Molar Mass by Freezing Point Depression

Introduction

When a nonvolatile solute is dissolved in a solvent, the freezing point of the solvent is lowered. This process is called Freezing Point Depression and is a colligative property of solutions. Colligative properties depend on the amount not the nature of the solute dissolved in the solvent. Sprinkling salt on icy roads to melt the ice is an example of Freezing Point Depression. One way to undestand why the temperature at which a mixture freezes lower that that of the pure solvent is to consider the vapor pressures.

The freezing point can be defined as the temperature where the vapor pressure of the solid and the liquid are the same. For water the normal (1 atmosphere) freezing point is at 0°C. If a solute is added to liquid water the vapor pressure of the water is lowered. The vapor pressure of the solid will also be lowered, and the vapor pressure of the solid is lowered even more than that of the liquid. Therefore the solution must be cooled for the two pressures to become equal again. The freezing point of the solution has been depressed.

Each solvent responds differently to the addition of a solute. This response is characterized by K_f, the molal freezing point depression constant. It is important that molality is used as the unit for concentration in temperature studies; as mass is temperature independent and volume (Molarity) is not. The equation for freezing point depression is:

ΔT_f = imK_f

Where:

ΔT_f = the absolute decrease in temperature. This is always a positive value.

i= the van’t Hoff factor. i = . For a

nonpolar solvent such as naphthalene, i = 1.

m = molality of the solution. (moles of solute)/(mass of solvent in kg).

K_f = molal freezing point depression constant.

K_f (naphthalene) = 6.80 °C/molal.

In this experiment, naphthalene will be used as the solvent and an unknown will be added The freezing point of pure naphthalen is 81.0 °C. From the decrease in freezing point of the solution versus pure naphthalene, the Molar Mass of the unknown will be determined.

Equipment:

Analytical balance ( measure to 0.001 g)	wide (fat) 6" test tube
600 mL beaker	wide mouth “gas” bottle
2 ring stands	test tube clamp
hot plate	Thermometer and thermometer clamp

Chemicals:

Naphthalene

Unknown (record the unknown number or letter)

Hazardous Waste container is provided for disposal of the naphthalene mixture. (Spill/Disposal F)

Procedure

1. Using a plastic boat, weigh out between 9.5 and 10.5 g of naphthalene to the nearest 0.001 g, record the mass and place the naphthalene into a dry fat 6" test tube.

Weigh out one sample of naphthalen only.

Using a plastic boat, weigh out between between 0.9 and 1.1g of the unknown compound to the nearest 0.001 g. record the mass and place the unknown in test tube containing your sample of naphthalene. Do not stir.

2. Place a hot plate on the base of the ring stand. Place a 600 mL beaker on the hot plate. Add 300 mL of water to the beaker and suspend the test tube in the 600 mL beaker using a test tube clamp.

Make sure that the water level in the beaker is above the level of the mixture in the test tube.

Now place the wire stirrer into the test tube and suspend a thermometer using a thermometer clamp in the naphthalene, so that the loop of the stirrer surrounds the thermometer.

Heat the water until the naphthalene melts, and heat to at least a temperature between 90-95°C.

2. Measure the temperature of the melted naphthalene/unknown mixture. This is the temperature at time zero. Start the stopwatch.

Quickly remove the test tube from the hot water using the attached clamp. Place the test tube into a wide mouth bottle and reclamp it to a second ringstand.

Gently stir the misture with the wire stirrer and record the temperature (to the nearest 0.2 C) of the liquid mixture every 30 seconds until the liquid starts freezing. Continue to stir as you take your readings.

Continue to take readings until about half the liquid has frozen.

Using the same sample: melt and solidify the misture one more time, recording a second set of data.

3. When you have completed the experiment the cleanup procedure is:

A. Warm the test tube in the hot water bath until all of the mixture has melted and all you have is a liquid in the test tube.

B. Remove the thermometer and stirrer and wipe them clean using a paper towel wetted with acetone.

C. Pour the molten naphthalene mixture into the specified waste container.

DO NOT POUR THE LIQUID INTO THE SINK !!!

D. Rinse the test tube with small amounts (2 mL) of acetone, pouring this waste solution into the waste container until the test tube is clean.

Disposal

The remaining melted contents of the test tube must be disposed of into the labeled - "MW by FPD" (Naphthalene & Acetone) Hazardous Waste container.

4. Using a computer, graph your data. Use the scatter plot option with time as the x-axis and temperature as the y-axis. Do not insert a trendline! This is a cooling curve. The temperature falls over time as the liquid cools. As solid begins to form the temperature levels out. If the system was isolated (no heat or mass lost) the freezing of the mixture would be level, constant temperature) as in Graph I. Since the system is open (both temperature and mass can be lost), the freezing temperature of the mixture will decrease as heat is lost to the surroundings (Graph II).

In each diagram the freezing point is the intersection of the two lines. These lines are obtained by drawing the best fit straight line through the two roughly straight-line portions of each cooling curve. Do not insert a trendline with excel!

Freezing Point Depression Pre-Lab

Name: __________________________________________________________

1. List health and safety hazards associated with naphthalene. The MSDS for naphthalene can be found on the web.

2. How do you dispose of the waste?

3. The melting point of pure biphenyl (C₁₂H₁₀) is found to be 70.00 °C. If 0.900 grams of codeine are added to 10.0 grams of biphenyl, the freezing point drops to 67.60 °C. If K_f for biphenyl is 8.00 °C/molal, what is the molar mass of codeine?

Molar Mass by Freezing Point Depression

Data Sheet

Name:________________________________________________________

Unknown number _________ Freezing point of naphthalene: 81.0 °C

Mass of Naphthalene _________ K_f (naphthalene) = 6.80 °C/molal

Mass of Unknown compound ________

Time

Temp: Mixture Run I

Temp: Mixture Run II

Graph your results (attach both graphs)

Show the work for your calculations.

Molar Mass of Unknown :_____________________________________

Partner’s Names ____________________________________________________