Electrolytes of colligative properties

As noted earlier, colligative properties of solutions are directly proportional to the concentration of solute particles. On this basis, it is reasonable to suppose that, at a given concentration, an electrolyte should have a greater effect on these properties than does a nonelectrolyte. When one mole of a nonelectrolyte such as glucose dissolves in water, one mole of solute molecules is obtained. On the other hand, one mole of the electrolyte NaCl yields two moles of ions (1 mol of Na+, 1 mol of Cl-). With CaCl three moles of ions are produced per mole of solute (1 mol of Ca2+, 2 mol of Cl-). 

With many electrolytes, AP is so large that the solid, when exposed to moist air, picks up water (deliquesces). This occurs with calcium chloride, whose saturated solution has a vapor pressure only 30% that of pure water. If dry CaCl2 is exposed to air in which the relative humidity is greater than 30%, it absorbs water and forms a saturated solution. Deliquescence continues until the vapor pressure of the solution becomes equal to that of the water in the air. 

The freezing points of electrolyte solutions, like their vapor pressures, are lower than those of nonelectrolytes at the same concentration. Sodium chloride and calcium chloride are used to lower the melting point of ice on highways their aqueous solutions can have freezing points as low as —21 and — 55°C, respectively. 

To calculate the freezing point lowering of an electrolyte in water, we use the general equation 

Similar equations apply for other coUigative properties. 

---

The colligative properties of electrolyte solutions are described by including the van t Hoff factor in the appropriate equation. For example, for changes in freezing and boiling points the modified equation is... 

COLLIGATIVE PROPERTIES OF ELECTROLYTES AS COMPARED WITH NON-ELECTROLYTE SOLUTES... 

COLLIGATIVE PROPERTIES OF ELECTROLYTE SOLUTIONS Review Questions... 

Why is the discussion of the colligative properties of electrolyte solutions more involved than that of nonelectrolyte solutions ... 

Describe electrolyte behavior and the four colligative properties, explain the difference between phase diagrams for a solution and a pure solvent, explain vapor-pressure lowering for nonvolatile and volatile nonelectrolytes, and discuss the van t Hoff factor for colligative properties of electrolyte solutions ( 13.5) (SPs 13.6-13.9) (EPs 13.59-13.83)... 

A U. luO-L solution is made by dissolving u.44i gof CaCl2(s) in water, (a) Calculate the osmotic pressure of this solution at 27 "C, assuming that it is completely dissociated into its component ions, (b) The measured osmotic pressure of this solution is 2.56 atm at 27 C. Explain why it is less than the value calculated in (a), and calculate the van t Hoff factor, i, for the solute in this solution. (See the A Closer Look box on Colligative Properties of Electrolyte Solutions in Section 13.5.) (c) The enthalpy of solution for CaCl2 is AH = —81.3 kj/mol. If the final temperature of the solution is 27 °C, what was its initial temperature (Assume that the density of the solution is 1.00 g/mL, that its specific heat is 4.18 J/g-K, and that the solution loses no heat to its surroundings.)... 

Colligative Properties of Electrolyte Solutions 540 Using Spectroscopic Methods to Measure Reaction Rates 564 Limitations of Solubility Products 726 Other Greenhouse Gases 764... 

Colligative properties of electrolyte solutions depend on the number of particles produced during the ionization or dissociation of the electrolyte. 

---