Osmotic nonelectrolytes

Osmotic pressure experiments provide absolute values for Neither a model nor independent calibration is required to use this method. Experimental errors can arise, of course, and we note particularly the effect of impurities. Polymers which dissociate into ions can also be confusing. We shall return to this topic in Sec. 8.13 for now we assume that the polymers under consideration are nonelectrolytes. 

The properties of a solution differ considerably from those of the pure solvent Those solution properties that depend primarily on the concentration of solute particles rather than their nature are called colligative properties. Such properties include vapor pressure lowering, osmotic pressure, boiling point elevation, and freezing point depression. This section considers the relations between colligative properties and solute concentration, with nonelectrolytes that exist in solution as molecules. 

Osmotic pressure, like vapor pressure lowering, is a colligative property. For any nonelectrolyte, ir is directly proportional to molarity, M. The equation relating these two quantities is very similar to the ideal gas law ... 

Chapters 7 to 9 apply the thermodynamic relationships to mixtures, to phase equilibria, and to chemical equilibrium. In Chapter 7, both nonelectrolyte and electrolyte solutions are described, including the properties of ideal mixtures. The Debye-Hiickel theory is developed and applied to the electrolyte solutions. Thermal properties and osmotic pressure are also described. In Chapter 8, the principles of phase equilibria of pure substances and of mixtures are presented. The phase rule, Clapeyron equation, and phase diagrams are used extensively in the description of representative systems. Chapter 9 uses thermodynamics to describe chemical equilibrium. The equilibrium constant and its relationship to pressure, temperature, and activity is developed, as are the basic equations that apply to electrochemical cells. Examples are given that demonstrate the use of thermodynamics in predicting equilibrium conditions and cell voltages. 

Van t Hoff introduced the correction factor i for electrolyte solutions the measured quantity (e.g. the osmotic pressure, Jt) must be divided by this factor to obtain agreement with the theory of dilute solutions of nonelectrolytes (jt/i = RTc). For the dilute solutions of some electrolytes (now called strong), this factor approaches small integers. Thus, for a dilute sodium chloride solution with concentration c, an osmotic pressure of 2RTc was always measured, which could readily be explained by the fact that the solution, in fact, actually contains twice the number of species corresponding to concentration c calculated in the usual manner from the weighed amount of substance dissolved in the solution. Small deviations from integral numbers were attributed to experimental errors (they are now attributed to the effect of the activity coefficient). 

The answer is b. (Hardman, pp 695-697.) A significant increase in the amount of any osmotically active solute in voided urine is usually accompanied by an increase in urine volume Osmotic diuretics affect diuresis through this principle. The osmotic diuretics (such as mannitol) are nonelectrolytes that are freely filtered at the glomerulus, undergo limited re absorption by the renal tubules, retain water in the renal tubule, and promote an osmotic diuresis, generally without significant Na excretion. Ln addition, these diuretics resist alteration by metabolic processes. 

Osmotic pressure is a colligative property and is dependent on the number of particles of solute(s) in a solution. The total number of particles of a solute in a solution is the sum of the undissociated molecules and the number of ions into which the molecule dissociates. The number of ions, in turn, depends on the degree of ionization. Thus, a chemical that is highly ionized contributes a greater number of particles to the solution than the same amount of a poorly ionized chemical. When a chemical is a nonelectrolyte such as sucrose or urea, the concentration of the solution depends only on the number of molecules present. The values of the osmotic pressure and other colligative properties are approximately the same for equal concentrations of different nonelectrolyte solutions. 

A solution prepared by dissolving 7.95 mg of a gene fragment in 25.0 mL of water has an osmotic pressure of0.295 torr at 25.0°C. Assuming the fragment is a nonelectrolyte determine the molar mass of the gene fragment. 

In this example, we need to determine the molar mass (g/mol) of the gene fragment. This requires two pieces of information—the mass of the substance and the number of moles. We know the mass (7.95 mg), thus we need to determine the number of moles present. We will rearrange the osmotic pressure relationship to n 77 V/RT. We know the solute is a nonelectrolyte so i = 1. We can now enter the given values into the rearranged equation and perform a pressure and a volume conversion ... 

As we saw in Section 17.5, the activity coefficient of a nonelectrolyte solute can be calculated from the activity coefficient of the solvent, which, in turn, can be obtained from the measurement of colligative properties such as vapor pressure lowering, freezing point depression, or osmotic pressure. We used the Gibbs-Duhem equation in the form [Equation (17.33)]... 

