17.13.10: Chapter 11

1.

A solution can vary in composition, while a compound cannot vary in composition. Solutions are homogeneous at the molecular level, while other mixtures are heterogeneous.

3.

(a) The process is endothermic as the solution is consuming heat. (b) Attraction between the K⁺ and $\text{N}{\text{O}}_3{}^{\text{−}}$ ions is stronger than between the ions and water molecules (the ion-ion interactions have a lower, more negative energy). Therefore, the dissolution process increases the energy of the molecular interactions, and it consumes the thermal energy of the solution to make up for the difference. (c) No, an ideal solution is formed with no appreciable heat release or consumption.

5.

(a) ion-dipole forces; (b) dipole-dipole forces; (c) dispersion forces; (d) dispersion forces; (e) hydrogen bonding

7.

Heat is released when the total intermolecular forces (IMFs) between the solute and solvent molecules are stronger than the total IMFs in the pure solute and in the pure solvent: Breaking weaker IMFs and forming stronger IMFs releases heat. Heat is absorbed when the total IMFs in the solution are weaker than the total of those in the pure solute and in the pure solvent: Breaking stronger IMFs and forming weaker IMFs absorbs heat.

9.

Crystals of NaCl dissolve in water, a polar liquid with a very large dipole moment, and the individual ions become strongly solvated. Hexane is a nonpolar liquid with a dipole moment of zero and, therefore, does not significantly interact with the ions of the NaCl crystals.

11.

(a) Fe(NO₃)₃ is a strong electrolyte, thus it should completely dissociate into Fe³⁺ and ${\text{NO}}_3{}^{\text{−}}$ ions. Therefore, (z) best represents the solution. (b) $\text{Fe}{({\text{NO}}_3)}_3(s)⟶{\text{Fe}}^{\text{3}}{\text{+}}^{}(aq)+3{\text{NO}}_3{}^{\text{−}}(aq)$

13.

(a) high conductivity (solute is an ionic compound that will dissociate when dissolved); (b) high conductivity (solute is a strong acid and will ionize completely when dissolved); (c) nonconductive (solute is a covalent compound, neither acid nor base, unreactive towards water); (d) low conductivity (solute is a weak base and will partially ionize when dissolved)

15.

(a) ion-dipole; (b) hydrogen bonds; (c) dispersion forces; (d) dipole-dipole attractions; (e) dispersion forces

17.

The solubility of solids usually decreases upon cooling a solution, while the solubility of gases usually decreases upon heating.

19.

40%

21.

2.8 g

23.

2.9 atm

25.

102 L HCl

27.

The strength of the bonds between like molecules is stronger than the strength between unlike molecules. Therefore, some regions will exist in which the water molecules will exclude oil molecules and other regions will exist in which oil molecules will exclude water molecules, forming a heterogeneous region.

29.

Both form homogeneous solutions; their boiling point elevations are the same, as are their lowering of vapor pressures. Osmotic pressure and the lowering of the freezing point are also the same for both solutions.

31.

(a) Find number of moles of HNO₃ and H₂O in 100 g of the solution. Find the mole fractions for the components. (b) The mole fraction of HNO₃ is 0.378. The mole fraction of H₂O is 0.622.

33.

(a) $X_{{\text{Na}}_2{\text{CO}}_{\text{3}}}=0.0119;$ $X_{{\text{H}}_2\text{O}}=0.988;$ (b) $X_{{\text{NH}}_4{\text{NO}}_3}=0.0928;$ $X_{{\text{H}}_2\text{O}}=0.907;$ (c) $X_{{\text{Cl}}_2}=0.192;$ $X_{{\text{CH}}_2{\text{CI}}_2}=0.808;$ (d) $X_{{\text{C}}_5{\text{H}}_9\text{N}}=0.00426;$ $X_{{\text{CHCl}}_3}=0.997$

35.

