Lab: Freezing Pt

Lab: Freezing Point Depression Introduction: Colligative properties depend on the number of particles present in a solution. Freezing point depression is one of the colligative properties of solutions discussed in this unit. Because ionic solutes dissociate into ions, they have a greater effect on the freezing point and boiling points than molecular (covalently bonded) solids of the same molal concentration. For example, the freezing point of water is lowered by 1.86° C with the addition of any nonvolatile molecular solute at a concentration of 1 molal. However, a 1 molal sodium chloride solution contains a 2 molal concentration of ions. Thus, the freezing-point depression for a 1.0 m solution of NaCl is 3.72° C, double that of a molecular solute. The relationship is given by the following equation:

Tf = Kf ● m ● n

Where Tf = the freezing point depression of the solution;

Kf = the molal freezing-point constant; m = the molality of the solution; n = the # of particles formed from the dissociation of each formula unit.

Hint: In this lab we will dissolve the ionic solid, NaCl, which has 2 particles (ions) per formula unit. Therefore, n=2 for this lab.

Purpose: Experimentally determine the molal freezing-point depression constant for aqueous solutions of sodium chloride (ionic).

Equipment & Materials: Beaker, 250 mL thermometer sodium chloride (salt) electronic balance weigh boat water insulated cup ice

Procedure: 1) In a 250 mL beaker, prepare a mixture of ice and water that totals 100 mL. Pour the ice water into an insulated cup. Measure the temperature at which equilibrium is reached between the ice and water

(i.e. when the temperature no longer is dropping). Record the temperature as T1 in table 1.

2) Add 6 grams of NaCl to the mixture. If you did not get exactly 6.00 g, make sure to record the exact amount you measured out and base your calculations off of that number (i.e. 6.02 g is acceptable). Carefully stir the solution. Measure the temperature at which equilibrium is reached.

(When the temp. is no longer dropping) Record the temperature as T2.

3) Add 6 more grams of NaCl to the mixture. Carefully stir the solution. Measure the temperature at which equilibrium is reached. (When the temp. is no longer dropping) Record the temperature as T3.

4) Add 6 more grams of NaCl to the mixture. Carefully stir the solution. Measure the temperature at which equilibrium is reached (when the temp. is no longer dropping). Record the temperature as T4. Data Table 1: Freezing Point Depression Data

Solution: Temperature (°C)of Tf of ice-water molality of each kf of water ice-water-salt mixture mixture (°C) solution (mol/kg) (°C/m)

T1 (0g solute)

T2 (6 g solute)

T3 (12 g solute)

T4 (18 g solute)

Calculations:

1) Determine the freezing-point depression, Tf , for each solution. This is the difference between the freezing/boiling point of the solution and the freezing/boiling point temperature of the pure solvent, water. Record in data table.

2) Calculate the molality (mol/kg) of each solution. Record in data table.

3) From the freezing point depression and the molality of each solution, calculate the molal

freezing-point depression constant, Kf, for each solution. (REMINDER: for the salt solution, the molality refers to the mol of individual solute particles. Sodium chloride dissociates in water to form 2 ions for every NaCl particle.)

4) Find the average value for the molal freezing point depression constant for water (kf).

5) Find your percentage error for each value obtained in #4 (average Kf), • the freezing-point depression constant of water is 1.86 °C/m

% Error = l (accepted value – experimental value) l x 100 l accepted value l

Conclusions & Post Lab Questions:

1) Summarize your results and your % error.

2) List three possible sources of error in this experiment.

3) Why was it necessary to measure the temperature of the ice and pure water mixture, instead of assuming it to be 0.0° C?

4) Why is salt sometimes used as a deicer on roads in cold areas?

5) Antifreeze containing ethylene glycol is also needed in automobiles in extremely hot climates. Using what you know about other colligative properties, explain why this is so.

6) Which of the following solutes would be most effective at lowering the freezing point of water? Explain your reasoning. a) sucrose b) sodium chloride c) calcium chloride d) aluminum chloride