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UCLA CHEM 20L - Revised Assignment

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Data AnalysisError AnalysisDiscussionConclusions2 (REVISED) - SOLUTIONS, CONCENTRATIONS, DETECTABILITY Topics for Study: Videotapes #5802 and 5806, concentrations, Beer’s Law Glossary: molarity, ppm, ppb, colorimeter, cuvette 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. * 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. Fall 2002 1BEER’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. 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 2.9979 x 108 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 = cn 1λ 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 6.6262 x 10-34 Js. Substituting the previous equation into this one gives E = hcnλ Thus, the energy of the radiation is directly proportional to the frequency and inversely proportional to the wavelength. Radiation of high energy (e.g., X-rays) has short wavelengths; low energy radiation (e.g., 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. Fall 2002 2The mathematical expression that describes the absorption of light by a substance can be stated as log PPo = -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.e., not absorbed) by the solution. This fraction is called the transmittance and is usually represented by the symbol T. When this substitution is made, the equation, commonly known as Beer’s Law is written as log T = -αbc For practical uses, the previous equation is slightly modified. The term "-log T" is given the name absorbance and represented by the symbol A. Since absorbance is dimensionless, the units on the right-hand side of this equation must cancel. If b is measured in centimeters and the


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UCLA CHEM 20L - Revised Assignment

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