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BufferChallenge_KEY - Chem 1A Fall 2007 Challenge Problem...

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Chem 1A Fall 2007 10/10-16/07 Challenge Problem - Buffers 1) In order to make carbonated water, soda makers dissolve carbon dioxide gas, CO 2 (g), into water in the following reaction: CO 2 (g) CO 2 (aq) a) Write the equilibrium expression (Henry’s Law constant), K H, for the reaction. 2 )] ( [ 2 CO H P aq CO K = b) How does K H vary with increasing temperature? Use the evidence in Figure 1 to justify your response. K H decreases as temperature increases. At higher temperatures, if comparing the same pressure (the denominator), the solubility of CO 2 (the numerator) is smaller for higher temperatures. 2) Using the data in Figure 1, which of the following conditions would be most optimal for making carbonated water? High Pressure Low Pressure High Temperature Low Temperature Justify your answer using Le Chatelier’s principle. Increasing the pressure of the gas (P CO2 ) drives the chemical equation towards creating more products, dissolved CO 2 , to sustain the equilibrium ratio, K H . According to Figure 1, decreasing temperature at constant pressure, results in increased dissolved CO 2 , so decreasing the temperature (or removing heat) must increase the K H , which drives equilibrium to favoring more products. Figure 1: Solubility of CO 2 (aq) in water vs. Pressure of CO 2 (g)
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Chem 1A Fall 2007 10/10-16/07 Challenge Problem - Buffers 3) Kristine and Karen are arguing about the best conditions for making soda water. Who do you agree with? Explain your reasoning. Kristine: We’ll get more CO 2 dissolved in water if we increase the air pressure above the water to 5 atm. Karen: We’ll get more CO 2 dissolved in water if we put 0.5 atm of CO 2 gas in a vacuum chamber above the water. **NOTE to GSI’s: The composition of air has been intentionally left out to see if students recognize the need for knowing the difference between partial and total pressure. When students ask, here is the composition of air: N 2 78.084% O 2 20.947% Ar 0.934% CO 2 0.033% I’d agree with Karen. The percent of CO 2 (g) in air is only around 0.033% so even though the total pressure in Karen’s scenario is less than the total pressure in Kristine’s scenario, the partial pressure of CO 2 (g) in Karen’s scenario is greater than the partial pressure of CO 2 (g) in Kristine’s scenario. 4) One can holds 355 mL of soda and has an internal pressure of about 2 atm at 0 o C. If you were to drink the entire can of ice cold soda, what volume of CO 2 gas would be evolved in your stomach at 35 o C, if we assume your stomach is at 1 atm of pressure and could expand to any volume? (K H = 0.117 mol/L atm at 0 o C, 0.056 mol/L atm at 35 o C) At 0 o C, soda in dissolved CO of mol x atm L mol x atm L mol P aq CO K CO H 2 2 08307 . 0
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