Vapor pressure lowering, molecular

M depends not on the molecular sizes of the particles but on the number of particles. Measuring colligative properties such as boiling point elevation, freezing point depression, and vapor pressure lowering can determine the number of particles in a sample. 

This molecular view of Figure 12-11 suggests that the extent of vapor pressure lowering will depend on the fraction of solvent molecules that has been replaced. In other words, the vapor pressure should be proportional to the mole fraction of the solvent. The molecular view also suggests that this effect does not depend on the nature of the solute, but only on its mole fraction. Experiments show that this is often the case, particularly for dilute solutions. A simple equation, Raoult s law, expresses this proportionality between vapor pressure and mole fraction V V /Jpuj-g solvent Raoulfs law states that the vapor pressure of a solution is the... 

The heat of vaporization and the heat of fusion of water are 540 and 80 cal/g respectively, (a) For a solution of 1.2 g of urea in 100 g of water, estimate (i) the boiling point elevation, (ii) the freezing point depression, (iii) the vapor pressure lowering at 100°C. Assume ideal-solution and ideal-gas behavior and assume urea to be nonvolatile. (b) Discuss the foregoing properties of a solution of 1.2 g of a nonvolatile solute of molecular weight 10 in 100 g of water. 

The identity of die molecular solute does not affect die vapor pressure lowering (as long as die solute does not ionize) it is the number of molecules per unit mass of solvent diat affects the extent of die vapor pressure lowering. Therefore, for all three parts of die problem,... 

The presence of a solute causes vapor-pressure lowering of a solvent. If the solute is nonvolatile (nonevaporating), the solution has a lower vapor pressure than the pure solvent does. (Review vapor pressure in Chapter 7.) From a molecular view, the solute particles at the surface of the liquid inhibit the movement of solvent molecules from going into the vapor phase, but do not inhibit solvent molecules in the vapor phase from returning to the liquid phase, so the rate of evaporation is lower than the rate of condensation until there are fewer solvent molecules in the vapor phase. For solving problems, the vapor pressure of any component (call it A) in the solution. Pa, is related to the vapor pressure of the pure substance, P, by Raoult s law ... 

Vapor-pressure lowering Applicable to molecular weights of up to 10. ... 

Vapor Pressure Osmometry The VPO became a popular method for the determination of the number-average molecular weight of nonvolatile solutes of less than about 20,000 g/mol and that tend to diffuse across the membrane in MO experiments [91], This method operates on the principle that the vapor pressure of a solution is lower than that of a pure solvent (P°) at constant pressure and temperature. This vapor pressure lowering (AP) is proportional to the molar mass of the solute (polymer) for dilute solutions. As it is known, the vapor pressure of a solvent in dilute solutions obeys the Raoult s law, Pj = Pfxj, where Pj is the partial vapor pressure of the solvent whose mole fraction in the solution is Xj. In terms of the mole fraction of the solute, Pj = P° (l - X2) or AP/pO = -X2. 

Vapor pressure osmometry Number average molecular weight Vapor pressure lowering <5 X 1 ... 

Ebulliometry n. Method of measuring molecular weight of polymers under 20,000 based on vapor pressure lowering and boding point elevation. Pethrick RA, Dawkins JW (eds) (1999) Modern techniques for polymer characterization. John Wiley and Sons, New York. 

Vapor-pressure lowering n. One of the col-ligative properties of a solution and the basis of a method for determination of the molecular mass of a solute. For a dilute solution, the solvent vapor pressure lowering is determined by (po—p)/po= 2> where po and p are the vapor pressures of the pure solvent and the solution, respectively, and X2 is the mole fraction of the solute. 

Note that X is smaller than 1, causing the positive entropy and negative contribution to the chemical potential stabilizing the solution. The vapor pressure lowering and other colligative properties are used to determine molecular masses of the solute 2. 

Vapor-pressure osmometry is from its name comparable to membrane osmometry by allowing the vapor phase to act like a semipermeable membrane, but it is based on vapor pressiue lowering or boiling temperature elevation. Since the direct measure of vapor pressure lowering of dilute polymer solutions is impractical because of the extreme sensitivity that is required, VPO is in widespread use for low-molecular and oligomer solutions (i.e., M less than 20,000 g/mol) by employing the thermoelectric method where two matched temperature-sensitive thermistors are placed in a chamber that is thermostatted to the measuring temperature and where the atmosphere is saturated with solvent vapor. If drops of pure solvent are placed on both thermistors, the thermistors will be at the same temperature (zero point calibration). If a solution drop is placed on one thermistor, a temperature difference AT which is caused by condensation of solvent vapor onto the solution drop occurs. From equilibrium thermodynamics it follows that this temperature increase has its theoretical limit when the vapor pressure of the solution is equal to that of the pure solvent, i.e., at infinite dilution. The obtained temperature difference is very small, about 10 K. 

Historically, chemists have used the group of colligative properties— vapor pressure lowering, freezing-point depression, boiling-point elevation, and osmotic pressure—for molecular mass determinations. In Example 14-9, we showed how this could be accomplished with osmotic pressure. Example 14-10 shows how freezing-point depression can be used to determine a molar mass and, with other information, a molecular formula. To help you understand how this is done, we present a three-step procedure in the form of answers to three separate questions. In other cases, you should be prepared to work out your own procedure. 

Ethyl Vinyl Ether. The addition of ethanol to acetylene gives ethyl vinyl ether [104-92-2] (351—355). The vapor-phase reaction is generally mn at 1.38—2.07 MPa (13.6—20.4 atm) and temperatures of 160—180°C with alkaline catalysts such as potassium hydroxide and potassium ethoxide. High molecular weight polymers of ethyl vinyl ether are used for pressure-sensitive adhesives, viscosity-index improvers, coatings and films lower molecular weight polymers are plasticizers and resin modifiers. 

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