INTERMOLECULAR FORCES AND
COLLIGATIVE PROPERTIES
Solids, Liquids, and Gases
Intermolecular Forces
Intermolecular forces are responsible for the internal “structure” of liquids and solids.
Intermolecular Forces —
♦ are always weaker than chemical bonds.
♦ are less directional than covalent chemical bonds, but more directional than ionic bonds.
♦ operate at longer ranges than covalent chemical bonds, but at shorter range than ionic bonds.
Types of Nonbonded Attractions
Dipole–dipole interactions
Ion-dipole interactions
Induced dipole interactions
Dispersion
Ex 1
What types of nonbonded interactions are present in the following:
(a) HI
(b)
N2
(c) CH3OH
(d)
O2 dissolved in water
Phase Changes
Vapor Pressure
Volatility, Vapor Pressure, and Temperature
Vapor Pressure and Boiling Point
Ex 2
Explain how each of the following affects the vapor pressure of a liquid:
(a) volume of the liquid
(b) surface area
(c) intermolecular attractive forces
(d) temperature
PROPERTIES OF SOLUTIONS
Solutions
Solubility
Solute-Solvent Interactions
Pressure Effects
Temperature Effects
Ways of Expressing Concentration
Mass Percentage, ppm, ppb
Ex 1
A disinfectant solution is made by dissolving 4.50 g of hydrogen peroxide (H2O2) in 143.2 g of
water. Calculate the mass percentage of hydrogen peroxide in the solution. What is the
concentration of hydrogen peroxide in parts per million?
Mole fraction, Molarity, Molality
Ex 2
What is the molality of hydrogen peroxide in the solution described in Example 1?
Colligative Properties
Colligative Properties are properties of solutions that depend primarily upon the
concentrations of solute particles rather than their nature. They include vapor pressure, boiling
point, melting point, and osmotic pressure.
Vapor Pressure Lowering
Ex 3
The vapor pressure of carbon tetrachloride (CCl4) at 50°C is 0.437 atm. When 7.42 g of a
pure, nonvolatile, and nondissociating substance is dissolved in 100.0 g of carbon tetrachloride,
the vapor pressure of the solution at 50°C is 0.411 atm. Calculate the mole fraction of solute.
Ex 4
The vapor pressure of pure liquid CS2 is 0.3914 atm at 20°C. When 40.0 g of rhombic sulfur is
dissolved in 1.00 kg of CS2, the vapor pressure of CS2 falls to 0.3868 atm. Determine the
molecular formula for the sulfur molecules dissolved in CS2.
Boiling-Point Elevation and the Effect of Dissociation
Ex 5
2+
Lead(II) nitrate dissociates to Pb and NO 3 ions in aqueous solution. Assuming no
association among ions and that the solution is ideal, calculate the boiling point of a solution
−
prepared by mixing 0.194 mol of Pb(NO3)2 and 1.171 kg of water.
Ex 6
When 39.8 g of a nondissociating, nonvolatile sugar is dissolved in 200.0 g of water, the boiling
point of the water is raised by 0.30°C. Estimate the molar mass of the sugar.(Kb = 0.52°C / m)
Freezing-Point Depression
Ex 7
List the following aqueous solutions in order of their expected freezing points:
pure water; 0.10 m sucrose (C12H22 O11); 0.03 m KCl; 0.02 m KBr; 0.10 m K2CO3
Ex 8
An aqueous solution contains 0.500 g Na2SO4. in 500.0 g of water. What is the freezing point
of this solution? (Kf = 1.86°C / m)