Boiling-point elevation through

The change in temperature, ATf, is once again proportional to the change in vapor pressure, ATj. For sufficiently small concentrations of solute, the freezing-point depression is related to the total molality, m (by analogy with the case of boiling-point elevation), through... 

An experiment is to be performed to determine the formula mass of a solute (KN03) through boiling-point elevation. 

Adding an impurity to a solvent makes its liquid phase more stable through the combined effects of boiling point elevation and freezing point depression. That s why you r irely see bodies of frozen salt water. The salt in the oceans lowers the freezing point of the water, making the liquid phase more stable and able to sustain temperatures slightly below 0°C. 

Since we know how the solution vapor pressure varies with concentration (the relationship being given by Equation 6.5-2) and temperature (through the Clausius-Clapeyron equation. Equation 6.1-3), we can determine the relationships between concentration and both boiling point elevation and freezing point depression. The relationships are particularly simple for dilute solutions x — 0, where x is solute mole fraction). 

Boiling point elevation AT = mK[, (the constants have been tabulated) Freezing point depression AT = —mK (the constants have been tabulated) A solution in contact with its pure solvent across a semi-permeable membrane experiences an increase in pressure as pure solvent flows through the membrane into the solution. This osmotic pressure can be measured quite accurately, and through the equation ttV = nRT permits determination of the molecular weight of the solute. 

This is very similar to the boiling-point elevation and freezing-point depression cases we considered in Section 8.10. In both of those cases the solute was inactive, either because its vapor pressure was 0.00 in those cases or because its permeability through the membrane is 0.00 in this case. The behavior of the solvent, which is close to pure, can be estimated by Raoult s law in all three cases. For the pure water, v, and y, are unity, and for the water in the solution, with mol fraction 0.98, Raoult s law is certain to be practically obeyed, so that y, is certain to be practically unity. The partial molar volume of water in pure water is practically the same as that in dilute solutions, so we may take the In of both sides and combine the two integrals, noting that the pressure of the salt water is greater than that of the freshwater, Ending... 

This is an example of a loss through volatilization (evaporation) under temperature conditions at which water would volatilize, hence the word drying. The loss can occur by elevating the temperature of the sample to just above the boiling point of water (although a different temperature may be specified) or through desiccation. This is not necessarily limited to water, however, as any sample component that volatilizes in the case of elevated temperature would be included in the weight loss. Thus it is called loss... 

---