Solutions: concentration units, 129 saturated

In this chapter, you learned about solutions. A solution is a homogeneous mixture composed of a solvent and one or more solutes. Solutions may be unsaturated, saturated, or supersaturated. Solution concentration units include percentage, molarity, molality, and mole fraction. The solubility of solids in liquids normally increases with increasing temperature, but the reverse is true of gases dissolving in liquids. The solubility of gases in liquids increases with increasing pressure. 

There are many ways of expressing the relative amounts of solute(s) and solvent in a solution. The terms saturated, unsaturated, and supersaturated give a qualitative measure, as do the terms dilute and concentrated. The term dilute refers to a solution that has a relatively small amount of solute in comparison to the amount of solvent. Concentrated, on the other hand, refers to a solution that has a relatively large amount of solute in comparison to the solvent. However, these terms are very subjective. If you dissolve 0.1 g of sucrose per liter of water, that solution would probably be considered dilute 100 g of sucrose per liter would probably be considered concentrated. But what about 25 g per liter—dilute or concentrated In order to communicate effectively, chemists use quantitative ways of expressing the concentration of solutions. Several concentration units are useful, including percentage, molarity, and molality. 

While concentration units indicate how much solute and solvent make up a solution at any given time, often it is desirable to know the concentration when the solution is saturated. A solution is saturated when the maximum amount of solute has dissolved in the solvent at a particular temperature. When the solvent holds less than its maximum amount of solute, the solution is said to be unsaturated when the solution holds more... 

Solubilities are defined either as the mole fraction of solute in solution, X, or by the amount of solute per unit volume, or concentration, C, at saturation. The concentration in moles per unit volume is given by ... 

AlO.Owi aqueous solution of sodium chloride is fed to an evaporative crystallizer operated under a partial vacuum. Evaporation of water concentrates the remaining solution beyond its saturation point at the crystallizer temperature and causes crystallization of NaCI. The crystallizer product is a slurry of solute crystals suspended in a saturated solution at 80°C The unit is to produce 1000 kg NaCI(s)/h. The solubility of NaCl in water is given by Figure 6.5-1. 

In contrast, several quantities are commonly used to measure the supersaturation. Some of these are P, Ln P, (P - 1) and (C - C ), where P = C/C and C is the concentration of the compound (i.e., TAG) in the solution (i.e., vegetable oil in a solution) at a given temperature. To achieve crystallization, C always must be higher than the concentration at saturation at the same temperature (C ). In the case of supersaturation, the units used to calculate C and differ according to the character of the solute, i.e., electrolyte or nonelectrolyte. The most common units used with TAG are molarities, molalities, and molar fractions. 

Several choices are available in defining the standard state of the solute. If the solute is a liquid which is miscible with the solvent (as, for example, in a benzene-toluene mixture), then the standard state is again the pure liquid. Several different standard states have been used for solutions of solutes of limited solubility. In developing a relationship between drug activity and thermodynamic activity, the pure substance has been used as the standard state. The activity of the dmg in solution was then taken to be the ratio of its concentration to its saturation solubility. The use of a pure substance as the standard state is of course of limited value since a different state is used for each compound. A more feasible approach is to use the infinitely dilute solution of the compound as the reference state. Since the activity equals the concentration in such solutions, however, it is not equal to unity as it should be for a standard state. This difficulty is overcome by defining the standard state as a hypothetical solution of unit concentration possessing, at the same time, the properties of an infinitely dilute solution. Some workers have chosen to... 

In making pH measurements with the quinhydrone electrode the solution should be saturated with quinhydrone, i. e.a the solid substance should be present. Biilniann and Jensen 7 have found that errors may arise if an unsaturated solution is used. Such errors will be about 0.01 pH unit if the quinhydrone concentration is one-tenth of that of the saturated solution, and the error increases if still less of the substance is present. 

Another practice is to refer to supersaturation in terms of degrees. This refers to the difference between the temperature of the solution and the saturation temperature of the solution at the existing concentration. A simpler way to explain this is that the degrees of supersaturation are simply the number of degrees a saturated solution of the appropriate concentration was cooled to reach its current temperature. This is generally not a good unit to use, however, it is often mentioned in the literature. 

Figure 5.11 Facilitated transport proteins in cell membranes. Unlike simple diffusion through the membrane bilayer, facilitated transport systems become saturated as the solute concentration difference increases. In this hypothetical example, the permeability of the membrane in simple diffusion is 0.4 (units of flux/concentration).

The above equation converts the mole fraction of a solute dissolved in water, up to saturation conditions, to the corresponding concentration of that solute in units of moles per liter. By knowing the molecular weight of a particular solute, it is straightforward to obtain the corrresponding concentration in milligrams per liter. 

When the adsorption of a molecule from solution onto a solid surface is considered, there are several quantitative and qualitative points that are of interest, including (1) the amount of material adsorbed per unit mass or area of solid, (2) the solute concentration required to produce a given surface coverage or degree of adsorption, (3) the solute concentration at which surface saturation occurs, (4) the orientation of the adsorbed molecules relative to the surface and solution, and (5) the effect of adsorption on the properties of the solid relative to the rest of the system. In all the above, it is assumed that such factors as temperature and pressure are held constant. 

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