PSI AP Chemistry Solutions Practice Problems Name Solutions: Mixtures, Solubility and Concentration Classwork 1. A student determined that there were 0.032 grams of oxygen gas dissolved in a 200.0 ml sample of lake water (D = 1.02 g/ml) at a temperature of 15C. a. How many oxygen atoms would exist in a 500.0 ml sample of this lake water? b. If hot water were dumped into the lake from a factory, what would be the effect on the mole fraction of oxygen in the sample? Justify your answer. c. What would be the molality and molarity of the 500.0 ml lake water solution? Ignore the presence of solute other than the oxygen gas. d. Describe what happened to the magnitude of the molality and molarity of the lake water after the hot water from the factory was added. Ignore the presence of solute other than the oxygen gas. 2. An organic compound with formula C 2 H 6 O was mixed with CCl 4 in one beaker and water in another beaker. Two layers were absorbed when the compound was mixed with water and one layer when mixed with CCl 4. a. Is the molecule more polar or more non-polar? Justify your answer. b. Draw a Lewis structure for the compound consistent with the observations of the experiment. c. Draw the Lewis structure of an isomer of this compound that would have been miscible in water. d. Propose how this compound could be separated from the CCl4. Be specific as to the technique AND the expected observations. Homework 3. Agree or disagree with the following statements and then justify your answer:
a. As the temperature of a solution increases, the mole fraction of solute decreases. b. An addition of 50 ml of distilled water would be required to dilute a 200 ml 0.12 M solution of HCl to a concentration of 0.08 M. 4. An aqueous mixture of ethylene glycol (C 2 H 6 O 2 ) and water is 50% water by volume. Assuming the total volume of the solution is known, what data would be required to determine the mole fraction of ethylene glycol in the mixture? 5. How many ml of 0.34 M Pb(NO 3 ) 2 would need to be added to a 100 ml solution of 0.50 M NaCl to a. Precipitate out all of the Cl- ions? b. Create exactly 1 gram of precipitate? Solutions: Formations and Properties Classwork 6. A student prepares 100 ml solutions of 0.1 M HCl, 0.1 M HC 2 H 3 O 2, and 0.1 M CaCl 2 : a. Rank the three solutions from highest to lowest electrical conductivity: b. Rank the three solutions from highest to lowest vapor pressure: c. Rank the three solutions from highest to lowest freezing point: 7. The vapor pressure of pure benzene at 100 C is 1500 mm Hg. The vapor pressure of pure hexane at 100 C is 1600 mm Hg. Which pure substance would have the weaker intermolecular forces? Justify your answer. 8. To 100 grams of water, a student added equimolar amounts of compounds X, Y, and Z and observed the freezing points of each. The results are below: Compound X Freezing Point: -5.58 C Compound Y Compound Z Freezing Point: - 3.72 C Freezing Point: -1.86 C Assuming the freezing point constant for water is 1.86 C/m, identify each of the following compounds as either KI, C 6 H 12 O 6, or MgCl 2
9. Explain the following observations in terms of particle interactions and chemical properties: a. Pure water makes a poor antifreeze and coolant in an automobile engine but the addition of ethylene glycol to the water in a roughly 50/50 ratio dramatically elevates the boiling point and lowers the freezing point. b. Adding large quantities of salt to water will reduce the cooking time of pasta (cooked in boiling water). Solutions: Formations and Properties Homework 10. A student needs to prepare 40.0 ml of a 0.25 M NaOH solution. a. Explain how such a solution could be prepared using solid NaOH, a balance, a beaker, distilled water, and a volumetric flask. b. When the NaOH dissolves, the temperature of the solution increases from 18.2 C to 24.5 C. Assuming the specific heat and density of the solution is approximately that of water, what is the heat of solution of NaOH in kj/mol? c. Draw a picture representation of the solution after the NaOH has dissolved in the water. Make sure to show each kind of species present and their orientation to each other. d. How would the picture representation be different for the dissolution of methanol (CH3OH) in water compared to the picture in c? e. Based on your answer to b, which must have had the greater bond enthalpies the solvent-solvent interactions or the solute-solvent interactions? Justify your answer. f. If the container in which the NaOH and water were mixed was not well insulated, would this have made the calculated heat of solution higher or lower than the theoretical value? Justify your answer. 11. A student prepares 0.1 M aqueous solutions of AlCl 3, MgCl 2, NaCl, and C 6 H 12 O 6. a. Which solution would be the strongest electrolyte? Justify your answer. b. How would the vapor pressures of these solutions compare to the vapor pressure of pure water? Explain. 12. Agree or disagree with the following statements and justify your answer. a. Antifreeze (a mixture of ethylene glycol C2H6O2 and water) has a higher vapor pressure and lower boiling point than pure ethylene glycol. b. A 0.2 M solution of glucose would be expected to have a higher boiling point than a 0.05 M AlCl3 solution.
