1Chemistry 222 Name__________________________________________ Spring 2008 Exam 1: Chapters 1-4 80 Points Complete two (2) of problems 1-3 and four (4) of problems 4-8. CLEARLY mark the problem you do not want graded. You must show your work to receive credit for problems requiring math. Report your answers with the appropriate number of significant figures. Do twoof problems 1-3. Clearly mark the problem you do not want graded. (10 pts each) 1. You need to prepare a 50.0 mL of solution that is 100.0 ppm magnesium. Clearly describe how you would prepare this solution starting from the points below. a. starting with solid magnesium chloride Remember, magnesium chloride is MgCl2(FW = 95.211 g/mol) 100 mgMg2+x 1 mol Mg2+x 1 mol MgCl2x95.211gMgCl2x1 g x0.0500 L= 0.01958g MgCl21 L24.305 g1 mol Mg2+1 mol MgCl21000 mgSo, dissolve 0.0196 g MgCl2in 50.0 mL of solution b. starting with a 0.100 M magnesium chloride solution Since each mole of MgCl2that dissociates liberates 1 mole of Mg2+, a 0.100 M MgCl2solution is also 0.100 M Mg2+100 mgMg2+x 1 mol Mg2+x0.050 Lx1 L =2.06 mL 1 L24.305 g0.100 mol Mg2+So, dilute 2.06 mL of 0.100 M MgCl2solution to 1.00 L 2. We ignore the contribution of buoyancy in virtually all of the mass measurements we make in the laboratory. How can we get away with this? Identify one situation where we would be unable to ignore buoyancy-introduced error. The buoyancy correction accounts for the varying volume of air displaced when a sample is weighed compared to the volume displaced when the balance was calibrated with calibration weights. When the density of the sample being weighed is similar to the density of the balance weights (8 g/mol), the error due to buoyancy is minimal (remember the plot we discussed in class). In general buoyancy errors are minimal because we have been weighing solid samples and because we do our critical weighing by difference. If we were to weigh samples of very low density (like water or organic solvents or especially gases), we should account for buoyancy errors.
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