ChemBytes - Chapter 6

Answers

1. What are the molality and the molarity of a solution of ethanol (C₂H₅OH) in water (H₂O) at 20°C if the mole fraction of the ethanol is 0.05? Assume that the density of the solution is 0.997 g/mL at 20°C. molality = 2.92 m; molarity = 2.5 M
2. Concentrated nitric acid is 69% HNO₃ by weight and has a density of 1.41 g/mL at 20°C. What volume and weight of concentrated nitric acid are needed to prepare 100 mL of 6 M acid? 38.9 mL HNO₃; 54.8 g
3. The freezing point depression constant for mercury(II) chloride (HgCl₂) is 34.3. For a solution of 0.849 g of mercury(I) chloride (Hg₂Cl₂) in 50 g of mercury(II) chloride, the freezing point depression is 1.24°C. What is the molecular weight of mercury(I) chloride in this solution? What is its molecular formula? MW = 470 g/mol; Hg₂Cl₂
4. Ten liters of dry air were bubbled slowly through water at 20°C and the observed weight loss (of water) was 0.172 g. By assuming that 10 liters of saturated water vapor formed during the experiment, calculate the vapor pressure of water (in torr) at 20°C. 17.4 torr
5. Ethanol and methanol form a solution that is very nearly ideal. The vapor pressure of ethanol is 44.5 torr and that of methanol is 88.7 torr, at 20°C. (a) Calculate the mole fractions of methanol and ethanol in a solution obtained by mixing 60 g of ethanol with 40 g of methanol. (b) Calculate the partial pressures and the total vapor pressure of this solution, and the mole fraction of ethanol in the vapor. a) X_ethanol = 0.51; X_methanol = 0.49; b) P_ethanol = 22.7 torr; P_methanol = 43.4 torr; P_T = 66.1 torr; X_{eth. vapor} = 0.343
6. At 55°C, ethanol has a vapor pressure of 168 torr, and the vapor pressure of methyl cyclohexane is 280 torr. A solution of the two, in which the mole fraction of ethanol is 0.68 has a total vapor pressure of 376 torr. Is this solution formed from its components with the evolution or absorption of heat? absorption of heat
7. The pressure of CO₂ in a 1.0 L bottle of soda is 1.5 atm. On opening the bottle and letting the gas escape, the pressure falls to 1.0 atm. If the temperature is 20°C, what volume of gas at one atmosphere escaped? Henry's law constant K_H = 1.26x10⁶ torr. 0.41 L
8. Assuming seawater to have a salt concentration of 0.52 M and a density of 1.024 g/cm³, determine what pressure in atmospheres is needed in order to cause seawater at 25°C to move through a semipermeable membrane in a reverse osmosis desalination plant. Consider only sodium and chloride ions in this calculation although seawater actually contains other ions such as calcium, magnesium and sulfate. At what temperature will the seawater freeze? P > 12.7 atm to reverse osmosis; freezing temperature = -0.97°C
9. Because of the marked sensitivity of osmotic pressure measurements, which produce relatively large values for very dilute solutions, molar masses for very large molecules such as proteins can be obtained. Now suppose that a typical synthetic nylon or natural protein was soluble to the extent of 0.1 g/L , and that the limit of your ability to measure osmotic pressure is 10^-3 torr at 298 K. Estimate the highest molar mass you can measure by this method. 2x10⁶ g/mol
10. What is the concentration of sap in a 200 foot tree?