10 8 m s-1. If the radiation is moving through any medium other than a vacuum, then the velocity is c/n where n, a dimensionless number, is the refractive index of the medium. The velocity and wavelength of the electromagnetic radiation are related by the frequency. Thus,
c/n = υλ or, upon rearrangement
υ =
c n
1
λ
where λ is the wavelength in meters and υ is the frequency. The units of υ are s-1.
Implicit in the above discussion of light is the assumption that it behaves like a wave. This picture of electromagnetic radiation only partially describes its total nature. In some instances, an interpretation of its behavior as a stream of particles known as photons is more convenient. The energy of the photon can be represented by the equation
E = hυ
where h is Plank's constant and is equal to 6 x 10-34 Js. Substituting the previous equation into this one gives
E =
hc nλ
Thus, the energy of the radiation is directly proportional to the frequency and inversely proportional to the wavelength. Radiation of high energy (e., X-rays) has short wavelengths; low energy radiation (e., microwaves) has long wavelengths.
The intensity of the radiation is independent of the frequency and the wavelength; it is proportional to the number of photons in the beam. Because this experiment deals with the interaction of visible light with matter, the remainder of this discussion will use only visible light as an example. The phenomenon is, however, generally applicable to the entire electromagnetic spectrum and is fundamental to all areas of spectroscopy.
When a beam of light is passed through a substance, some of the energy is often absorbed by the substance. This causes a decrease in intensity of the transmitted beam. If the substance absorbing the light is a solute in solution then changing the concentration of the solution changes the amount of solute in the path of light. Alternatively, we can change the amount of substance in the light beam by increasing the thickness of the solution through which the light beam passes. The mathematical expression that describes the absorption of light by a substance can be stated as
log
P
Po = -αbc
where α is the absorption coefficient which is characteristic of the absorbing species; b is the thickness of the solution through which the light beam passes, c is the concentration of the solution in moles/L, and P/Po is the fraction of light transmitted (i., not absorbed) by the
(a) Visible spectrum of MnO 4 - showing (b) Beer’s Law plots for MnO 4 - for several wavelengths. maximum absorbance at 525 nm Dotted line shows points for solution of same concentration as spectrum in (a)
PRE-LAB ASSIGNMENT (CHECK CCLE FOR THE UPDATED GUIDELINES)
What is the molar concentration of a solution prepared by dissolving 0 g of KCl in a 50-mL volumetric flask?
Given 5-mL and 10-mL pipets and 50-mL and 100-mL volumetric flasks, what are the possible concentrations you could make in one dilution of the KCl solution above? What are the maximum and minimum concentrations that you could make by doing a second dilution? 3.. How long is one ppm of a kilometer? One inch is a ppm of how many miles? (Hint: how many miles = 10 6 inches?) A second is a millionth of how many days?
Calculate the percent transmittance of a solution if its absorbance is 0.
Calculate the absorbance of a solution if the percent transmittance 74%
What is the purpose of the “blank” solution in this experiment?
Set up your notebook for the experiment including, the title, a references to the source. Include any changes to the procedure that you know you will make.
Write a short summary of the purpose of the experiment including a brief flow chart summary of the key procedures of the experiment
Under a heading “Safety” provide MSDS information for Brilliant Blue FCF
Note: Start a NEW page in your notebook for item 10. The previous material will be turned in at the beginning of the period; this page will be turned in at the end of the lab period.
400 500 600 700
A b s o r b a n c e
Wavelength (nm)
Concentration
480 nm
630 nm
450 nm
525 nm
A b s o r b a n c e
- Set up three data tables to record
- the specific equipment used for the serial dilutions and the concentrations of the resulting
solutions. Leave a space in the table after the first entry for an explanation of the scheme that you will use to get to a solution within the specified concentration range.
the measured absorbances of the three most dilute solutions.
the spectral data (wavelengths and absorbances) for the solution chosen. (Be sure to indicate
in the table title, which solution is used.
- Read chapter III.
PROCEDURE
Prepare a 100-mL volumetric solution. Weigh the packet of food coloring dye to the nearest tenth of a milligram. Record the weight. Following Procedure 2, Method 1, carefully transfer the dye to the 100-mL volumetric flask. Re-weigh the empty packet to determine the weight of dye that you have transferred into the volumetric flask.
What procedural errors could cause this weight to not be an accurate record of the solid in the volumetric flask? Evaluate whether you made any of these errors? If so, begin again.
Complete the final procedural step in the preparation of this volumetric solution, which we will then refer to as Solution 1.
