exam 1 - Chemistry 222 Spring 2008 Exam 1: Chapters 1-4 80...

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1 Chemistry 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 two of 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 MgCl 2 (FW = 95.211 g/mol) 100 mg Mg 2+ x 1 mol Mg 2+ x 1 mol MgCl 2 x95.211g MgCl 2 x 1 g x0.0500 L = 0.0195 8 g MgCl 2 1 L 24.305 g 1 mol Mg 2 + 1 mol MgCl 2 1000 mg So, dissolve 0.0196 g MgCl 2 in 50.0 mL of solution b. starting with a 0.100 M magnesium chloride solution Since each mole of MgCl 2 that dissociates liberates 1 mole of Mg 2+ , a 0.100 M MgCl 2 solution is also 0.100 M Mg 2+ 100 mg Mg 2+ x 1 mol Mg 2+ x0.050 L x1 L = 2.06 mL 1 L 24.305 g 0.100 mol Mg 2+ So, dilute 2.06 mL of 0.100 M MgCl 2 solution 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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2 3. A statistical analysis is an essential component in the evaluation of experimental results.
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This note was uploaded on 04/07/2008 for the course CHEM 222 taught by Professor Lamp during the Spring '08 term at Truman State.

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exam 1 - Chemistry 222 Spring 2008 Exam 1: Chapters 1-4 80...

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