CHEM 106General Chemistry II

 

EXERCISE 2.Freezing Point Depression

In lecture we discussed the phenomenon of freezing point depression and boiling point elevation for a bulk solvent containing a solute.†† The general formula for the difference in freezing or boiling temperature compared to that of the pure solvent is given as:

Δ T = Km

The concentration is given in molality, m= (moles solute/ kg solvent), temperature is given in Celsius degrees, andK is the freezing point or boiling point constant specific to the solvent. The relationship holds for any non-dissociating solute dissolved in a solvent.When the solute is capable of dissociation upon dissolution, such as with table salt NaCl, an additional correction factor is introduced called the Vanít Hoff factor.

Δ T = i Km

The Vanít Hoff factor, i, is dependent upon the concentration of the solution and the number of ions produced upon complete dissociation of the solute in the solvent.For very dilute solutions the Vanít Hoff factor is approximately equal to the number of ions formed upon dissolution.For NaCl, two ions, Na+ and Cl- are formed upon dissolution, so in dilute solutions, the Vanít hoff factor is about 2, corresponding to 2 moles of ions dissolved for I mole of salt added to solution.When the concentration is increased there is a greater probability that the dissociated ions can re-associate to form NaCl molecules.The expected range for the Vanít Hoff factor for NaCl in solution would be between 1 and 2.This can be experimentally determined.

 

Procedural Outline:Sugar solutions

ō      Prepare a 0.15 m solution of sucrose in 100 ml

ō      Using this solution, fill a test tube provided to about half full.

ō      Place the 1 hole cork/ stopperin the test tube and insert the thermometer.

ō      Be sure the thermometer bulb is immersed in the solution.

ō      Place the test tube into the graduated cylinder ( acts as a stand only) and stand the cylinder in the 1000 ml beaker.Fill the beaker with ice, then cover the ice with rock salt to cool the test tube and its contents.Keep adding ice and rock salt as needed.

ō      In regular degree intervals, record Temperature vs. Time

ō      You may stop recording after a solid ice appears in the test tube.

ō      Prepare a cooling curve plotting temp vs time.

ō      Record the temperature at which ice crystals appear. This will not necessarily be the lowest observed temperature, as supercooling may occur.†† The temperature where ice crystals appear is used in calculations.

Pure Water:As a control, repeat the above procedure using distilled water only, no solute added.†† Prepare a cooling curve, temperature vs time.†††

Calculations:Based on the solution concentration calculate the expected freezing point depression.Kf = 1.83 oC/m.†† Compare this to your observed freezing point and do a percent difference calculation.

Vanít Hoff factor determination:

Repeat the process using 0.15 m NaCl solution.Compare the calculated freezing point to the observed freezing point.The ratio of observed over expected will be the Vanít Hoff factor, i.Prepare a cooling curve for the salt solution as before.