plest help to plot the graph and determine the concentration of the the unknown from the calibration curve

EXPERIMENT 8 SPECTROPHOTOMETRIC ANALYSIS OF TRANSITION METAL CATIONS PURPOSE The purpose of this experiment is to determine the concentration of an unknown copper(ll) solution by spectrophotometric analysis. INTRODUCTION The Spectronic 20 colourimeter is an extremely versatile instrument that is useful for spectrophotometric or colourimetric determinations of solutions. Spectrophotometric determination of an aqueous ion is many times difficult because concentrations may be very low. If the sons do not produce an intensely coloured solution, they can sometimes be converted to complex ions that are brightly coloured, absorbing light in the UV/visible range. The copper(II) ion can be converted to the intensely coloured complex, Cu(NH3)2, by the addition of concentrated aqueous ammonia. The absorbance at various concentrations is used to plota graph to determine the concentration of copper(II)ions in an unknown solution. PROCEDURE 1. 2. Switch on the spectrophotometers to warm-up set max 645 nm. Clean and dry six test tubes and number them 1-6. The first four test tubes will be used as the reference solutions to calibrate the Spectronic 20 spectrophotometer Prepare calibration standards and samples of the coordination compound prepared earlier according to the directions in Table 8.1. Mer the contents of each test tube thoroughly. Concentration of standard Cu? solution: 0.2 M Table 8.1: Preparation of Calibration Standards and Unknown Samples Test Tube No Contents 1 100 mL of IM HINO) from the horse 4.0 mL of standard solution 1.0 LI MENING from the burene. Mix well 70 mL of standard routes and 30 mL of M ENO from the burete Mix well 4 100 ml. of standard Curstice 5 About 0.05 gefunktion sample and 100 LIMHINO, from the bute. Mix well. Centrifuge the samples and filter of 6 About 1 of unkne sample and 100 ml of MIINO, from the batte Mix well. Centrifuge the samples and files of 3. Number six clean, dry cuvettes 1-6. Transfer the contents of the test tubes into the cuvettes accordingly (about 90% full). Measure the absorbance of each solution at 645 nm. Note that the content of test tube 1 is your blank solution. Enter your data in Table 8.2. Concentration of standard Cu? solution: 0.2 M Table 8.1: Preparation of Calibration Standards and Unknown Samples Test Contents Tube No. 1 2 3 4 10.0 mL of 1 M HNO, from the burette. 4.0 mL of standard Cu?" solution and 6.0 mL of 1 M HNO, from the burette. Mix well. 7.0 mL of standard Cu? solution and 3.0 mL of 1 M HNO, from the burette. Mix well. 10.0 mL of standard Cu2+ solution. About 0.05 g of unknown sample and 10.0 mL of 1 M HNO, from the burette. Mix well. (Centrifuge the samples and filter oft) About 0.1 g of unknown sample and 10,0 mL of 1 M HNO, from the burette. Mix well. (Centrifuge the samples and filter oft) 5 L RESULTS Table 8.2: Absorbance Data Test Tube Concentration (M) Absorbance (nm) No. 1 1 0.055 2 0.251 3 3 0.397 4 1 0.582 5 0.060 6 0.058 QUESTION 1. Plot a graph of absorbance versus concentration (calibration curve) of known Cu solutions 1. Determine the concentration of the unknown from the calibration curve. 2. What is the general relationship between concentration and absorbance? 3. Explain how the concentration of the unknown is determined