FREEZING POINT DEPRESSION LAB

Introduction:Theory

In this lab, you will use the freezing point depression of 1.0 m aqueous solutions to determine the solutes identity. Solutes used in this lab will be sucrose, sodium chloride, calcium chloride, and aluminum sulfate. The solutions will be frozen in a 250 mL beaker containing a mixture of ice and table salt. The freezing point depression will be the difference between water’s normal freezing point (0°C) and the solution’s freezing point both determined in the lab (∆T= sfp-nfp). The freezing point depression is related to concentration as ∆T= kf • m• i (where kf is the freezing point depression constant for water, (1.86 °C/m), m is the molality (n solute / kg solvent), and i is the number of moles of particles formed per mole of solute. From lab data we will calculate i and use it to identify the solution.

Prelab

1)Determine the correct formula for each of the above compounds and write a dissociation equation for each.

2)Make a data table to record the brine slurry temperature and the freezing point of water on your thermometer. You will also need space in your table to record the unknown solutions letter, solution freezing point (sfp), freezing point depression (∆T), moles of particles produced per mole of solute (i), and formula (of solute as determined in lab).

Procedure (You may write this in your lab notebook as you perform the lab)

1.In a 250 mL beaker form layers of 1-2 cm of ice followed by about a tsp. of salt until the beaker is full.

2.Stir with a glass stir rod until a slurry (like a slurpy) is formed, and measure and record its temperature to the 10ths place. Wrapping it with newspaper or paper toweling as insulation will aid in maintaining this cold temperature.

3.Fill a small test tube (18x150 mm) about halfway with deionized water.

4.Place a clean thermometer in the water in the test tube, and holding the top of the tube and thermometer, carefully swirl it in your brine slurry until the temperature stops falling. Caution: if the water or solution freezes completely, the tube and thermometer may break.

5.Check the tube for signs of freezing (ice crystals) and record the freezing point to the 10ths place. This may vary slightly from what you know as thermometers vary in the accuracy of their calibration. Using the same thermometer for the rest of this lab will negate this inaccuracy.

6.Repeat this procedure for each of the unknown lettered solutions.

7.Calculate i (show detailed calculations in the calculations section ofyour lab) and use it to identify the solute in each solution.

Data Table Example:

Solution / Normal f.p. (nfp) / Solution f.p. (sfp) / ∆T / i / Identity
H2O / n/a / n/a / n/a / n/a
Slurry / n/a / n/a / n/a / n/a
A / n/a
B / n/a
C / n/a
D / n/a