Which of the following 0.10 M solutions will have the lowest freezing point? (Assume complete dissociation for strong electrolytes.)
Show the answer and explanation
Correct answer
B. CaCl2
Principle or equation
Freezing point depression is a colligative property: ΔTf = i * Kf * m, where i is the van't Hoff factor (number of particles per formula unit). The solution with the highest i will have the lowest freezing point.
Why this answer is correct
For 0.10 M solutions, the effective particle concentration is i * 0.10. NaCl dissociates into 2 ions (i=2). CaCl2 dissociates into 3 ions (i=3). Glucose does not dissociate (i=1). Acetic acid is a weak electrolyte, partially dissociates (i slightly >1 but less than 2). Therefore, CaCl2 has the highest i and thus the lowest freezing point.
Example
0.10 M NaCl has i=2, so effective concentration = 0.20 M; 0.10 M CaCl2 has i=3, effective = 0.30 M; so CaCl2 has greater freezing point depression.
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