c. More energy is required to boil salt water than distilled water d. When methanol (CH3OH) dissolves in water, covalent bonds within the water molecules are broken to allow the solvent to form attractions with the solute. Answers 1. A student determined that there were 0.032 grams of oxygen gas dissolved in a 200.0 ml sample of lake water (D = 1.02 g/ml) at a temperature of 15C. A. How many oxygen atoms would exist in a 500.0 ml sample of this lake water? 3.02 x 10 21 atoms O B. If hot water were dumped into the lake from a factory, what would be the effect on the mole fraction of oxygen in the sample? Justify your answer. The mole fraction of oxygen would decrease as the gas is less soluble at high temperatures so the moles of oxygen gas would decrease while the solvent amount would stay unchanged. C. What would be the molality and molarity of the 500.0 ml lake water solution? Ignore the presence of solute other than the oxygen gas. M =.00500 mol/l molality = 0.00490 mol/kg solvent D. Describe what happened to the magnitude of the molality and molarity of the lake water after the hot water from the factory was added. Ignore the presence of solute other than the oxygen gas. The molarity will decrease as the higher temperature increases the volume. The molality will remain unchanged as neither moles or kg are affected by temperature. 2. An organic compound with formula C 2 H 6 O was mixed with CCl 4 in one beaker and water in another beaker. Two layers were absorbed when the compound was mixed with water and one layer when mixed with CCl 4. A. Is the molecule more polar or more non-polar? Justify your answer. The molecule is non-polar as it was miscible in CCl4 (a non-polar compound)
B. Draw a Lewis structure for the compound consistent with the observations of the experiment. C. Draw the lewis structure of an isomer of this compound that would have been miscible in water. D. Propose how this compound could be separated from the CCl 4. Be specific as to the technique AND the expected observations. Distill the mixture. The compound would vaporize first due to weaker London dispersion forces due to having fewer electrons. 3. Agree or disagree with the following statements and then justify your answer: A. As the temperature of a solution increases, the mole fraction of solute decreases. The mole fraction would remain unchanged as neither the moles of solute or solvent are influenced by temperature. If anything, the mole fraction of solute would increase due to evaporation of solvent. Disagree A. An addition of 50 ml of distilled water would be required to dilute a 200 ml 0.12 M solution of HCl to a concentration of 0.08 M. Disagree, an additional 100 ml of water would be required. 4. An aqueous mixture of ethylene glycol (C 2 H 6 O 2 ) and water is 50% water by volume. Assuming the total volume of the solution is known, what data would be required to determine the mole fraction of ethylene glycol in the mixture?
