Chemistry 14BL- Week 2 Notes
Chemistry 14BL- Week 2 Notes CHEM 14BL
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This 13 page Class Notes was uploaded by Daniel Ochs on Monday April 11, 2016. The Class Notes belongs to CHEM 14BL at University of California - Los Angeles taught by Casey in Spring 2016. Since its upload, it has received 33 views. For similar materials see General and Organic Chemistry Laboratory I in Chemistry and Biochemistry at University of California - Los Angeles.
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Date Created: 04/11/16
Lecture 2 4/6/16 Volumetric Glassware • Volumetric means report to hundreth decimal place • 5-mL volumetirc pipet: 5.00 mL • 25-mL volumetric buret: 25.00 mL • 100-mL volumetric ﬂask: 100.00 mL • Acceptable range for a 10-mL volumetric pipet • 9.96 to 10.04 Analyzing Real Samples • Various Methods to choose from • Method choice depends on what we want the precision to be 1- Gravimetric Analysis • “Weight Relations” • Very precise quantitaitve technique • Precision— quantitative analysis (+/- 0.1%) • Disadvantages • Difﬁcult: must be able to selecitvely isolate the component to be measured as a solid compoind • Time Consuming • Expensive 2- Volumetric Analysis • Precision— quantitative analysis (+/- 0.5%) • Disadvantages • Time Consuming (though faster than gravimetric analysis) • Expensive 3- Spectrophotmetric Analysis • Beer’s Law • Precision— semi-quantitative (+/- 5 to 10%) • Advantages • Rapid • Easy to establish quality control meausres for reliability • Lends itself to on-line monitroring • Disadvantages • Have to set up chemistry so that the absorbtion of light by chemical species of interest will have a very different absorbtion proﬁle than the other constituents in the solution • See Theory on Beer’s Law in Handout Fundamental Law of Spectrophotometry • For dilute solutions, the following relation between the intensity of incident light (Io) and the transmitted light (I) holds: I/Io = 10^(-eLC) • Io= intensity of incident light beam • I= intensity of transmitted light beam • e= Molar absorpitivity constant (unit in L/mol cm) • Depends on the indentity of the chemical species and the wavelength selected • C= Concentration (mol/L) of the chemical species • L= Depth of the solution (cm) trnasversed by the light Transmittance, Absrobance, and Concentration • Transmittance: T = I/Io [ranges from 0 to 1] • I/Io = 10^(-eLC) = T • Absorbance: A = -logT • Beer’s Law: A = eLC • e and C are related to the chemical species being analyzed • A is directly proportional to C Beer’s Law: A = eLC • A is detemined in the labratory during the experiement • Falls in range between 0.1 to 1.0 • C is the unknown quantity • e is a physical characteristic of the chemical species being measured • e depends on wavelength • Only a constant if and only if: • Wavelength must be ﬁxed during analysis • Sample has a low (ambigous) concentration • L is a Constant • See Graphical Representations of Beer’s Law Graphs in handout Two Major Problems in Beer’s Law • Beer’s Law doesn’t work if the concentration of the solution is too high (A is greater than 1.0) • Solution: dilute the sample • Need to take care of background interference • Solution: use a blank solution • Blank solution: contains ALL the chemical species except for the one you want to measure • Example of blank solution in Handout Beer’s Law Practice Problems Concept of Direct Proprotionality • For the same chemical species and same wavelength: • A / A’ = eLC / eLC’ • Since both e and L are under these conditions: • A / A’ = C / C’ Standard Solution • A standard solution has a precisely known concentration • Standard solution concentration can be calculated without using Beer’s Law by using: • M1 V1 = M2 V2 • Beer’s Law Calibration Graph • Concntration of unknown = unknown absorbance / slope
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