The Foundations of Chemistry

Problem-Solving Tip:Selection of a van’t Hoff Factor

Use an ideal value for the van’t Hoff factor unless the question clearly indicates to do

otherwise, as in the following example and in some of the end-of-chapter exercises. For

a strong electrolyte dissolved in water, the ideal value for its van’t Hoff factor is listed

in Table 14-3. For nonelectrolytes dissolved in water or any solute dissolved in common

nonaqueous solvents, the van’t Hoff factor is considered to be 1. For weak electrolytes

dissolved in water, the van’t Hoff factor is a little greater than 1.

TABLE 14-3 Actual and Ideal van’t Hoff Factors, i, for Aqueous Solutions of

Nonelectrolytes and Strong Electrolytes

ifor ifor

Compound 1.00 mSolution 0.100 mSolution

nonelectrolytes 1.00 (ideal) 1.00 (ideal)

sucrose, C 12 H 22 O 11 1.00 1.00

If 2 ions in solution/formula unit 2.00 (ideal) 2.00 (ideal)

KBr 1.77 1.88
NaCl 1.83 1.87

If 3 ions in solution/formula unit 3.00 (ideal) 3.00 (ideal)
K 2 CO 3 2.39 2.45

K 2 CrO 4 1.95 2.39

If 4 ions in solution/formula unit 4.00 (ideal) 4.00 (ideal)

K 3 [Fe(CN) 6 ] — 2.85

Percent Ionization and iValue from Freezing Point Depression Data

EXAMPLE 14-12 Colligative Property and Weak Electrolytes

Lactic acid, C 2 H 4 (OH)COOH, is found in sour milk. It is also formed in muscles during
intense physical activity and is responsible for the pain felt during strenuous exercise. It is
a weak monoprotic acid and therefore a weak electrolyte. The freezing point of a 0.0100 m
aqueous solution of lactic acid is 0.0206°C. Calculate (a) the ivalue and (b) the percent
ionization in the solution.

Plan for (a)

To evaluate the van’t Hoff factor, i,we first calculate meffectivefrom the observed freezing
point depression and Kffor water; we then compare meffectiveand mstatedto find i.

Solution for (a)

meffective0.0111 m

i1.11

0.0111 m



0.0100 m

meffective



mstated

0.0206°C



1.86°C/m

Tf



Kf

E

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