The molecular weight of Brilliant Blue FCF is 789 g/mol. Calculate the concentration of Solution 1 in units of molarity (M), weight percent (w/v%), ppm, and ppb. How precisely do you know this value?
Perform serial dilutions. Using the 10-mL volumetric pipet, transfer 10 mL of Solution 1 to a second 100-mL volumetric flask. Fill the bulb approximately two-thirds full with distilled water. Mix well. Dilute to the mark. Mix well by inverting the flask back and forth for several minutes.
Calculate the concentration of this diluted solution, which you should label as Solution 2, in the four sets of units used earlier.
Transfer Solution 1 to a clean, dry container and thoroughly rinse the inner walls of the volumetric flask three or four times with distilled water from the wash bottle. Design a scheme to continue diluting solutions beginning with Solution 2 so that you prepare a solution with a concentration between 5 x 10 -7 M and 8 x 10 -7 M.
Assume you did not make any procedural errors. The manufacturer’s tolerance for a weight determined by difference on the analytical balance is ±0 mg; the error in using the volumetric flask is ±0 mL
Does the volumetric flask need to be dry? Why or why not?
You will probably not need Solution 1 again. However, it is good technique to not discard any solutions until you are sure that the experiment has worked correctly. Why is it necessary to put Solution 1 in a dry container?
NOTEBOOK REPORT (CHECK CCLE FOR THE UPDATED GUIDELINES)
Data Analysis Record all data and identifying information directly in your lab notebook.
Describe your serial dilution procedure. Explain the rationale for your procedure. (That is, explain why you rejected or did not consider other possible schemes.)
Using millimeter graph paper, plot a Beer’s Law calibration curve based on your calculated concentrations and absorbances that you measured for the three solutions at 620NM
Using millimeter graph paper, plot the spectrum (Absorbance vs wavelength) of the diluted solution that you chose.
The value for the molar extinction coefficient for Brilliant Blue FCF is reported in the literature as 1 x 10 5 M-1 cm-1. Using this value and your measured absorbances, calculate the measured concentration for each of your three solutions.
Error Analysis Calculate the inherent error for each of the solutions you prepared in sections 1 and 2. (See Sect III in your lab manual.)
Calculate the percentage difference between the calculated and measured values for the concentrations of your diluted solutions.
Discussion and Extrapolation
Explain any difference between your calculated values for the concentrations of your three solutions and the concentrations you determined from the absorbances.
Set up a table with columns for the wavelengths you used for your spectrum and the absorbances of the solutions you used for your calibration curve. Fill in your experimental data. (This should be one column and one row of the table.).. Based on the fact that Absorbance is proportional to concentration, calculate the predicted absorbances for your solutions at 550 nm, and the absorbances you would have obtained for your solution closest in concentration to the one you measured.
Sketch on your Beer’s Law plot, the line you would have obtained if you had taken your measurements at 550 nm rather than 620 nm.
Sketch on your experimental spectrum plotted above, the spectrum that you would have expected for one of your other diluted solutions.
Explain why the maximum absorbance is used whenever possible when taking Beer’s Law measurements.
Conclusions
Briefly summarize your experimental data in terms of its fit to a linear Beer’s Law plot.
Practical Considerations
Assume you have a 1 M stock solution for making a set of solutions with concentrations that are 0, 0 and 0 M. You have only one 100-mL volumetric flask. Which order should you make the solutions from the stock solution in order to minimize error. Why? Hint: Assume rinsing is not thorough and that you leave one drop (0 mL) of the previous solution in the volumetric flask each time.
SOLUTIONS, CONCENTRATIONS, DETECTABILITY
MASTERY CHART
Performance self-assessment novice need a lot more practice
apprentice would like more practice
master ready to be tested
Manipulative skills
use an analytical balance ___ ___ ___
make a volumetric solution ___ ___ ___
use a volumetric pipet ___ ___ ___
perform serial dilutions ___ ___ ___
take measurements on a colorimeter ___ ___ ___
Theoretical skills
calculate the molar concentration of a solution ___ ___ ___
calculate serial dilution concentrations ___ ___ ___
convert between the units of molarity, weight/volume percent, ppm, ppb
___ ___ ___
use a calibration curve to determine a concentration ___ ___ ___
relate a Beer’s Law plot and a spectrum ___ ___ ___
MH expt2 new 1 Sum 16 - Professor - Henary
Course: General and Organic Chemistry Laboratory I (Chem 14BL)
University: University of California Los Angeles
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