Nonvolatile Solutes, Boiling-Point Elevation

Example 8.15 One mol of sugar (sucrose, C12H22O11, M= 342.3 g/mol whose vapor pressure / surcmse 0) is dissolved in 1000 g (1000/18 = 55.6 mol) of water. What is the vapor pressure of this solution at 100°C = 212°F At what temperature will this solution boil at 1 atm  

FIGURE 8.20 DePreister s K-factor chart, for low temperatures. There is a similar chart for higher temperatures. An SI equivalent is in [16]. This nomograph is intended only for preliminary estimates, but is widely used because it is simple. (From DePreister, C. L. Light hydrocarbon vapor-liquid distribution coefficients. Applied Thermodynamics, CEP Symp Ser. 7-49 1-43 (1953). Reproduced with permission of the American Institute of Chemical Engineers.) 

To find the temperature at which the solution will boil, we see on the figure that we must raise the temperature to 

Interpolating in the steam table [12] we find T=212.92°F = 100.51°C. We may restate this that the boiling-point elevation caused by this dissolved, nonvolatile solute is 0.92°F = 0.51°C.  

The amounts of species a and b in this example (1 mol of solute, lOOOg of solvent, which produce a solute concentration of 1.00 molal) were chosen to match the common presentation of boiling point elevation in chemistry textbooks. They normally report the boiling-point elevation coefficient for various solvents. For water at one atmosphere the molal boiling-point elevation constant Ai 0.51°C it has different values for other solvents. It is common in chemistry books to write that as 

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When a solution of a nonvolatile solute is heated, it does not begin to boil until the temperature exceeds the boiling point of the solvent. The difference in temperature is called the boiling point elevation, ATb. 

Boiling point elevation is a direct result of vapor pressure lowering. At any given temperature, a solution of a nonvolatile solute has a vapor pressure lower than that of the pure solvent. Hence a higher temperature must be reached before the solution boils, that is, before its vapor pressure becomes equal to the external pressure. Figure 10.8 (p. 270) illustrates this reasoning graphically. 

Boiling point elevation (ATb) Increase in the boiling point caused by addition of a nonvolatile solute, 269-271 Bomb calorimeter Device used to measure heat flow, in which a reaction is carried out within a sealed metal container, 202-203... 

Because the presence of a nonvolatile solute lowers the vapor pressure of the solvent, the boiling point of the solvent rises. This increase is called boiling-point elevation. The elevation of the boiling point has the same origin as vapor-pressure lowering and is also due to the effect of the solute on the entropy of the solvent. 

By a similar set of arguments, it can be demonstrated that the boiling point elevation for dilute solutions containing a nonvolatile solute is given by the expression... 

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. 

Solutions containing nonelectrolyte nonvolatile solutes have higher boiling points than the pure solvent. The boiling point elevation (ATb) is directly proportional to the solvent s boiling point elevation constant (Ky) times the molality (m) of the solute in moles per kg of solvent ... 

Calculate the boiling-point elevation for a 0.650 m solution of fructose, fruit sugar, C6H12O6, in water. Fructose is nonvolatile and nonionic. 

Consider a 1.00 m solution and a 2.00 m solution of glucose, C6H12O6, a nonvolatile solute, in water, (a) Which solution has the greater boiling-point elevation (b) Which one has the higher boiling point ... 

Given a liquid solution of a nonvolatile solute, estimate the solvent vapor-pressure lowering, the boiling-point elevation, and the freezing-point depression, and list the assumptions required for your estimate to be accurate. 

Since the coefficients of x in these two equations are constant, it follows that for dilute solutions of nonvolatile, nonreactive, nondissociative solutes, both boiling point elevation and freezing point depression vary linearly with solute mole fraction. 

Derive Equation 6.5-4 for the boiling-point elevation of a dilute solution of a nonvolatile solute with mole fraction x in a solvent that has a pure-component vapor pressure p (r).Todo so. suppose that when the pressure is Pq, the pure solvent boils at temperature [so that Pq = Ps( bo)] and the solvent in the solution boils at Tbs > Tbo- Further suppose that at temperature Tw the effective vapor pressure of the solvent is P = (Ps)e(Fbo) < Po- (See diagram.)... 

Figure 2.8 Freezing point depression and boiling point elevation for a binary system with a nonvolatile solute.

The molal boiling point elevation constant, S), is the difference in boiling points between aim nonvolatile, nonelectrolyte solution and a pure solvent. It is expressed in units of °C/w and varies for different solvents. Values of A j, for several common solvents are found in Table 15-4. Note that water s A j, value is 0.512°C/w. This means that a m aqueous solution containing a nonvolatile, nonelectrolyte solute boils at 100.512°C, a temperature 0.512°C higher than pure water s boiling point of 100.0°C. 

A lab technician determines the boiling point elevation of an aqueous solution of a nonvolatile, nonelectrolyte to be 1.12°C. What is the solution s molality ... 

Calculate the molar mass of a nonvolatile solute from the changes it causes in the colligative properties (vapor-pressure lowering, boiling-point elevation, freezing-point lowering, or osmotic pressure) of its dilute solution (Section 11.5, Problems 41-56). 

In Chemistry Lecture 4, we saw that the addition of a nonvolatile solute will lower tire vapor pressure of the solution in direct proportion to the number of particles added, as per Raoult s law. The vapor pressure has an important relationship to the normal boiling point. When the vapor pressure of a solution reaches the local atmospheric pressure, boiling occurs. Thus, the boiling point of a substance is also dranged by the addition of a solute. The addition of a nonvolatile solute lowers the the vapor pressure and elevates the boiling point. The equation for the boiling point elevation of an ideally dilute solution due to the addition of a nonvolatile solute is ... 

The boiling-point constant, kb, depends on the solvent and has units of K-kg-mol (Table 8.8). -> The boiling-point elevation equation holds for nonvolatile solutes in dilute solutions that are approximately ideal. 

Figure 14-14 Because a nonvolatile solute lowers the vapor pressure of a solvent, the boiling point of a solution is higher and the freezing point lower than the corresponding points for the pure solvent. The magnitude of the boiling point elevation, ATj, is less than the magnitude of the freezing point depression, ATf.

Boiling point elevation The increase in the boiling point of a solvent caused by dissolution of a nonvolatile solute. 

Boiling point elevation constant, A constant that corresponds to the change (increase) in boiling point produced by a one-molal ideal solution of a nonvolatile nonelectrolyte. CoUigative properties Physical properties of solutions that depend on the number but not the kind of solute particles present. Colloid A heterogeneous mixture in which solute-like particles do not settle out also called colloidal dispersion. 

Note that in order for boiling-point elevation to occur, the solute must be nonvolatile, but no such restriction applies to freezing-point depression. For example, methanol (CH3OH), a fairly volatile liquid that boils at only 65°C, has sometimes been used as an antifreeze in automobile radiators. 

The boiling point of a substance is the temperature at which its vapor pressure is equal to that of the surroundings. The boiling point of a solvent is elevated by the presence of a nonvolatile solute. This boiling-point elevation, like the freezing-point depression, is directly proportional to the molality of the solute particles ... 

A common application of freezing-point depression and boiling-point elevation experiments is to provide a means to calculate the molar mass of a nonvolatile solute. What data are needed to calculate the molar mass of a nonvolatile solute Explain how you would manipulate these data to calculate the molar mass of the nonvolatile solute. 

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