EXAM PRACTICE QUESTIONS Chapter 14 1. How many grams of sucrose (C12H22O11) must be dissolved in 750. mL of water to prepare a 0.250 molal solution? 64.1 g 2. If the mole fraction of CH3OH in a solution with only water is 0.0250, what is the molality of the CH3OH? 1.42 m 3. In a home ice cream freezer, we lower the freezing point of the water bath surrounding the ice cream by dissolving NaCl in water to make a brine solution. A 15.0% brine solution is observed to freeze at -10.888 qC. What is the van't Hoff factor, i, for this solution? 1.94 4. Indicate (by writing yes or no in the blanks below) which of the following pairs will form a solution when mixed. KI in CCl4 LiF in water no CH3OH in water yes CH3OH in C6H6 yes vegetable oil in mineral oil no yes 5. What is the vapor pressure of an aqueous solution containing 10.0% (w/w) ethylene glycol (62.0 g/mol) at 25 qC. PH2O = 24.3 torr at 25 qC. 23.5 torr 6. At 25 qC a solution consists of 0.450 mole pentane, C5H14, and 0.250 mol of cyclopentane, C5H10. What is the lowering of the vapor pressure of pentane in this solution? The vapor pressure of the pure liquids at 25 qC are 451 torr for pentane and 321 torr for cyclopentane. 161 torr 7. When 1.150 grams of an unknown electrolyte dissolves in 10.0 grams of water, the solution freezes at -2.16 qC. What is the molecular weight of the unknown compound? Kf for water is 1.86 qC/m. 99.1 g/mol8. The osmotic pressure of 1.00 L of an aqueous solution of a nonelectrolyte solute is 1.17 atm at 0.00 qC. The density of the solution is 1.09 g/mL. The chemical formula of the nonelectrolyte is C3H6O3. What is the % m/m of the nonelectrolyte in the solution? 0.431% m/m 9. Which solution would produce the lowest freezing point? 0.075 m sodium sulfate 10. A 0.0490 m aqueous NaBr solution freezes at -0.173 qC. What is the apparent percent dissociation if NaBr in this solution? Kf = 1.86 qC/m for water NaBr o Na+ + Br-
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