Using Colligative Properties to Find Solute Molar Mass

Using Colligative Properties to Find Solute Molar Mass 

Each colligative property relates concentration to some measurable quantity—the number of degrees the freezing point is lowered, the magnitude of osmotic pressure created, and so forth. From these measurements, we can determine the amount (mol) of solute particles and, for a known mass of solute, the molar mass of the solute as well. 

In principle, any of the colligative properties can be used to find the solute s molar mass, but in practice, some systems provide more precise data than others. For example, to determine the molar mass of an unknown solute by freezing point depression, you would select a solvent with as large a molal freezing point depression constant as possible (see Table 13.5). If the solute is soluble in acetic acid, for instance, aim concentration of it depresses the freezing point of acetic acid by 3.90°C, more than twice the change in water (1.86°C). 

SAMPLE PROBLEM 13.8 Determining Molar Mass from Osmotic Pressure 

Problem Biochemists have discovered more than 400 mutant varieties of hemoglobin, the blood protein that carries oxygen throughout the body. A physician studying a variety associated with a fatal disease first finds its molar mass (Tt). She dissolves 21.5 mg of the protein in water at 5.0°C to make 1.50 mL of solution and measures an osmotic pressure of 3.61 torr. What is the molar mass of this variety of hemoglobin  

Plan We know the osmotic pressure (II = 3.61 torr), R, and T (5.0°C). We convert II from torr to atm, and T from °C to K, and then use Equation 13.13 to solve for molarity (A/). Then we calculate the amount (mol) of hemoglobin from the known volume (1.50 mL) and use the known mass (21.5 mg) to find At. 

Solution Combining unit conversion steps and solving for molarity from Equation 13.13  

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Using Colligative Properties to Find Solute Molar Mass Volatile Nonelectrolyte Solutions Strong Electrolyte Solutions... 

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