Freezing-point depression molar mass determination

When using the freezing-point depression method of determining the molar mass of a nonelectrolyte, what information is needed in addition to the above ... 

Following Eq. (8.12) the freezing point depression is proportional to the cryoscopic constant and the concentration of the solute (m2 (mol/kg)) in the molality scale. Since the cryoscopic constant K ry of the solvent depends on the molar mass, the melting temperature T and the enthalpy of fusion, the various liquids show different values. From Eq. (8.12) it can be seen that the measurement of the freezing point depression allows the determination of the molar mass of unknown compounds, if the melting temperature and the enthalpy of fusion is known. 

Experiment 28 Colligative Properties 1 Freezing-Point Depression and the Determination of Molar Mass... 

Colligative properties, particularly freezing point depression, can be used to determine molar masses of a wide variety of nonelectrolytes. The approach used is illustrated in Example 10.9. 

In carrying out a molar mass determination by freezing point depression, we must choose a solvent in which the solute is readily soluble. Usually, several such solvents are available. Of these, we tend to pick one that has the largest kf. This makes ATf large and thus reduces the percent error in the freezing point measurement From this point of view, cyclohexane or other organic solvents are better choices than water, because their kf values are larger. 

The freezing-point depression data are used to determine the molar mass. 

Determination of molar mass by freezing-point depression... 

D—To calculate the molar mass, the mass of the solute and the moles of the solute are needed. The molality of the solution may be determined from the freezing-point depression, and the freezing-point depression constant (I and II). If the mass of the solvent is known, the moles of the solute may be calculated from the molality. These moles, along with the mass of the solute, can be used to determine the molar mass. 

Determining Molar Mass Using Freezing Point Depression... 

When determining a molar mass from freezing-point depression, it is possible to make each of the following errors (among others). In each case, predict whether the error would cause the reported molar mass to be greater or less than the actual molar mass. 

X V iution), the determination of the molar mass of a solute requires a measurement of mass, volume, temperature, and osmotic pressure. Osmotic pressures are generally large and can be determined quite accurately, thus yielding accurate molar masses. Boiling-point elevations and freezing-point depressions are usually small and not very accurate, so molar mass determinations based on those measures often are not accurate. 

Table 3.6 lists Kf and Kb for several solvents. In general, the higher the molar mass of the solvent, the larger the values of Kf and Kb. If the freezing point depression and boiling point elevation constants are known, the molecular weight of the dissolved solute, M2, can be determined ... 

Determination of molar mass by freezing-point depression test tube, thermometer, pipet, beaker, stirrer, stopwatch, ice... 

One of the laboratory requirements for the course, and also the topic of former test questions, is the determination of the molar mass of a substance from the freezing-point depression. Actually, any of the colligative properties can be used to determine the molar mass, but the only one that you are required to know is the freezing-point depression method. It is easier to illustrate the technique within the framework of a problem, so the discussion of this process will be done within a sample problem. 

Partial Molar Volume 1 0. Cryoscopic Determination of Molar Mass 1 1. Freezing-Point Depression of Strong and Weak Electrolytes 12. Chemical Equilibrium in Solution... 

The expressions commonly used for determination of molalities or molar mass from freezing-point depressions are derived with the following approximations ... 

For the determination of very high molar masses, freezing-point depressions, boiling-point elevations, and vapor-pressure lowerings are too small for accurate measurement. Osmotic pressures are of a convenient order of magnitude, but measurements are time-consuming. The technique to be used in this experiment depends on the determination of the intrinsic viscosity of the polymer. However, molar-mass determinations from osmotic pressures are valuable in calibrating the viscosity method. 

Like the boiling-point elevation, the observed freezing-point depression can be used to determine molar masses and to characterize solutions. 

Considering your answer to Exercise 67, which colliga-tive property, freezing-point depression or osmotic pressure, would be better for determining the molar masses of large molecules Explain your answer. 

Both freezing-point depression and boiling-point elevation can be used to determine whether a species of known molar mass dissociates in solution (Fig. 11.13), as the following example shows. 

Describe how you would use freezing-point depression and osmotic pressure measurements to determine the molar mass of a compound. Why are boiling-point elevation and vapor-pressure lowering normally not used for this purpose ... 

The molar mass of benzoic acid (CgH5COOH) determined by measuring the freezing-point depression in benzene is twice what we would expect for the molecular formula, C7H5O2. Explain this apparent anomaly. 

It would be impossible to determine the molar mass of this solute with a freezing-point depression experiment. (See Supplementary Problems 11 and 19.) ... 

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). 

The freezing point depression constants of the solvents cyclohexane and naphthalene are 20.1°C/ and 6.94°C/ , respectively. Which solvent would give a more accurate result if you are using freezing point depression to determine the molar mass of a substance that is soluble in either one Why ... 

Equation (13.22) provides a simple relation between the freezing-point depression and the molal concentration of solute in a dilute ideal solution, which is often used to determine the molar mass of a dissolved solute. If W2 kg of a solute of unknown molar mass, M2, are dissolved in w kg of solvent, then the molality of solute is m = W2/WM2. Using this value... 

---