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The extent to which the vapor pressure of a solvent is lowered and the boiling point is elevated depends on the total number of solute particles present in a given amount of solvent, not on the mass or size or chemical identities of the particles. A 1 m aqueous solution of sucrose (342 g/mol) and a 1 m aqueous solution of ethylene glycol (62 g/mol) will exhibit the same boiling point because each solution has one mole of solute particles (molecules) per kilogram of solvent.

Calculating the boiling point of a solution

What is the boiling point of a 0.33 m solution of a nonvolatile solute in benzene?

Solution

Use the equation relating boiling point elevation to solute molality to solve this problem in two steps.

This is a diagram with three boxes connected with two arrows pointing to the right. The first box is labeled, “Molality of solution,” followed by an arrow labeled, “1,” pointing to a second box labeled, “Change in boiling point,” followed by an arrow labeled, “2,” pointing to a third box labeled, “New boiling point.”
  1. Calculate the change in boiling point.
    Δ T b = K b m = 2.53 ° C m −1 × 0.33 m = 0.83 ° C
  2. Add the boiling point elevation to the pure solvent’s boiling point.
    Boiling temperature = 80.1 ° C + 0.83 ° C = 80.9 ° C

Check your learning

What is the boiling point of the antifreeze described in [link] ?

Answer:

109.2 °C

Got questions? Get instant answers now!

The boiling point of an iodine solution

Find the boiling point of a solution of 92.1 g of iodine, I 2 , in 800.0 g of chloroform, CHCl 3 , assuming that the iodine is nonvolatile and that the solution is ideal.

Solution

We can solve this problem using four steps.

This is a diagram with five boxes oriented horizontally and linked together with arrows numbered 1 to 4 pointing from each box in succession to the next one to the right. The first box is labeled, “Mass of iodine.” Arrow 1 points from this box to a second box labeled, “Moles of iodine.” Arrow 2 points from this box to to a third box labeled, “Molality of solution.” Arrow labeled 3 points from this box to a fourth box labeled, “Change in boiling point.” Arrow 4 points to a fifth box labeled, “New boiling point.”
  1. Convert from grams to moles of I 2 using the molar mass of I 2 in the unit conversion factor.
    Result: 0.363 mol
  2. Determine the molality of the solution from the number of moles of solute and the mass of solvent, in kilograms.
    Result: 0.454 m
  3. Use the direct proportionality between the change in boiling point and molal concentration to determine how much the boiling point changes.
    Result: 1.65 °C
  4. Determine the new boiling point from the boiling point of the pure solvent and the change.
    Result: 62.91 °C
    Check each result as a self-assessment.

Check your learning

What is the boiling point of a solution of 1.0 g of glycerin, C 3 H 5 (OH) 3 , in 47.8 g of water? Assume an ideal solution.

Answer:

100.12 °C

Got questions? Get instant answers now!

Distillation of solutions

Distillation is a technique for separating the components of mixtures that is widely applied in both in the laboratory and in industrial settings. It is used to refine petroleum, to isolate fermentation products, and to purify water. This separation technique involves the controlled heating of a sample mixture to selectively vaporize, condense, and collect one or more components of interest. A typical apparatus for laboratory-scale distillations is shown in [link] .

Figure a contains a photograph of a common laboratory distillation unit. Figure b provides a diagram labeling typical components of a laboratory distillation unit, including a stirrer/heat plate with heat and stirrer speed control, a heating bath of oil or sand, stirring means such as boiling chips, a still pot, a still head, a thermometer for boiling point temperature reading, a condenser with a cool water inlet and outlet, a still receiver with a vacuum or gas inlet, a receiving flask for holding distillate, and a cooling bath.
A typical laboratory distillation unit is shown in (a) a photograph and (b) a schematic diagram of the components. (credit a: modification of work by “Rifleman82”/Wikimedia commons; credit b: modification of work by “Slashme”/Wikimedia Commons)

Oil refineries use large-scale fractional distillation to separate the components of crude oil. The crude oil is heated to high temperatures at the base of a tall fractionating column , vaporizing many of the components that rise within the column. As vaporized components reach adequately cool zones during their ascent, they condense and are collected. The collected liquids are simpler mixtures of hydrocarbons and other petroleum compounds that are of appropriate composition for various applications (e.g., diesel fuel, kerosene, gasoline), as depicted in [link] .

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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