Chapter 8

Hexane, \(\left(\mathrm{C}_{6} \mathrm{H}_{14}\right)\) and cyclohexane \(\left(\mathrm{C}_{6} \mathrm{H}_{12}\right)\) form an ideal solution. The vapor pressure of hexane is 151 Torr and that of cyclohexane is 98 Torr at \(25.0^{\circ} \mathrm{C}\). Calculate the vapor pressure of each of the following solutions and the mole fraction of each substance in the vapor phase above those solutions: (a) \(0.2 \mathrm{~S} \mathrm{~mol} \mathrm{C}_{6} \mathrm{H}_{14}\) mixed with \(0.65 \mathrm{~mol} \mathrm{C}_{6} \mathrm{H}_{12}\) (b) \(10.0 \mathrm{~g}\) of hexane mixed with \(10.0 \mathrm{~g}\) of cyclohexane.

Complete the following statements about the effect of intermolecular forces on the physical properties of a substance: (a) The higher the boiling point of a liquid, the (stronger, weaker) its intermolecular forces. (b) Substances with strong intermolecular forces have (high, low) vapor pressures. (c) Substances with strong intermolecular forces typically have (high, low) surface tensions. (d) The higher the vapor pressure of a liquid, the (stronger, weaker) its intermolecular forces. (c) Because nitrogen, \(\mathrm{N}_{2}\), has (strong, weak) intermolecular forces, it has a (high, low) critical temperature. (f) Substances with high vapor pressures have correspondingly (high, low) boiling points. (g) Because water has a relatively high boiling point, it must have (strong, weak) intermolecular forces and a correspondingly (high, low) enthalpy of vaporization.

Hydrogen peroxide, \(\mathrm{H}_{2} \mathrm{O}_{2}\), is a syrupy liquid with a vapor pressure lower than that of water and a boiling point of \(152^{\circ} \mathrm{C}\). Account for the differences between these properties and those of water.

Explain the effect that an increase in temperature has on each of the following properties: (a) viscosity; (b) surface tension; (c) vapor pressure; (d) evaporation rate.

Explain how the vapor pressure of a liquid is affected by each of the following changes in conditions: (a) an increase in temperature; (b) an increase in surface area of the liquid; (c) an increase in volume above the liquid; (d) the addition of air to the volume above the liquid.

The normal boiling point of water is \(100^{\circ} \mathrm{C}\). Suppose a cyclonic region (a region of low pressure) moves into the area. State and explain what happens to the boiling point of the water.

(a) Calculate the mass of \(\mathrm{CaCl}_{2}-6 \mathrm{H}_{2} \mathrm{O}\) needed to prepare a \(0.10 \mathrm{~m} \mathrm{CaCl}_{2}(\mathrm{aq})\) solution, using \(2.50 \mathrm{~g}\) of water. (b) What mass of \(\mathrm{NiSO}_{4} \cdot 6 \mathrm{H}_{2} \mathrm{O}\) must be dissolved in \(500 \mathrm{~g}\) of water to produce a \(0.22 \mathrm{~m}\) \(\mathrm{NiSO}_{4}(\mathrm{aq})\) solution?

Explain in thermodynamic terms why a solute raises the boiling point and lowers the freceing point of a solution.

When determining a molar mass from freczingpoint depression, it is possible to make each of the following errors (among others). In each case, predict whether the error would cause the reported molar mass to be greater or less than the actual molar mass. (a) There was dust on the balance, causing the mass of solute to appear greater than it actually was. (b) The water was measured by volume, assuming a density of \(1.00 \mathrm{~g} \cdot \mathrm{cm}^{-3}\), but the water was warmer and less dense than assumed. (c) The thermometer was not calibrated accurately, so the temperature of the freering point was actually \(0.5^{\circ} \mathrm{C}\) higher than recorded. (d) The solution was not stirred sufficiently so that not all the solute dissolved.

Suggest a reason why saltwater fish die when they are suddenly transferred to a freshwater aquarium.