Objectives Design an experimental procedure to determine the concentration of ca
ID: 480583 • Letter: O
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Objectives Design an experimental procedure to determine the concentration of caffeine in a commercial product using techniques learned from two previous experiments Learn rules of scientific writing - prepare a lab report Introduction Beer's Law (Abs_lambda = epsilon*b*c, where Abs_lambda - absorbance value at wavelength lambda, epsilon = absorptivity constant, b = path length of cuvette (usually 1 cm), c = concentration) tells us that the absorbance of light by a compound at a certain wavelength is proportional to the concentration of that compound. Beer's law is a "limiting law" and only applies to quite dilute solutions. Therefore, when using Beer's law one wants to keep all the absorbance values between 0-1 absorbance units, ideally 0-0.7 abs units. As long as one knows the units they are working with, the concentration units can be anything and will dictate what the units on e are. The overall objective is to use a class generated calibration curve to determine how the extraction of caffeine from a bag of tea is dependent on the extraction technique used by the tea drinker. What is the reported molar absorptivity constant (e) at 270 nm for caffeine dissolved in water? List your reference source and make sure and include the units in your answer. Convert the molar absorptivity into units of (mg/ml)^-1 cm^-1. Show your work. Consider the preparation of five standard solutions to make. Identify 5 concentrations (in mg/mL) that you will make and measure the absorbance of to generate a calibration 'curve' for caffeine. Propose a sample data table to include in your laboratory notebook. Consider how you will make all of those solutions? Weigh out 5 different caffeine amounts and dilute to the appropriate volume? Make a standard stock solution and use a serial dilution technique? Make a standard stock solution and use systematic (parallel) dilutions? Identify one of the above options and explain why it will give the best accuracy/precision, minimize materials/waste, etc. Show explicit work for how to generate all five solutions you want to use in #3.Explanation / Answer
Spectroscopy requires very dilute solutions. It is very difficult to weigh such small amount every time and can give rise to serious error. Thus , dilution is a better techniques. Now let us compare serial dilution and parallel dilution techniques.
A serial dilution consists of a series of solutions where each solution made becomes the source solution for the next solution in the series. The disadvantage is that any measurement errors made early on will be propagated through the remaining solutions.
Parallel dilution consists of a series of solutions where all are made from the original solution. Errors are less critical than in serial dilution.
Thus the best method would be systematic or parallel dilution.
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