This expression is analogous to Eiq. (2.3), in that (1 — (p) expresses the contribution of the solvent and In y+ that of the electrolyte to the excess Gibbs energy of the solution. The calculation of the mean ionic activity coefficient of an electrolyte in solution is required for its activity and the effects of the latter in solvent extraction systems to be estimated. The osmotic coefficient or the activity of the water is also an important quantity related to the ability of the solution to dissolve other electrolytes and nonelectrolytes. 

The osmotic coefficient of the binary nonelectrolyte solution was obtained from isopiestic measurements... 

Maron,S.H., Nakajima,N. A theory of the thermodynamic behavior of nonelectrolyte solutions. III. The osmotic pressure of polymer solutions. J. Polymer Sci. 42, 327-340 (1966). 

The same van t Hoff responsible for the / factor showed that the osmotic pressure of a nonelectrolyte solution is related to the molarity, 3W, of the solute in the solution ... 

Solutions are usually classified as nonelectrolyte or electrolyte depending upon whether one or more of the components dissociates in the mixture. The two types of solutions are often treated differently. In electrolyte solutions properties like the activity coefficients and the osmotic coefficients are emphasized, with the dilute solution standard state chosen for the solute.c With nonelectrolyte solutions we often choose a Raoult s law standard state for both components, and we are more interested in the changes in the thermodynamic properties with mixing, AmjxZ. In this chapter, we will restrict our discussion to nonelectrolyte mixtures and use the change AmjxZ to help us understand the nature of the interactions that are occurring in the mixture. In the next chapter, we will describe the properties of electrolyte solutions. 

The osmotic pressure, n, of a dilute solution of a nonelectrolyte is given by an equation formally equivalent to the ideal gas law. 

An organic compound is known to be nonvolatile and a nonelectrolyte. A 0.35-g sample is dissolved in water and diluted to 150 mL. The osmotic pressure is measured as 0.04 atm at 25°C. What is the approximate mass number for this compound ... 

Sample A solution is made by placing 220.0 g of glucose (C6H1206, molar mass = 180.16 g) in a volumetric flask and adding distilled water to equal 1.00 L of solution. Calculate the osmotic pressure at 25°C. Glucose is a nonelectrolyte. 

Osmotic pressure may be considerable for even highly dilute solutions for example, for an aqueous nonelectrolyte solution with x2 = 0.001 at 200 K, we have... 

Figure 2-9. Relationship between concentration and osmotic pressure at 20°C for a nonelectrolyte (sucrose) and two readily dissociating salts (NaCl and CaCl2). The different initial slopes indicate the different degrees of dissociation for the three substances and are consistent with the Van t Hoff relation (Eq. 2.10). Data for osmotic pressure are based on the freezing point depression. (Data source Lide, 2008.)...

Osmotic pressure can be quite large, even for very dilute solutions. Consider an aqueous solution containing mole fraction xi = 0.001 of a nonelectrolyte solute species at 298.15 K (25°C). Then... 

If the sucrose solution in the aforementioned membrane sac were replaced with a sodium chloride solution of the same molarity, the solution in the manometer would reach equilibrium at a point almost twice as high as that observed with sucrose because sodium chloride dissociates into two ions per molecule. If ion activity is unrestricted, the sodium chloride solution would have twice as many osmoticaUy active particles (osmoles) for the same molecular concentration as the sucrose solution. In reality, the number of active particles is less than this (0.93 for NaCl), as explained later in this chapter. The total number of individual (solute) particles present in a solution per given mass of solvent, regardless of their molecular nature (i.e., nonelectrolyte, ion, or coUoid), determines the total osmotic pressure of the solution. In blood plasma, for example, nonelectrolytes such as glucose and urea and even proteins contribute to the osmotic pressure of this body fluid. 

The total osmolality or osmotic pressure of a solution is equal to the sum of the osmotic pressures or osmolalities of all solute species present. The electrolytes Na", Cr, and HCO3, which are present in relatively high concentration, make the greatest contribution to serum osmolality. Nonelectrolytes such as glucose and urea, which are present normally at lower molal concentrations, contribute less, and serum proteins contribute less than 0.5% of the total serum osmolality because even the most abundant protein is present at millimolar concentrations. 

The osmolality of stool "water wiU normally be that of serum (i.e., 290mosm/kg), but the contribution of electrolytes and of nonelectrolytes to the total osmolality will vary depending on the cause of the diarrhea. Fecal osmotic (osmolal) gap (FOG) expresses the difference between the theoretical normal osmolality (290 mosm/kg) and the contribution of Na and as follows ... 

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