The density of ethylene glycol and water must be known to convert the volume to grams and then to moles. 5. How many ml of 0.34 M Pb(NO 3 ) 2 would need to be added to a 100 ml solution of 0.50 M NaCl to A. Precipitate out all of the Cl- ions? 0.0735 L = 73.5 ml A. Create exactly 1 gram of precipitate? 0.0106L = 10.6 ml 6. A student prepares 100 ml solutions of 0.1 M HCl, 0.1 M HC 2 H 3 O 2, and 0.1 M CaCl 2 : a. Rank the three solutions from highest to lowest electrical conductivity: 0.1 M CaCl2, 0.1M HCl, 0.1 M HC2H3O2 b. Rank the three solutions from highest to lowest vapor pressure: 0.1 M HC2H3O2, 0.1 M HCl, 0.1 M CaCl2 c. Rank the three solutions from highest to lowest freezing point: 0.1 M HC2H3O2, 0.1 M HCl, 0.1 M CaCl2 7. The vapor pressure of pure benzene at 100 C is 1500 mm Hg. The vapor pressure of pure hexane at 100 C is 1600 mm Hg. Which pure substance would have the weaker intermolecular forces? Justify your answer. Hexane, it has the higher rate of evaporation due to weaker particle interactions. 8. To 100 grams of water, a student added equi-molar amounts of compounds X, Y, and Z and observed the freezing points of each. The results are below: Compound X Freezing Point: -5.58 C Compound Y Compound Z Freezing Point: - 3.72 C Freezing Point: -1.86 C Assuming the freezing point constant for water is 1.86 C/m, identify each of the following compounds as either KI, C 6 H 12 O 6, or MgCl 2 X = MgCl 2, Y = KI, Z = C 6 H 12 O 6
9. Explain the following observations in terms of particle interactions and chemical properties: a. Pure water makes a poor antifreeze and coolant in an automobile engine but the addition of ethylene glycol to the water in a roughly 50/50 ratio dramatically elevates the boiling point and lowers the freezing point. Adding solute diminishes the rate of evaporation, thereby lowering the vapor pressure and requiring more energy to raise the vapor pressure to atmospheric pressure. Furthermore, the addition of solute makes it more difficult to crystallize the pure solvent thereby lowering the freezing point. b. Adding large quantities of salt to water will reduce the cooking time of pasta (cooked in boiling water). Adding salt to the water lowers the vapor pressure thereby raising the boiling point and shortening the cooking time. 10. A student needs to prepare 40.0 ml of a 0.25 M NaOH solution. a. Explain how such a solution could be prepared using solid NaOH, a balance, a beaker, distilled water, and a volumetric flask. Mass out 0.40 grams of NaOH and place in beaker. Add enough water to dissolve the NaOH and transfer to volumetric flask. Add distilled water to hit line. b. When the NaOH dissolves, the temperature of the solution increases from 18.2 C to 24.5 C. Assuming the specific heat and density of the solution is approximately that of water, what is the heat of solution of NaOH in kj/mol? -1,058 J / 0.01 moles = 105,800 J =- 105.8 kj/mol c. Draw a picture representation of the solution after the NaOH has dissolved in the water. Make sure to show each kind of species present and their orientation to each other. Student must show NaOH broken into Na + and OH - ions with Na + interacting with negative O atoms of water molecule and OH - interacting with positive H of water molecule d. How would the picture representation be different for the dissolution of methanol (CH 3 OH) in water compared to the picture in c? Methanol will not dissociate into ions.
e. Based on your answer to b, which must have had the greater bond enthalpies the solvent-solvent interactions or the solute-solvent interactions? Justify your answer. Since the dissolution is exothermic, more energy must be released from the solute-solvent interactions than absorbed by the breaking of solutesolvent interactions. f. If the container in which the NaOH and water were mixed was not well insulated, would this have made the calculated heat of solution higher or lower than the theoretical value? Justify your answer. Less heat would be absorbed by the water so dt would be less, so dh would be less. 11. A student prepares 0.1 M aqueous solutions of AlCl 3, MgCl 2, NaCl, and C 6 H 12 O 6. a. Which solution would be the strongest electrolyte? Justify your answer. AlCl 3 dissolves into most ions. b. How would the vapor pressures of these solutions compare to the vapor pressure of pure water? Explain. The vapor pressures would all be lower as the solute solvent interactions make it require more energy for the water molecules to evaporate 12. Agree or disagree with the following statements and justify your answer. a. Antifreeze (a mixture of ethylene glycol C 2 H 6 O 2 and water) has a higher vapor pressure and lower boiling point than pure ethylene glycol. Disagree, the solute-solvent interactions diminish the vapor pressure and raise the boiling point. b. A 0.2 M solution of glucose would be expected to have a higher boiling point than a 0.05 M AlCl 3 solution. Agree, the effective M of the AlCl3 is equal to 0.05*3 = 0.15 M which is still less than the glucose solution, despite the fact that glucose has the smaller Van t Hoft factor. c. More energy is required to boil salt water than distilled water Agree, the increased solute-solvent interactions are harder to break thereby decreasing the vapor pressure and increasing the boiling point.
d. When methanol (CH 3 OH) dissolves in water, covalent bonds within the water molecules are broken to allow the solvent to form attractions with the solute. Disagree it is the intermolecular forces between water molecules that need to be weakened to allow for solute-solvent interactions to form.