I. Determining Molar Mass by Freezing-Point Depression in Naphthalene
PROP 507, Determining Molar Mass by Freezing-Point Depression in
Naphthalene
Chemistry 1046L
II. Purpose of Experiment
The following experiment ask that you complete three consecutive procedures. The first being
to determine the freezing of pure naphthalene, then the freezing point of a solution containing
measured masses of naphathalene and biphenyl and finally to determine the molar mass of and
unknwown substance.
III. Background/Introduction
When a pure liquid is at a higher temperature then its surroundings, the liquids temperture will
begin to drop, releasing heat. As the liquid continues to drop it will eventually reach its freezing
point. In this moment the liquid will enter a transition phase change, where the surrouding
solution closest will go from being a liquid to a liquid plus solid. As the temperature continues to
drop pass the freezing point it becomes ‘supercooled’, resulting in crystallization. At this point,
the liquid will continue to solidify.
We can state that a pure solution reaches its freezing point when it has reached equillibrium,
whereas a solvent in a solution cannnot because it will not freeze at a constant temperature.
When solutions are ‘ideal’ the freezing point of the the solvent in the solution is lower then the
proportion of the solute particles. An ideal solution is one in which the vapor pressure that sits
above a solution is proportional to the number of moles of the solvent.
Colligative properties are proporites of solutions that depend entiretly on the number of
dissolved particles in a solution, but not on the identities of the solutes. A representation of this
can be seen in a mixture of salt water vs. pure water. The freezing point of salt water is lower
that that of pure water, due to the presence of salt that is being dissolved in the water.
IV. Summary of Experimental Procudures
This experiment is divided into three parts as follows:
Part I of this experiment deals with determing the freezing point of pure Naphtalene. You are
asked to set up the equipment you will be using to carry out the experiment as shown in figure 3
in the lab manual. You are to
