Colligative properties molar mass determination

Vapour pressure osmometry is the second experimental technique based on colligative properties with importance for molar mass determination. The vapour pressure of the solvent above a (polymer) solution is determined by the requirement that the chemical potential of the solvent in the vapour and in the liquid phase must be identical. For ideal solutions the change of the vapour pressure p of the solvent due to the presence of the solute with molar volume V/1 is given by... 

Determination of Molar Masses from Colligative Properties... 

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. 

A laboratory experiment on colligative properties directs students to determine the molar mass of an unknown solid. Each student receives 1.00 g of solute, 225 mL of solvent and information that may be pertinent to the unknown. 

Molar masses can also be determined using other colligative properties. Osmotic pressure measurements are often used, particularly for solutes of high molar mass, where the concentration is likely to be quite low. The advantage of using osmotic pressure is that the effect is relatively large. Consider, for example, a 0.0010 M aqueous solution, for which... 

Use colligative properties to determine molar mass of a solute. 

HOWTO USE COLLIGATIVE PROPERTIES TO DETERMINE MOLAR MASS... 

The lowering of freezing point and the generation of osmotic pressure both depend on the total concentration of solute particles. Therefore, by using the colligative property to determine the amount of solute present, and knowing its mass, we can infer its molar mass. 

The osmotic pressure is a colligative property and mathematically can be represented as 71 = (nRTIV) i, where It is the osmotic pressure in atmospheres n is the number of moles of solute R is the ideal gas constant 0.0821 L atm/Kmol T is the Kelvin temperature Vis the volume of the solution and i is the van t Hoff factor. Measurements of the osmotic pressure can be used to calculate the molar mass of a solute. This is especially useful in determining the molar mass of large molecules such as proteins. 

The colligative properties, described in Section 3.4.1 to Section 3.4.3, have been used to determine the molar mass of unknown chemical compounds. Pharmaceutical scientists and pharmacists may apply this concept in the preparation of isotonic (meaning of equal tone) solution dosage forms. These solution dosage forms can be applied to sensitive and delicate organs such as the eye, nose, or ear or directly injected into the body (i.e., blood vessels, muscles, lesions, etc.). They should have, when administered, the same osmotic pressure as body fluids. Otherwise, transport of body fluids inside and outside the cell tissues will occur, causing discomfort and damage to the tissue. Osmolarity of body fluids is approximately 0.307 osmol/L or 307 mosmol/L. 

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. 

Osmotic pressure can be used to characterize solutions and determine molar masses just as the other colligative properties can however, osmotic pressure is particularly useful because a small concentration of solute produces a relatively large osmotic pressure. 

The colligative properties are of importance by themselves, but they can also be used to determine the molar mass of a solute, since they all depend on the molar concentration and since the mass concentration generally is known. To this end, the determination of the freezing point often is most convenient. Because of nonideality, determinations should be made at several concentrations and the results extrapolated to zero. For determination of the molar mass of macromolecules, osmotic pressure measurement is to be preferred, since membranes exist that are not permeable for macromolecules, while they are for small-molecule solutes, and even small quantities of the latter have a relatively large effect on the colligative properties. Actually, a difference in osmotic pressure is thus determined, the difference being due to the macromolecules only. 

In Chapter 12 you learned about the colligative properties of solutions. Which of the colligative properties is suitable for determining the molar mass of a polymer Why ... 

This is a very large pressure indeed Compared to other colligative properties, osmotic pressure measurements allow very precise determinations of molar masses. ... 

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). 

Colligative properties are related to the number of dissolved solute particles, not their chemical nature. Compared with the pure solvent, a solution of a nonvolatile nonelectrolyte has a lower vapor pressure (Raoult s law), an elevated boiling point, a depressed freezing point, and an osmotic pressure. Colligative properties can be used to determine the solute molar mass. When solute and solvent are volatile, the vapor pressure of each is lowered by the presence of the other. The vapor pressure of the more volatile component is always higher. Electrolyte solutions exhibit nonideal behavior because ionic interactions reduce the effective concentration of the ions. 

The measurement of colligative properties is one of the classical methods for determining the molar mass of solute. Although all of these properties have been used at one time or another to measure the molar mass of a polymer, only the osmotic effect is large enough to be generally useful. 

Because chemical methods are rather limited, the most widely used techniques for measuring the molar mass of a polymer are physical. Methods that depend on the colligative properties of dilute solutions can be used to determine the molar mass of a substance. These include ... 

Because of this peculiarity, the colligative properties can be used to determine the amount of substance Wb of sample of an unknown substance B and, therefore, if the mass of the sample is known, also the molar mass Mb = m /n. Let us take a quick look at this by considering the example of lowering of freezing point. Xb ns/riA is valid at high dilution, and because of a = rriA/MA, we have Xb Wb MA/rriA- The quotient n- IrriA corresponds to the molality b- (compare Sect. 1.5). Inserting these expressions in Eq. (12.14) for the freezing-point depression results in... 

The colligative properties of solutions provide a useful means of determining molar mass. Any of the four colligative properties can be used, as shown in Sample Exercises 13.11 and 13.12. 

D5.3 All of the colligative properties are a function of the concentration of the solute, which implies that the concentration can be determined by a measurement of these properties. See eqns 5.33,5.34,5.36,5.37, and 5.40. Knowing the mass of the solute in solution then allows for a calculation of its molar mass. For example, the mole fraction of the solute is related to its mass as follows ... 

Using Colligative Properties to Determine Molar Mass... 

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