Freezing Point Depression
Problem: Determine how much a 1.0 molal salt solution
lowers the freezing point of water.
you ever wondered how antifreeze lowers the freezing point of water in the cooling systems of automobiles? In the late nineteenth
century the French chemist Francois Raoult noted that the vapor pressure of a solvent was lowered by the addition of a solute
(Raoult's Law), and that the freezing points of solutions also were lowered.
the following mathematical expression that related freezing point depression and molality (moles of solute per kilogram of
solvent) was developed.
Kf ´ msolute ´ i
this equation, DT is the freezing point depression, Kf is the molal freezing point constant for the solvent, and m
is the molality of the particles in the solution.
you can see from the equation, the lowering of the freezing point depends on the concentration of dissolved particles present.
In the case of a non-electrolyte, like antifreeze, the molality of the non-electrolyte and the molality of particles in solution
are the same (a 1:1 ratio). For electrolytes, like NaCl, the molality of particles is equal to the molality of the electrolyte
times the number of ions in the chemical formula of the compound. The correction factor (i) is needed due to the attraction
between solvent particles. This correction factor is called the van't Hoff Factor. The accepted van't Hoff Factor (i) for NaCl is 1.9.
a marker, label a test tube and the 250mL beaker "D" for distilled water. Then label a test tube and a plastic cup "N" for
the 250mL beaker labeled "D" with 200mL of distilled water.
a pipet, transfer the distilled water into the test tube labeled "D" to a height of 3cm.
the plastic cup labeled "N". Record its mass.
50.0mL of distilled water into the plastic cup.
the plastic cup with the water. Record its mass.
exactly 2.92 grams of sodium chloride. Record this mass.
the sodium chloride to the plastic cup labeled "N" containing 50.0mL of water.
a stirring rod, mix the solution until all of the salt has dissolved.
10. Using a pipet, transfer
the sodium chloride solution into the test tube labeled "N" to a height of 3cm.
11. Fill the 400mL beaker with
ice to the 300mL mark. Cover the ice with a dusting of sodium chloride crystals. Stir this ice-salt mixture with a stirring
rod until it reaches a constant temperature at or below -10oC.
12. Hold the test tube containing
distilled water with a test tube holder, and place it into the ice-salt bath. Using the thermometer, gently stir the water
until ice crystals first appear. Hold the thermometer at the very top of the water. Record this temperature.
13. Hold the test tube containing
sodium chloride solution with a test tube holder, and place it into the ice-salt bath. Using the thermometer, gently stir
the water until ice crystals first appear. Hold the thermometer at the very top of the solution. Record this temperature.
Calculate the mass of the water, in kilograms.