6. What is the freezing point of a solution (water) that boils at 105.0 o C?? 7.

Question

6.   What is the freezing point of a solution (water) that boils at 105.0oC??

7. The Henry's law constant of helium gas in water at 30oC is 3.7 X 10-4 M/atm and the constant for Nitrogen (N2) at 30oC is 6.0 X10-4 M/atm

          a. How many M of helium are dissolved in a solution if it is at 3 atm?

          b. How many M of Nitrogen are dissolved in a solution if it is at 75 atm?

8.   List the following aqueous solutions in order of decreasing freezing point:

0.04 m glycerine                 0.020 m Kbr                     0.03 m Phenol

Explanation / Answer

Solution 6

First use the boiling pt. elevation equation: ?T = Kb * m.
?T is difference in temp. between the boiling point of the solution and the boiling point of the pure solvent (water).
Kb is boiling point elevation constant (the Kb for water is 0.512 *C/m).
m is concentration in units of molality

Figure out the molality of the solution so you can use it later on to calculate its freezing point.

The boiling point of pure water is 100 *C. So ?T = (b.p. of solution) - (b.p. of pure water) = 106.5 *C - 100 *C = 6.5 *C.

Now just plug and chug to find molality.
6.5 = 0.512 * m

I get a molality (m) value of 12.7.

Now use the molality just calculated to figure out the freezing pt. of the solution.
Use the freezing point depression equation: ?T = Kf * m.
It's the same as the first equation, only that Kf refers to a freezing point depression constant for a given solvent (water's Kf is 1.86).
?T refers to the difference between the freezing pt. of pure solvent (water's f.p. is 0 *C) and the freezing pt. of the solution.

Plug and chug to find ?T:
?T = 1.86 * (12.7) = 23.61 *C.
Now you can set up this equation:
?T = (freezing pt. of water) - (freezing pt. of solution)
23.61 *C = 0 *C - (freezing pt. of solution)

After a little algebra, I get the freezing pt. of the solution to be -23.61 *C.

Solution 8

Freezing point depression is a colligative property meaning that it depends only on the number of particles present in the solution.

The material with the fewest heterogeneous particles in solution is the phenol solution. It will have the highest freezing point.

The solutions of glycerin and KBr would appear to have the same number of particles in solution, so their freezing points should be quite close.

However, KBr does not quite produce 2 particles per mole, so it will be slightly higher in freezing point than the glycerin solution.

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