Introduction

We know from experience that adding salt to ice makes the ice thaw. This is the reason why we spread salt onto icy roads. The reason for the thawing is an example of a colligative property. Freezing occurs as solvent molecules cluster together to form solid particles that grow until all of the liquid is converted to solid. Solute molecules or ions disrupt the normal freezing process, altering the equilibrium and causing the solvent to freeze at a lower temperature. In this experiment we will use water as the solvent and isopropyl alcohol (also called propan-2-ol or isopropanol with the formula CH₃CH₂(OH)CH₃) as the solute. We can find the temperature of freezing by monitoring the cooling curve. As a solution cools its temperature will decrease until it reaches a temperature where freezing can occur. At this point energy is removed from the substance in order to convert it from a liquid to a solid but its temperature will remain constant. The change in freezing point, deltaTf is the freezing point temperature of the solvent with solute added minus the freezing point temperature of the pure solvent and is known to be directly proportional to the molality, m.

Where k_f is a constant that depends on the solvent (for water it has a value of –1.86ºC/m)

Molality, m, is defined as:

The consequence of these two equations is that if the weight of solvent is known and the change in temperature is measured, it should be possible to calculate the moles of solute (in our case isopropyl alcohol). If we know the weight of solute that was added then we can calculate the weight for one mole of isopropyl alcohol, which is the same thing as the molar mass (usually expressed as g/mol). Whether this works well as a method of determining molecular weights is something that you will have to decide based on your results.
 
 

Prelab Assignment

In your laboratory notebook answer the following questions;

1. Would you expect deltaTf to be a positive or negative number?
2. Use an appropriate source to find the density and molar mass of isopropyl alcohol.
3. In a similar experiment, 40.00g of water is found to freeze at 0.120?C and a mixture of 40.00g water and 5.00g of an unknown liquid (non-electrolyte) freezes at –4.36?C. What is the molar mass of the unknown liquid?

The apparatus that we will use will be similar to the one shown in the photograph above. You will need a large test tube, a two-holed stopper that fits the tube, a means of measuring the temperature (CBL with temperature probe) and a length of wire to stir the solution. Bend the end of the wire in a circle to create a stirrer and run the other end through one of the holes in the stopper. Push the temperature probe through the other hole in the stopper. Insert the stopper into the test tube so that the probe reaches to nearly the bottom of the tube. In a 600ml beaker add approximately equal amounts of ice and rock salt with a little water. While this is cooling set up a CBL with temperature probe attached to channel 1. Attach a calculator to the CBL. On the calculator select prgrm, 1-Chembio, enter, enter, 1-set up probes, number probes = 1, enter, 1-temp, channel number = 1, enter, 2 collect data, 2 time graph, time between samples = 30, number samples = 60, enter, 1: use time setup, Ymin=-15, Ymax=10, Yscl=1. Do not hit enter again until you are ready to start the experiment. Using a regular thermometer, measure the temperature of the ice bath. Try to get the temperature as close as possible to -10ºC (at least -6ºC). You may need to pour out some water and add extra ice and salt a few times. When the bath is cold enough, place 30ml of distilled water into the test tube. Measure the temperature of the water before placing it in the ice and find the density of water at this temperature. Add your designated amount of isopropyl alcohol into the test tube, attach the stopper with the temp probe and stirrer and place into the ice bath. When you are ready press enter on the calculator to start taking measurements. Ensure that the liquid inside the tube is completely immersed in the ice portion of the ice bath (note: the salt at the bottom will be warmer). A graph should appear on the calculator. Write down the temperature from the CBL every 30 seconds into your lab book. Record the temperature after it has not changed for 5 successive readings and/or when ice starts to form in the test tube (you may need to move the test tube to the side of the beaker it see the ice). Note that the temperature may dip below the freezing point temperature for a short time, this is known as supercooling. Repeat the experiment using different volumes of solute mixed with 30mL of water and finally with 30 ml of distilled water only. Calculate the temperature change, deltaTf, for each run and enter your data into a computer spreadsheet in the lab.

Assignment

The assignment for this experiment is to perform the calculations necessary to find the molar mass for isopropyl alcohol. To do this, you will first have to calculate the molality based on your temperature change, then use the density of water in order to convert the volume used into kilograms, Kg. The number of moles of isopropyl alcohol can then be found from the molality. Using the density of isopropyl alcohol that you found in the prelab assignment, you can then calculate the molar mass in g/mol. Your calculated value for molar mass will be compared to the class average in the recitation period. Submit all of your calculations on a printed page, compare your value with the accepted value and suggest reasons for any discrepancy.