In a 1 M solution, the mole is contained in exactly 1 L of solution. In a 1 m solution, the mole is contained in exactly 1 kg of solvent.

37.

(a) Determine the molar mass of HNO₃. Determine the number of moles of acid in the solution. From the number of moles and the mass of solvent, determine the molality. (b) 33.7 m

39.

(a) 6.70 $\times$ 10⁻¹ m; (b) 5.67 m; (c) 2.8 m; (d) 0.0358 m

41.

1.08 m

43.

(a) Determine the molar mass of sucrose; determine the number of moles of sucrose in the solution; convert the mass of solvent to units of kilograms; from the number of moles and the mass of solvent, determine the molality; determine the difference between the boiling point of water and the boiling point of the solution; determine the new boiling point. (b) 100.5 °C

45.

(a) Determine the molar mass of sucrose; determine the number of moles of sucrose in the solution; convert the mass of solvent to units of kilograms; from the number of moles and the mass of solvent, determine the molality; determine the difference between the freezing temperature of water and the freezing temperature of the solution; determine the new freezing temperature. (b) −1.8 °C

47.

(a) Determine the molar mass of Ca(NO₃)₂; determine the number of moles of Ca(NO₃)₂ in the solution; determine the number of moles of ions in the solution; determine the molarity of ions, then the osmotic pressure. (b) 2.67 atm

49.

(a) Determine the molal concentration from the change in boiling point and K_b; determine the moles of solute in the solution from the molal concentration and mass of solvent; determine the molar mass from the number of moles and the mass of solute. (b) 2.1 $\times$ 10² g mol⁻¹

51.

No. Pure benzene freezes at 5.5 °C, and so the observed freezing point of this solution is depressed by ΔT_f = 5.5 − 0.4 = 5.1 °C. The value computed, assuming no ionization of HCl, is ΔT_f = (1.0 m)(5.14 °C/m) = 5.1 °C. Agreement of these values supports the assumption that HCl is not ionized.

53.

144 g mol⁻¹

55.

0.870 °C

57.

S₈

59.

1.39 $\times$ 10⁴ g mol⁻¹

61.

54 g

63.

100.26 °C

65.

(a) $X_{{\text{CH}}_3\text{OH}}=0.590;$ $X_{{\text{C}}_2{\text{H}}_5\text{OH}}=0.410;$ (b) Vapor pressures are: CH₃OH: 55 torr; C₂H₅OH: 18 torr; (c) CH₃OH: 0.75; C₂H₅OH: 0.25

67.

The ions and compounds present in the water in the beef lower the freezing point of the beef below −1 °C.

69.

$\text{Δ}\text{bp}=K_{\text{b}}m=\left(1.20\text{°}\text{C}\text{/}m\right)\left(\frac{9.41\text{g}\times \frac{1\text{mol Hg}{\text{Cl}}_2}{271.496\text{g}}}{0.03275\text{kg}}\right)=1.27\text{°}$
The observed change equals the theoretical change; therefore, no dissociation occurs.

71.

Colloidal System	Dispersed Phase	Dispersion Medium
starch dispersion	starch	water
smoke	solid particles	air
fog	water	air
pearl	water	calcium carbonate (CaCO3)
whipped cream	air	cream
floating soap	air	soap
jelly	fruit juice	pectin gel
milk	butterfat	water
ruby	chromium(III) oxide (Cr2O3)	aluminum oxide (Al2O3)

73.

Colloidal dispersions consist of particles that are much bigger than the solutes of typical solutions. Colloidal particles are either very large molecules or aggregates of smaller species that usually are big enough to scatter light. Colloids are homogeneous on a macroscopic (visual) scale, while solutions are homogeneous on a microscopic (molecular) scale.

75.

If they are placed in an electrolytic cell, dispersed particles will move toward the electrode that carries a charge opposite to their own charge. At this electrode, the charged particles will be neutralized and will coagulate as a precipitate.