EXPERIMENT 7

1
EXPERIMENT 7

• COLLIGATIVE PROPERTIES FREEZING POINT DEPRESSION

2
PURPOSE

• To determine the effect of an unknown solute on
  the freezing point of cyclohexane.
• To calculate the molar mass of the unknown solute.

3
Place about 20 g of rock salt (like the amount
shown in the picture) in a tall 400 mL beaker.
4
Add about 50 mL of distilled water.
5
Stir and dissolve most of the salt.
6
Fill the beaker with ice and stir again.
7
and clamp it into the salt bath.
Place the stopper
and the insulation into the culture tube
8
Place a mark on the inner test tube at about 7.5
cm height.
9
Put the test tube in a 250 mL Erlenmeyer flask
and weigh it.
10
Take the test tube to the fumehood and pour
cyclohexane into the test tube to the mark.
11
Weigh the flask plus the test tube with
cyclohexane.
12
Place the test tube in a 400 mL beaker with ice
water and let it stand until some of the
cyclohexane freezes.
13
Remove the test tube from the ice water and warm
it by holding it in your hand until the solid
melts.
14
Insert the thermometer. When the temperature
reaches about 10 C, place the test tube with
thermometer into the culture tube.
15
Stir occasionally the ice water with a stirring
rod.
Stir the solution CONSTANTLY and record the time
and temperature of the cyclohexane at 30-second
intervals.
Watch for the formation of crystals and mark your
data at the point where they first form.
Continue taking data until the temperature
remains almost constant for 2 to 3 minutes.
Remove the test tube and warm it with your hand
until it reaches about 10 C.
Return it to the culture tube and repeat the
procedure, recording time and temperature for 2
minutes past the freezing point.
16
DO NOT weigh all three samples out at once!
You will require three samples of unknown solute,
each weighing about 0.10 g (weigh out between
0.08 and 0.12 g to 0.001 g).
17
Use your hands to warm the test tube containing
the cyclohexane.
18
Transfer carefully all the sample to the
cyclohexane.
19
Remove the test tube from the culture tube, warm
it with your hands add the second sample of the
unknown. Repeat the freezing point procedure.
Stir to dissolve the solute and warm or cool the
cyclohexane until the temperature is about 8-9 C.
Remove the test tube from the culture tube, warm
it and add the third portion and repeat the
procedure.
Repeat the freezing point procedure.
20
ATTENTION Do not pour the cyclohexane solution
down the sink. Dispose of it in the appropriate
container in fumehood 1.