13.3.5 Colligative Properties of Ionic Solutions

Question 1

Colligative Properties of Ionic Solutions

Answer 1

• The van’t Hoff factor is a correction factor used in relationships involving colligative properties of a solution to account for the dissociation of solute particles

Question 2

note

Answer 2

• If a solute added to a solution dissociates, the number of particles in solution is greater than the original number of particles of solute.
• For example, when sodium chloride (NaCl) dissociates in solution, two particles are present (a sodium ion and a chloride ion) for each particle of NaCl.
• The van’t Hoff factor ( or i) is a correction factor used in
  relationships involving colligative properties of a solution to account for the dissociation of solute particles. For example, the van’t Hoff factor for NaCl is 2, while the van’t Hoff factor for iron(III) bromide (FeBr3) is 4.
• Problem: When 0.494 g of K3Fe(CN)6 is dissolved in 100.0 g of water, the freezing point is lowered by 0.11˚C. How many ions are formed from the dissociation of K3Fe(CN)6?
• First, the number of moles of K3Fe(CN)6 must be calculated from the number of grams and the molar mass of K3Fe(CN)6.
  This value is then used to calculate the molality of the
  K3Fe(CN)6 solution (0.015 m).
• Plugging the known values into the freezing point depression equation yields a van’t Hoff factor of about 4. This value indicates that K3Fe(CN)6 dissociates into four particles in solution: three potassium ions, and one hexacyanoiron(III) ion (Fe(CN)6 3+).

Question 3

A substance is dissolved in water to form a 0.2 M solution. This solution boils at 100.1°C. Is this substance an electrolyte in solution (Kb for water is 0.513)?

Answer 3

No

Question 4

In some electrolytes, the ions do not exist completely independently, even when ionization is complete. Temporary ion pairs form. Because of temporary ion pair formation, the apparent values of ν may be ____

Answer 4

smaller than expected

Question 5

In the expression delta Tb = vKbm, the symbol v stands for the van’t Hoff factor. This factor represents the average number of particles contributed per formula unit. What is the value of v for aluminum chloride, AlCl3?

Answer 5

4

Question 6

Reverse osmosis is the process of applying a pressure of greater than the osmotic pressure of a solution in order to force the solvent molecules back through the semipermeable membrane to the pure solvent side. The process can be used to purify seawater and make drinking water. Seawater is a solution of mostly NaCl with a molarity of 0.60 M. Calculate the minimum pressure needed for reverse osmosis to occur at 25°C. Assume the NaCl is completely ionized.

Answer 6

29 atm

Question 7

Suppose you calculate the molecular weight of a substance using the freezing point depression. However, a subsequent conductivity test shows the substance to be ionic. Your original calculation of the molecular weight would be ____

Answer 7

too low

Question 8

Which choice has the aqueous solutions arranged in order of increasing amount of freezing point depression?

Answer 8

1.0 M glucose, 1.0 NaCl, 1.0 M MgCl2

Question 9

Solutes that ionize when dissolved in water are called electrolytes. A solute was dissolved in water to form a solution with a concentration of 0.15 M. The boiling point of the solution was found to be 100.2°C. Is the solute an electrolyte and how many particles does it break into?

Answer 9

Yes, 3

Question 10

Calcium choloride, CaCl2, is sometimes used as a road de-icer. The solubility of CaCl2 in cold water is 74.5g per kilogram of water. Assume v is 3.0 and determine the lowest temperature for which CaCl2 would be effective (kf for water is 1.86)

Answer 10

−3.7°C

Question 11

Assuming complete ionization, how much will the freezing point of water be depressed by a 0.1 m solution of CaCl2? Kf = 1.86°C • kg / mol

Answer 11

0.56°C

Question 12

What is the boiling point of a solution that contains 5.0 g of NaCl dissolved in 25 g of water (Kb for water is 0.51)?

Answer 12

103.5°C