202-NYB-05 (Group 06.)
By: Jordan Hribar
Partner: Anthony Cuillierier
Teacher: Daniel Baril
Experiment Done: February 23rd, 2011
Experiment Number 2.
Some of the properties unique to solutions depend only on the number of dissolved particles and not their identity. Such properties are called colligative properties. The colligative property that will be examined in this experiment will be the freezing point depression as an example of a colligative property. Every liquid has a freezing point: the temperature at which a liquid undergoes a phase change from liquid to solid. When solutes are added to a liquid, forming a solution, the solute molecules disrupt the formation of crystals of the solvent. That disruption in the freezing process results in a depression of the freezing point for the solution relative to the pure solvent. The amount of the freezing point depression can be calculated using Raoult’s Law:
Note that the sign of the change in freezing point is negative because the freezing point of the solution is less than that of the pure solvent. Molality must be used to measure the concentration of the solute because it is temperature independent:
Whereby, sssssss = freezing temperature of pure Cyclohexane. f freezing temperature of the napthalene-cyclohexane solution. D * Note the possible presence of Van't Hoff’s factor: , in your freezing point calculations.
In this experiment, the freezing points of three substances will be measured before calculations: 1. Pure Cyclohexane, C6H12 g 2. A solution composed of cyclohexane and naphthalene, C10H8 and d 3. A solution of an unknown solid dissolved in cyclohexane The freezing point of the third solution will be used to find the molar mass of the unknown solid.
Refer to Laboratory manual page 10 to 12.
One modification to the procedure is that part B and C is combined into one laboratory experiment to be performed consecutively instead of at separate time frames.
= (0.3402g /128.18 g/mol) C10H8k [ 1 ]A (18.7828g / 1000) C6H12
= 0.1413 mol . kg-1 naphthalene/cyclohexane solution
[ 2 ]A
D = cyclohexane - cyclohexane + naphthalene
Kf = (6.3320 °C – 2.9180 °C) Kf = 24.1613 kg . °C . mol -1 cyclohexane 0.1413 mol . kg-1
[ 3 ]A
= | 24.1613 – 20.2000 | x 100 = 19.6104 % error 20.2000
m = (6.3320 °C – 4.3400 °C) [ 1 ]B f 24.1613 kg . °C . mol -1 m = 0.0824 mol . kg-1 unknown/cyclohexane solution