1. Molality Measurement of Concentration of Solutions
(Session 20)
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2. Number of moles in a compound = grams
Molality is represented with the letter m (non Cap) and the solutions are known as Molal.
The Molality is defined as the number of moles of solute dissolved in a kilogram of dissolvent.
m = solute moles
Kg of dissolvent
A solution of 3 m of KOH has 3 moles of KOH dissolved in a kilogram of dissolvent.
Number of moles in a compound = grams
Molecular Weight
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3. Ciencias de la Tierra II
Activity 1
Prepare a watery solution of 1 m of calcium bromide.
1 mol of CaBr2 = 200 g of CaBr2
Furthermore 200 g of CaBr2 are dissolved in exactly 1 kilogram of water.
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4. Ciencias de la Tierra II
Activity 2
Prepare the following solutions:
A solution of sodium hydroxide 0.5 m
A solution of sulfuric acid 1.4 m
A glucose solution 0.8 m
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5. Elevation of the boiling point
Tb -Tb0 = Kb * m
Where:
bf is the mole constant for the elevation of the boiling point of a pure dissolvent.
m is molality.
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6. Decrease in the freezing point
Tf0 -Tf = Kf * m
Where:
Kf is the constant mole of the decrease of the boiling point of a pure dissolvent.
m is molality.
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7. Ciencias de la Tierra II
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 point
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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8. Ciencias de la Tierra II
Problems
Calculate the freezing point and the boiling point of a solution of 100 g of ethylene antifreeze, C2H6O2 in 900 g of H2O.
Calculate the freezing point of a solution that has 600 g of CHCl3 and 42 g of eucalyptol, C10H18O , a fragrant substance that is found in eucalyptus leafs.
Order the following watery solutions in increasing order of their boiling points:
Glycerin 0.03m
KBr 0.02m
Benzoic acid HC7H5O2 0.03m
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9. 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.
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