BSK1412 Physical Chemistry Laboratory
EXPERIMENT 2: DETERMINATION OF THE MOLAR MASS OF A COMPOUND BY FREEZING POINT
DEPRESSION
Aim: To determine the molar mass of an unknown solute by measuring the freezing point depression
of a solution of this solute in a solvent as compared to the freezing point of the pure solvent.
Apparatus Required: Beakers (500mL), test tube (25 x 150mm), thermometer, glass rod and
hotplate.
Chemicals Required: T-Butanol, hot water, ice and unknown material.
Principle: The decrease in freezing point, Δ𝑇𝑓 (freezing point depression) for a near ideal solution
can be expressed by:
Δ𝑇𝑓 =𝑘𝑓·m
where 𝒌𝒇 is the molal freezing point depression constant of the solvent (express in °C kg/mol or °C
/m ). m is the moles of the solute dissolved in the solvent (express in moles of solute/kg solvent).
Molar mass can be determined by:
𝓜 = (𝑘𝑓·g)/( Δ𝑇𝑓·kg solvent)
* 𝒌𝒇 for t-butanol = 9.10 °C kg/mol or 9.10 °C /m
Procedure:
a. Freezing point of pure t-butanol
1. Place a 150 ml beaker on a top loader balance and tare it.
2. Place a dry test tube in the beaker and record the mass.
3. Fill the test tube about half full with t-butanol, put aside after record the mass.
4. Place about 250 ml of hot tap water in one 400 ml beaker.
5. Place the test tube containing the t-butanol in the hot water, about 40 °C. (warm the t-butanol
in the water bath).
6. Stir the t-butanol (up and down motion) with the glass rod (CAUTION: Careful when use the
thermometer).
7. Place about 250-300 ml of ice in another 400 ml beaker and enough cold tap water to just cover
the ice.
8. Once the temperature of the t-butanol has warmed to about 40 °C, transfer the test tube to the
ice water bath (Make sure that most of the t-butanol is below the surface of the ice-water bath.
May add more ice if needed).
9. Immediately begin to take temperature readings and record them in every 15 seconds with
continuous stirring until the t-butanol has solidified.
10. Stop stirring as solid starting to form and continue to record the temperature every 15 seconds
for one more minute. CAUTION: Do not try to pull the thermometer from the frozen t-
butanol to avoid broken.
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BSK1412 Physical Chemistry Laboratory
b. Freezing point of solution:
11. Place the test tube contain with t-butanol in a 150 ml beaker. CAUTION: Test tube may be
heavy because it consists of t-butanol in the previous experiment.
12. Place the beaker with test tube on a balance and tare the balance.
13. 0.5 g of unknown put into the test tube and record the mass.
14. Reheat the sample to about 40 °C and this is sample Solution 1. (Make sure all unknown
material is dissolved in the t-butanol. May use glass rod to do so.)
15. Repeat step 8 to 10.
16. Add another 0.5 g of unknown to the test tube and record the mass. This is sample Solution 2.
17. Repeat step 5 to 10 (melting and temperature recording steps).
Treatment of Data:
1. Mass of test tube:
2. Mass of test tube and t-butanol:
3. Mass of t-butanol:
4. Mass of t-butanol in kg:
5. Mass of first sample of unknown:
6. Mass of second sample of unknown
7. Total mass of unknown in second solution freezing point determination:
Solution 1 (t-butanol Solution 2 (t-butanol
t-butanol pure solvent + first sample portion) + second sample portion)
time temperature time temperature time temperature
0
15
30
...
...
300
