will not ask questions of that type. The test will ask questions only about the
relative degree of the effect and that recognize that the effect occurs. As stated, the
relative degree of the effect depends only on the number of particles dissolved.
However, the number of those particles also depends on the nature of the solute.
Using a solute that is an ionic solid and that completely ionizes in an aqueous
solution introduces a greater number of particles than when a nonionizing
molecular solute is dissolved. Notice in the preceding chart that a 1 molal
solution of NaCl yields a solution with 2 moles of particles because:
Thus, a 1 molal solution of NaCl has 2 moles of ion particles in 1,000 grams
of solvent. In contrast, a 1 molal solution of a sugar solution has only 1 mole of
molecular particles in that same mass of solvent because the sugar does not
dissociate into ions. Although the colligative property of lowering the freezing
point and raising the boiling point depends primarily on the concentration of
particles and not the type of particles, the number of particles influencing the
property does vary with the type of solute dissolved.
Example
Rank the following aqueous solutions based on the range of temperature in which
the solution is a liquid. Rank the solutions from largest to smallest range.
0.50 m KI 0.50 m MgCl 2 0.75 m C 2 H 5 OH 1.00 m CaCl 2The solution that has the greatest range of temperature over which the
solution is a liquid is the one that has the lowest freezing point and the highest
boiling point. All but the ethyl alcohol (C 2 H 5 OH) solution contain ionic solutes
that would dissociate into the number of ions as dictated by their formulas. That
means that the total number of moles of solute particles per 1,000.0 grams of
water in each solution would be 1.00, 1.50, 0.75, and 3.00 respectively.
Therefore, the solutions should be ranked as shown below:
#1. (greatest range) 1.00 m CaCl 2#2. 0.50 m MgCl 2#3. 0.50 m KI#4. (smallest range) 0.75 m C 2 H 5 OH