39 - AP CHEMISTRY Lab 8-1 Spectrophotometric Iron Analysis...

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AP CHEMISTRY Lab 8-1 Spectrophotometric Iron Analysis Pre-Lab Questions- (Must be completed before lab work begins.) 1. List the purpose for each of these substances in the iron analysis. a. 1, 10-phenanthroline ( o -phen) b. NaCH 3 CO 2 c. NH 3 OH + Cl - Write a balanced equation for its reaction with Fe +3 . The oxidation half-reaction of NH 3 OH - is 2NH 3 OH + (aq) N 2 (g) + 2H 2 O(l) + 4H + (aq) + 2e - 2. A concentration of 2 ppm Fe +2 means 2 g Fe +2 in 10 6 g solution. Assuming the density of the solution is 1.0 g/mL, express 2 ppm Fe +2 in a. μ g Fe +2 / mL solution b. mg Fe +2 / L solution 3. What is the color of a solution that absorbs 600 nm EM radiation? 4. If [Fe( o -phen) 3 ] +2 appears red-orange, what is the approximate wavelength for its maximum absorption? 5. A sample containing 18.0 mg of iron dissolves in 250 mL of solution. One mL is then withdrawn and diluted to 10 mL. What is the iron concentration in the diluted sample? Express your answer in μ g Fe +2 / mL and in ppm Fe. 6. A test solution is prepared by pipeting 2.0 mL of an original sample into a 250 mL volumetric flask and then diluting to the mark. The concentration of iron in the test solution is determined (from a calibration curve) to be 1.66 μ g / mL or 1.66 ppm. What is the concentration in the original sample? Express your answer in μ g Fe +2 / mL and in ppm Fe. INTRODUCTION - The principle underlying a spectrophotometric method of analysis involves the interaction of electromagnetic (EM) radiation with matter. The ultraviolet, visible, and infrared regions of the EM spectrum are the most common used in analyses; in this experiment the visible region is used. The wavelength range for the visible spectrum is about 400 nm to 700 nm; the 400 nm radiation approximates a violet color while the 700 nm region has a red color. Every chemical species (atoms, molecules, or ion) possesses a characteristic set of electronic, vibrational, and rotational energy states. Because they are characteristic of a chemical species, energy transitions between these states are often used to identify their presence and/or concentration in a mixture. This unique set of energy states for chemical specie is therefore analogous to the unique set of fingerprints possessed by each person—both can be used for characteristic identifications. The absorption of EM radiation from the visible spectrum is a result of the excitation of an electron from a lower to a higher state in the chemical specie. The energy of the radiation that is absorbed is equal to the difference between these two energy states. The species (atom, molecule, or ion) that absorbs the radiation is in an excited state . The EM radiation that is not absorbed, and therefore passes through the sample, is detected by an EM detector (either our own eye or an instrument). The absorbed energy, E , is related to the wavelength, λ , of the EM radiation by the equation, E = h c λ , where H is Planck’s constant and c is the speed of light. When our eye is the detector, the color that we see is the EM radiation, which the sample does NOT absorb.
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39 - AP CHEMISTRY Lab 8-1 Spectrophotometric Iron Analysis...

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