1. Ciencias de la Tierra II
Application of the colligative properties Determination of Molecular Masses
(Session 22)
Ciencias de la Tierra II

2. Colligative Properties
With the colligative properties of the solutions we can determine the molar masses or molecular of solutes dissolved in them.
This is an important application when we want to identify a unknown solute because from the molecular mass and percentage composition can determine the chemical formula of a solute.
Ciencias de la Tierra II

3. Ciencias de la Tierra II
The colligative properties decrease od the freezing point and elevation of the boiling point are used to determine the molar or molecular masses with the methods know as:
Cryoscopy: decrease of the freezing point
Ebullioscopy: increase of the boiling point
Ciencias de la Tierra II

4. Decrease of the freezing point: Cryoscopy
Δ Tc = Kc * m
Where:
Δ Tc= Kc * solute grams /PM
solvent kilograms
PM = Kc * solute grams
Δ Tc * solvent grams
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5. Elevation of the boiling point: Ebullioscopy
Δ Te = Ke * m
Where:
Δ Te= Ke * solute grams/PM
solvent kilograms
PM = Ke * solute grams
Δ Te * solvent kilograms
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6. Ciencias de la Tierra II
Problem #1
When in 150 g of water g of fructose (sugar of fruits) are dissolved, we can observe in the resulting solution has a freezing point of– C. What is the molecular mass of the fructose?
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7. Ciencias de la Tierra II
Problem #2
A solution that contains 22.0 g of ascorbic acid (vitamin C) in 100 g of water freezes at C. Which is molecular mass of ascorbic acid?
Ciencias de la Tierra II

8. Ciencias de la Tierra II
Problem #3
A solution has 22.0 g ascorbic acid (vitamin C) in 100 g of water it freezes at C. What is the molecular mass of ascorbic acid?
Ciencias de la Tierra II

9. Molal constants of elevation of boiling point (Ke) and decrease of the freezing point (Kc)
Dissolvent
Normal boiling point (°C)
Ke (°C/m)
Normal freezing poin (°C)
Kc (°C/m)
Water
100.00
0.52
0.00
1.86
Benzene
80.10
2.53
5.50
5.12
carbon tetrachloride
76.80
5.02
-22.30
29.80
Ethanol
78.40
1.22
1.99
Chloroform
61.20
3.63
-63.50
4.68
Brown, T. et al. (1991)
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10. Ciencias de la Tierra II
Bibliography
Brown, T. et al. (1991) Chemistry: The Central Science. 5th ed. United States of America: Prentice Hall.
Burns, R. (1996) Chemistry Fundaments. 2nd. ed. United States of America: Prentice Hall.
Chang, R. (1992). Chemistry. 4th ed. United States of America: McGraw Hill.
Ciencias de la Tierra II