Determination of Molecular Mass by Freezing Point Depression

Because of interactions between molecules of different substances, solutions behave differently than pure substances in many ways. One of the most obvious, and useful effects, is the change in a solvent’s freezing and boiling temperatures upon the addition of a solute. This effect is exploited in car engines, where the boiling and freezing temperatures of water are changed by the addition of a solution of ethylene glycol. This solution of water and a high weight alcohol is referred to as engine coolant, but is more commonly known, now for obvious reasons, as antifreeze. You will investigate the effect of the addition of a solute on a pure substance in this experiment. You will also use the measured freezing point depression to calculate the molecular mass of unknown liquid.

Materials

Thermometer

2 test tubes

250 mL beaker

clamp

ring stand/ring

ice

rock salt

unknown liquid

LabPro/Temp. probe

Safety

  1. The unknown liquid is slightly toxic. If you spill any on your skin, wash the area with soap and water. Do not ingest it.

Procedure

  1. Set LoggerPro to record temperatures every second for 10 minutes (600 seconds).
  2. Set up the cooling bath apparatus as shown in figure 1 (on the board).
  3. Fill your test tube to a height of between 1 and 2 inches with distilled water. Record the exact mass of water used.
  4. Prepare the ice water bath by adding roughly equal amounts of rock salt and ice to the Styrofoam cup. Periodically stir the mixture until it consistently reads about -10 C. If it does not cool enough, pour off some of the water. Make sure that the cup is nearly full. The level of the ice bath must be above the level of liquid in your tube for the experiment to work.
  5. When you are ready to begin recording data, immerse the tube in the ice water mixture.
  6. Start recording the temperature when you immerse the tube. Use the probe to periodically stir the mixture. After the liquid has completely frozen, stop recording data.
  7. Use the software to determine the freezing point (range) of the distilled water. While you do this, remove the tube from the ice bath and allow the water to melt (immerse in a beaker of warm water to speed the process).
  8. Add the unknown until you have added about 25% or so new volume of the unknown liquid to the tube. Determine the exact mass of the unknown you measure out.
  9. Repeat step 5. When you are finished, pour the liquid down the sink and wash out your test tube.

Analysis

  1. Use LoggerPro to determine the freezing points of both the water and the mixture.
  2. If you did not record your two plots on the same axes, do this: export the data from both graphs as a text file, then open it in Excel and plot both sets of data on the same graph (don’t do this in class—it takes too much time).
  3. From the two freezing temperatures, determine the change in freezing temperature, Tf, for your water-alcohol solution.
  4. Determine the molality of your solution. Kf for water = -1.86C/m.
  5. Use the molality to determine the molecular weight of unknown liquid.
  6. See Mr. Grunden to determine the identity of your solute. Calculate its theoretical molar mass. Compare your results to the theoretical mass and determine your percent error.
  7. Turn in: graphs, data, calculations, conclusion (including results and error analysis), procedure.

Notes on calculations:

  1. To calculate the molar mass of the unknown, we need the following equations:

(1)

(2)

By rearranging (2), we get

Since m = moles/kg, we can substitute (1) for # of moles in molality (m):

Finally, we can rearrange to solve for the molar mass in terms of the mass of the unknown liquid and water we added and the change in T that we measured: