LAB VISIT 5: CRYOSCOPY

CRYOSCOPY(freezing point)

Objective:

With this experiment our aim is obtaining a graph in which we will show the variation of the melting point of an aqueous solution as a function of its concentration and, also, calculating the molecular mass (mm) of one of the solute from the results that we will obtain.

Background information:

For understanding and developing this experiment we may know that the freezing point of a solution is without exception lower than the one of the pure solvent, with the proportional drop to the molality of the solution:

ATf = Kf * m

Solvent = 5 grams of water.

Materials:

• Distilled water (H20)
• Sugar (C12H22O11)
• Test tubes
• A digital thermometer
• Ice
• Salt (NaCl)
• Balance
• 250 mL beaker

Table:

Grams of sugar	Tf (ºC)	ATf (ºC)	Molality (mol/kg)
0.0	0.6	-----------------------	0.0
0.5	0.3	0.3	0.29
1.0	-1.9	2.5	0.58
1.5	-2.5	3.1	0.87
2.0	-4.5	5.1	1.16
2.5	-6.8	7.4	1.46

Calculations (molecular mass, moles and molality):

Sucrose : C12 H22 O11

Carbon: 12  Hydrogen: 1 Oxygen: 16

Molecular Mass=  12 (12) + 1 (22) + 16 (11) = 144 + 22 + 176 = 342 g/mol

Moles: mass/molecular mass

Molality = moles / kg of solvent

0.5 grams: (0.5 : 342)/ 0.005 = 0.001/ 0.005 = 0.29

1.0 grams: (1.0 : 342)/ 0.005 = 0.002/ 0.005 = 0.58

1.5 grams: (1.5 : 342)/ 0.005 = 0.004/ 0.005 = 0.87

2.0 grams: (2.0 : 342)/ 0.005 = 0.005/ 0.005 = 1.16

2.5 grams: (2.5 : 342)/ 0.005 = 0.007/ 0.005 = 1.46

Graphs:

Conclusion:

As we can see on the table, as the amount of sugar (in grams) increase, the freezing point is colder, so you need a colder temperature to freeze the solution. As we lower the temperature, the molecules have less energy to move. Adding sugar makes it harder to slow the molecules down, so the temperature has to be lower than pure water to freeze. Adding more sugar it takes more energy out. The sugar slow down the molecules and the temperature is lower than water. This way it makes more energy going out. We can see that in the first graph the results of the molality is increasing, when mass and molality increase relatively while in the second graph we can see how mass increase and the freezing point decrease. The second one is inversely proportional. Molality is higher because its related with the sugar, not with the temperature. In the molality column, we can see that as the amount of sugar increases, the molality increases, because in the formula the number of moles changed depending on the amount of sugar, that in this case was decreasing. In the other graph, its inversely proportional because its the relation of the grams of sugar and the freezing point, making it descent because as we first said, the sugar makes the freezing point be lower.