MHS Chemistry
Freezing Point Depression

The purpose of this lab is to determine how much the freezing point of a pure substance changes when impurities are added.  We will first determine the freezing point of pure naphthalene (C8H10), then add a small amount of p-dichlorobenzene (C6H4Cl2) and determine the new freezing point.

Recall that a liquid freezes when it becomes solid; it does not necessarily have to become cold.

The relationship between amount of impurities and freezing point is given by the equation
        DT = km
where DT is the amount the freezing point lowers or “depresses”, k is a constant unique to every substance, and m is molality, defined as “moles of solute per kilogram of solvent.”

Purpose
The purpose of this activity is to determine the freezing point depression constant for naphthalene, and to compare our experimental value to the accepted value.

Procedure

  1. Label a medium test tube and determine it’s mass.
  2. Fill the test tube about ¾ full with naphthalene.  Record the combined mass.
  3. Melt the naphthalene in a warm water bath.  Watch for a constant temperature while it melts.  When the naphthalene is all melted, remove it from the bath and monitor the temperature while gently agitating the liquid.  Record the constant temperature at which there was both liquid and solid present.
  4. Use the bath to melt the naphthalene again, and add about a spatula full of p-dichlorobenzene.  Make sure both substances are entirely melted and the solution is stirred.
  5. Remove the test tube from the bath and monitor the temperature while gently agitating the liquid.  Record the constant temperature at which there was both liquid and solid present.
  6. Heat the test tube again so you can remove the thermometer cleanly from the mixture.  Wipe the thermometer dry and return it to the rack.
  7. When the test tube has cooled again, record the mass of it with all the contents.
  8. Clean up the lab station; check with your teacher for disposal.
 
Data
test tube # __________
1. mass of test tube __________ g
2. mass of test tube + naphthalene __________ ___
3. freezing point of pure naphthalene __________ ___
5. freezing point of naphthalene + p-dichlorobenzene __________ ___
7. mass of test tube + naphthalene + p-dichlorobenzene  __________ ___

 
 
Results (attach all calculations)
a. mass of p-dichlorobenzene __________ ___
b. moles of p-dichlorobenzene __________ ___
c. mass of naphthalene __________ Kg
d. molality of solution __________ ___
e. change in freezing point __________ ___
f. k from experiment __________ ___
g. actual k __________ C/m
h. % error __________ %

[Freezing Point Depression score sheet][MHS Chem page]