hw2solution_pdf

1 t 1 1 t 2 parenleftbigg 8 314 j mol k

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1 T 1 - 1 T 2 = parenleftbigg 8 . 314 J mol · K parenrightbigg ln parenleftbigg 760 torr 120 torr parenrightbigg 1 300 K - 1 353 . 1 K = 3 . 07 × 10 4 J / mol 026 3.3points What is the aqueous solubility of air at 1 atm and at 20 C? The solubility of air in water at 20 C is 0 . 00079 mol / L / atm and the average molar mass of air is 28 . 97 g / mol. Correct answer: 22 . 8863 mg / L. Explanation: S air in water = 0 . 00079 mol / L / atm MW air (average) = 28 . 97 g / mol S = k H × P = (0 . 00079 mol / L / atm) (1 atm) × (28 . 97 g / mol) parenleftbigg 1000 mg 1 g parenrightbigg = 22 . 8863 mg / L . 027 3.3points A 100 mL solution that is 0.02 M in Na + can be prepared by adding how many g of Na 2 SO 4 (of MW 142 g) to water and diluting to a final volume of 100 mL? 1. 0.28 g 2. 0.045 g 3. 0.028 g 4. 23.8 g 5. 0.14 g correct Explanation: V soln = 100 mL [Na + ] = 0 . 02 M MW Na 2 SO 4 = 142 g / mol V final soln = 100 mL Molarity is moles solute per liter of solution. We want a solution that is 0.02 M in sodium ion, or 0 . 02 mol Na + 1 L soln . We use this ratio to determine the moles of Na + in 100 mL of the solution: ? mol Na + = 100 mL soln × 1 L soln 1000 mL soln × 0 . 02 mol Na + 1 L soln = 0 . 002 mol Na + The sodium ion comes from the dissociation of Na 2 SO 4 . There are two Na + per Na 2 SO 4 . We use this ratio to find the moles Na 2 SO 4 necessary to prepare the solution: ? mol Na 2 SO 4 = 0 . 002 mol Na + × 1 mol N 2 SO 4 2 mol Na + = 0 . 001 mol Na 2 SO 4 We use the molar mass of Na 2 SO 4 to con- vert to grams: ? g Na 2 SO 4 = 0 . 001 mol Na 2 SO 4 × 142 g Na 2 SO 4 1 mol Na 2 SO 4 = 0 . 142 g Na 2 SO 4 028 3.3points At 20 C the vapor pressure of dry ice is 56.5 atm. If 10 g of dry ice (solid CO 2 ) is placed in an evacuated 0.25 L chamber at a con- stant temperature of 20 C, will all of the solid sublime? 1. Only if it is at the triple point

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casey (rmc2555) – Homework 2 – holcombe – (51395) 10 2. No 3. Yes correct Explanation: The dry ice will sublime until the pressure is equal to the vapor pressure. There are 0.227 moles of CO 2 . If all sublimes the pressure will be less than the vapor pressure, so the solid will never reach equilibrium with the gas, and the system will be all gas. 029 3.3points A 741 mL sample of hydrogen was collected over water at 24 C on a day when the at- mospheric pressure was 756 torr. The vapor pressure of water under these conditions is 17.54 torr. What volume will the dry hydro- gen (no water vapor) occupy at STP? 1. 0 . 992725 L 2. 0 . 529453 L 3. 0 . 441211 L 4. 0 . 827271 L 5. 0 . 661817 L correct Explanation: Sample: V = 741 mL = 0 . 741 mL T = 24 C = 297 K P total = 756 torr P H 2 O = 17 . 54 torr P total = P H 2 O + P H 2 P H 2 = P total - P H 2 O = 756 torr - 17 . 54 torr = 738 . 46 torr Dry H 2 : V = ? T = 0 C = 273 K P Dry H 2 = 760 torr P Dry H 2 V Dry H 2 P sample V sample = n Dry H 2 RT Dry H 2 n sample RT sample n Dry H 2 = n sample V Dry H 2 = T Dry H 2 P sample V sampe T sample P Dry H 2 = (273 K)(738 . 46 torr)(0 . 741 mL) (297 K)(760 torr) = 0 . 661817 L 030 3.3points On a hiking expedition with Bear Grylls, you ‘accidentally’ end up in a huge chasm 2 km below sea level. Bear, being the resourceful TV star that he is, builds a fire and promptly starts to heat some water to cook the local snake for dinner. At what temperature do you expect Bear’s water to boil?
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