quant2 - Chemistry 222 Fall 2007 Exam 2: Chapters 5-8, 27...

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1 Chemistry 222 Name__________________________________________ Fall 2007 Exam 2: Chapters 5-8, 27 80 Points Complete two (2) of problems 1-3, problem 4, and three (3) of problems 5-8. CLEARLY mark the problems 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. A saturated solution of BaSO 4 (K sp = 1.1x10 -10 ) that originally held a volume of 1.00 L is allowed to evaporate until the solution volume is 0.500 L. How does the new concentration of Ba 2+ compare to the concentration in the original solution? Justify your response. Since the solution was initially saturated, a decrease in solution volume will force an increase in the concentrations of Ba 2+ and SO 4 2- in solution, leading to supersaturation. This increase will cause the K sp for barium sulfate to be exceeded, resulting in the precipitation of BaSO 4 , and a decrease in the concentration of Ba 2+ and SO 4 2- back to their original values. Therefore, the [Ba 2+ ] after evaporation will be the same as its initial concentration. 2. Consider the table of activity coefficients on the last page of this exam. As you move from left to right across any row on the table, the values for activity coefficient decrease. As you move down in a given column, the activity coefficient also decreases. Clearly describe the phenomena that cause these trends. No calculations are necessary. The cause for both trends is rooted in electrostatic interactions between the ion of interest and other materials in solution. Moving left to right across a row in the table, the ionic strength of the solution increases. As µ increases, there is a greater probability that the ion of interest will undergo an interaction with a third-party ion in solution, thus decreasing its activity in the reaction of interest. Moving down in the table, the charge on the ion is increasing. This increase in charge provides for stronger electrostatic interactions with all other ions in solution, also reducing activity.
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quant2 - Chemistry 222 Fall 2007 Exam 2: Chapters 5-8, 27...

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