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
WhereTf = 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
electronic balance
insulated cup
thermometer
weigh boat
ice
sodium chloride (salt)
water
Procedure:
1) In a 250 mL beaker, prepare a mixture of ice and water that totals 150 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 / Total mass of NaCl dissolved(g) / Temperature of ice-water-salt mixture
(°C) / Tf of ice-water mixture
(°C) / Molality (m) of each solution (mol/kg) / kf of water
(°C/m)
T1
(~ 0 g solute) / 0.00 grams / 0.4 °C
T2
(~6 g solute) / 6.02 grams / -3.7 °C
T3
(~12 g solute) / 12.03 grams / -7.9 °C
T4
(~18 g solute) / 18.01 grams / -14.6 °C
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:
Purpose of labDescribe major steps in the method as a means of justifying your conclusion
(What did you do/measure and why?
Conclusion—re-state result and percent error (either show how percent error was calculated here, or show work in calculations section)
Comment on the accuracy and precision of your data.
Limitation/source of error / How does this error affects your result (makes it too high/too low/random) and why? / Possible solution or way to “fix” or minimize this error (must be SPECIFIC and REALISTIC)