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# Determining the Concentration of an Unknown Solution CoCl2.docx

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School
Western University
Department
Chemistry
Course
Chemistry 2223B
Professor
Tom Haffie
Semester
Winter

Description
Determining the Concentration of an Unknown Solution CoCl2 6H20 Using Spectroscopy Data Collection: First, we must determine the λmax. We must use the λmax for the experiment. We found the λmax by adjusting the wavelength to find the wavelength that has the highest absorption with the lowest transmission. Absorption Spectrum Wavelength (nm) Absorption Transmission 400 0.07 79 450 0.42 35 500 0.81 16 550 0.36 43 600 0.11 83 650 0.07 83 0.9 Absorption Rate at Different Wavelengths 0.8 0.7 0.6 0.5 0.4 Absorption 0.3 0.2 0.1 0 400 450 500 550 600 650 Wavelength (nm) As shown in the graph above, the absorption peaks somewhere between 500 – 525 nm. We can now zero in and use the absorption spectrum at every 10 nm to find a more precise λmax. Absorption Spectrum at 10nm Wavelength (nm) Absorption Transmission 490 0.78 17 510 0.85 15 520 0.79 16 From the chart above, we can see that the wavelength 510nm gives the highest absorption and the lowest transmission. This means that we will be using this transmission for the rest of the experiment. Our test tube #1 was 100% of our cobalt chloride solution. Test tube #2 had 80% of our cobalt chloride solution. Test tube #3 had 60% of our cobalt solution and so on. Our cobalt chloride solution was made to be a 0.14 molarity solution. Using Beer’s Law Tube # Concentration (molarity) Absorbance at 510nm 1 0.14 0.89 2 0.112 0.72 3 0.084 0.55 4 0.056 0.40 5 0.028 0.19 6 0 0.00 We made our stock solution by figuring out the molar mass of CoCl2 H2O The molar mass of CoCl2 H2O = 237.93 g/mol We want a 0.14 molarity and since we are using 100mL, the molarity becomes 0.014 0.014 x 237.93 = 3.33 g Therefore we need 3.33grams of cobalt chloride but we made 3.34 grams because it was hard to get that precise. (3.34-3.33)/3.33 = 0.003 or 0.3% error. Absorpance of Light Compared to the Concentration of the Cobalt Chloride Solution (Caibration Curve) 1 y = 6.474x 0.9 0.8 0.7 0.6 0.5 0.4 Absorpance 0.3 0.2 0.1 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 Concentration (molarity for 100mL) A = * b * c Using this formula (Beer’s Law), we should see a linear relationship between the absorbance and the concentration. This is because b is negligible because we are using curvettes and they are typically 1 cm in width, so it is often left out of the equation. This leaves A * c absorbance = some constant multiplied by the concentration. Using excel, I graphed the absorbance versus the concentration and with a line of best fit, I found the constant to be 6.474 I can no
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