Vapour pressure, partial

As a rough approximation it may be assumed that one mg. of water contained in one litre of gas at 25-30° exerts a 1 mm. partial vapour pressure. Obviously, the lower the residual water content or the vapour pressure, the more intense is the ultimate drying capacity of the substance. 

The pressure exerted by a vapour, either alone or in a mixture of gases. When a mixtrure of gases is involved the partial (vapour) pressure is a measure of the relative concentrations of the constituents of the mixture. The saturated vapour pressure, the pressure of a vapour in contact with its liquid form is often termed simply vapour pressure. 

Butler, J.A.V., Thomson, D.W., and Maclennan, W.H. The free energy of the normal aliphatic alcohols in aqueous solution. Part I. The partial vapour pressures of aqueous solutions of methyl, n-propyl, and rrbutyl alcohols. Part II. The solubilities of some normal aliphatic alcohols in water. Part III. The theory of binary solutions, and its application to aqueous-alcoholic solutions, J. Chem. Soc. (London), 136 674-686, 1933. 

Fig. 116. Diagram by Trogus [5] /—Formation of the addition (Knecht) compounds, //—Nitrationjof cellulose (in % N), ///—Freezing temperatureof HNO3-H2O solutions, IV— Partial vapour pressure of HNO3 in aqueous nitric acid. Hatched area — strong swelling of cellulose fibre. Cross hatched area — solution of cellulose fibre...

When the ratio between both partial vapour pressures changes, the value V changes. The electronics in the car react to this by adjusting the air fuel ratio. 

Consider a binary solution of two components A and B, forming an ideal solution. Let aA and aB be the activities, and xA and xB be the mole fractions of the constituents A and 5, respectively. According to Raoult s law, the partial vapour pressure (pA) pf constituent A is given by... 

Figure 33.1(c) sketches a model of the surface of the liquid mixture of liquids A and B. The relative numbers of each type of liquid molecule (A or B) on the surface will be proportional to xA and xb respectively. Since the vapour pressure exerted when there are no B molecules present (i.e. in pure liquid A) is P then having B molecules present reduces the ability of A molecules to return the full vapour pressure P . Since the proportion of liquid A is xA then the partial vapour pressure returned will be reduced to xA.P. For B molecules a similar argument leads to the conclusion that a partial vapour pressure xB.P should arise from xb molecules of B when co-existent withxA molecules of A within the liquid phase. The total vapour pressure, P, is given by equation (33.3). Thus, Raoult s Law, in the form of the latter equation, is rationalised for this system of two liquids. 

The intercepts of these tangents with the ordinate axes (Frame 3) representing P have no chemical or other significance whatsoever, the purpose of drawing the tangents is merely a means of providing a simple linear relationship between xsoiute (= Xj) and the partial vapour pressure exerted by the (volatile) solute, P, which will be applicable only to a small range of composition. 

Figure 33.3 Henry s Law behaviour for solutes in ideal dilute solutions. Reproduction of Figure 33.2 but with tangents (C K, DL, HJ and Gl) appropriately drawn to the partial vapour pressure curves at the ends where they are acting as solute. ...

Later Sapozhnikov [16] developed a theory, based on density, electrical conductivity and partial vapour pressure measurements. He assumed a state of equilibrium between the hydrates of the two acids in their mixtures ... 

Fig.. 3. Isobars of partial vapour pressure of nitric acid in nitric acid-sulphuric acid-water mixtures (Sapozhnikov [16]).

Experimental detefrmination of partial vapour pressures. The experimental determination of the partial pressures over a mix-... 

When the solute is only sparingly soluble but volatile, measuring the partial vapour pressure may help to determine d/i. For the dilute systems under consideration Dalton s law for the pressure in the mixture, (p + P W = RTln + n ), see 11.2.17.3), is valid. Then we may use 11.2.17.191 for the chemical potential ... 

Equation 5.4 is the basis of a more detailed and fundamental study of the swelling process achieved through the study of resin-water vapour sorption isotherms obtained isopiestically i.e. at equal total pressures and equal resin water content). The isopiestic vapour pressure technique takes account of variable activity of the water in the resin (and therefore IT) by allowing the resin to come to equilibrium with water vapour at different partial vapour pressures P. It is assumed that two resins of the same structural type, but with different degrees of crosslinking, have the same water activity at the same equivalent water content. At equilibrium between resin and vapour phases the water activity in resins (1) and (2) are given by ... 

This step is intended to reduce residual moisture to levels allowing no microbial growth or chemical reactions of the end product. The amount of residual moisture present in a product depends on its desorption isotherms. Such isotherms in turn depend on various factors including the product temperature, pressure chamber, partial vapour pressure in the container and nature of the interaction of the water vapour with the interstitial material formed in the freezing step. The computer should be fed with information on the target sample component. For example, if the component of interest is a protein, then overdrying may alter its configuration and decrease the potency of the end product. Consequently, the computer should control not only the final product temperature but also the partial water vapour pressure and the duration of the desorption step. 

In an ideal solution, the relationship between the partial vapour pressure of a component i in the vapour phase, and the mole fraction of that component in solution, is expressed by Raoult s law as... 

Fig. 21.1 shows the partial vapour pressures and the total pressure of a perfect solution as a function of composition. In the case of a binary solution (21.8) may be written... 

Fig. 21.1. Total and partial vapour pressure of a perfect solution at constant T.

As we have already seen, activity coefficients in liquid solutions may be derived from partial vapour pressure curves. As an example we give in fig. 21.6 the activity coefficients of methylal and carbon disulphide in mixtures of these substances, calculated from the data in fig. 21.5. 

That is to say, if the tangent to one partial vapour pressure curve passes through one origin, then, at the same composition, the tangent to the other curve must pass through the other origin. 

Ap2=Ap+plAx2 The smaller the value chosen for Ax2, the more precise is the method.t Fig. 21.11 shows the excellent agreement obtained between observed and calculated partial vapour pressures for the system ethanol H- chloroform at 45 °C. 

Equation (24.4) shows that the heat of mixing can be calculated from the temperature variation of the activity coefficients. This may be done by measuring the partial vapour pressures of the solution at different temperatures. It is generally preferable, however, to employ a direct calorimetric method, by which h is found without having to employ the activity coefficients. 

We see that the activity coefficients are both less than unity. Starting from (25.40) and (21.34) the partial vapour pressure of A and B can be calculated easily. The results are given in fig. 25.7 which shows the way in which the negative deviations from Raoult s law depend upon the differences in molecular size. 

Using the literature data for the concentrations of the non-hydrated acid in the HN03-H20 system, its states in the HN03-H2S04-H20 system, and its partial vapour pressures, the applicability of Henry s law has been demonstrated to non-hydrated nitric acid in these systems.128 On the basis of a study129 of the systems HN03-H20-M(N03)n (MB+ = Na+, Cu2+, Be2+, U02+, La3+, Fe3+, or Al3+), the salting-out activity of the nitrates is characterized by the separation coefficient I described by ... 

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