Freezing Point Depression and Molality for Solutions

A brief introduction to freezing
point depression
THE COLLIGATIVE
PROPERTIES OF
MOLALITY AND
FREEZING POINT
DEPRESSION
grownextgen.org
What is molality?
A quantitative method to measure solution concentration.
http://www.swiftutors.com/admin/photos/molality-formula.png
Moles of solute:  grams of
solute/molar mass of solute
Kilograms of solvent for water
(which is what we will use):
milliliters of water = grams of
water/1000
We can use this to calculate our salt
water concentration.
Let’s calculate the moles of NaCl used to make
ice cream.
Using 60.00 g of NaCl
Molar Mass of NaCl = 58.5 g/mol
1 sodium atom weights 23.0 g/mol and 1 chlorine atom weighs 35.5 g/mol as
listed on the periodic table
60.00 g NaCl/58.5 g/mol  =
                               1.03 moles of NaCl
Let’s calculate the mass of the solvent
(ice/water)
Using 960.00 g of ice
960 g ice = .960 kg ice
Now let’s calculate the molality…
                  Molality = 1.03 mol/.960 kg  = 1.07 mol/kg or 1.07 m
What is freezing point depression?
Freezing point depression (     T
f
 
)
:  change in the freezing point of a
solution
It is how far the freezing point of a solution will lower (depress) due
to the addition of solute particles
http://calculator.swiftutors.com/freezing-point-depression-calculator.html
Where:  m = molality of the solution (calculated on previous
slide)
K
f
 = freezing point constant for the solvent
 
for water = 1.86
o
C/m
i = number of particles the solvent will dissociate
 
-for ionic compounds, it is equal to the number of
ions
 
-for covalent compounds it is equal to ONE
For our salt solution?
 
http://calculator.swiftutors.com/freezing-point-depression-calculator.html
NaCl is ionic and will dissociate into 2 ions (Na
+
 and Cl
 -
)
 
so i = 2
K
f
 = freezing point constant for the solvent
 
for water = 1.86
o
C/m
m = 1.07 m
     T
f
 
 = 2  x 
1.86
o
C/m  x 1.07 m  =  3.98 
o
C
Which means that by adding 60 g of salt to the 960 g of ice,
the temperature of the solution now should be depressed
3.98 
o
C  below normal freezing point (which is 0
o
C)
                    …new solution may chill down to – 3.98
o
C
So what does that mean for us?
Ice cream…we can depress the freezing point down to freeze the
milk/sugar solution in the bag and enjoy
A few final words…and looking further
Freezing point depression is also used to determine brine solutions,
rock salt, quick melt (CaCl
2
) on roads/walk in winter to “melt” the
ice
It really doesn’t melt the ice, but rather depresses the freezing point of the
water (ice)
Reason why these solutes don’t work in very cold temperatures
Won’t depress the freezing point that far down
Extensions:
Could investigate effectiveness of various ionic compounds for melting ice
Could investigate environmental impacts of various de-icers
Could investigate other bioproducts to act as deicers
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Introduction to molality and freezing point depression in solutions. Molality is a way to measure solution concentration, calculated using moles of solute and kilograms of solvent. By calculating the moles of NaCl in a salt solution and the mass of the solvent (ice/water), the molality can be determined. Freezing point depression, the change in freezing point due to adding solute particles, is calculated based on molality and the freezing point constant of the solvent. For a salt solution, the new freezing point can be determined by considering the dissociation of ions.

  • Molality
  • Freezing Point Depression
  • Solutions
  • Concentration
  • Chemistry

Uploaded on Sep 25, 2024 | 1 Views


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  1. THE COLLIGATIVE PROPERTIES OF MOLALITY AND FREEZING POINT DEPRESSION A brief introduction to freezing point depression grownextgen.org

  2. What is molality? A quantitative method to measure solution concentration. http://www.swiftutors.com/admin/photos/molality-formula.png Moles of solute: grams of solute/molar mass of solute Kilograms of solvent for water (which is what we will use): milliliters of water = grams of water/1000 We can use this to calculate our salt water concentration.

  3. Lets calculate the moles of NaCl used to make ice cream. Using 60.00 g of NaCl Molar Mass of NaCl = 58.5 g/mol 1 sodium atom weights 23.0 g/mol and 1 chlorine atom weighs 35.5 g/mol as listed on the periodic table 60.00 g NaCl/58.5 g/mol = 1.03 moles of NaCl

  4. Lets calculate the mass of the solvent (ice/water) Using 960.00 g of ice 960 g ice = .960 kg ice

  5. Now lets calculate the molality Molality = 1.03 mol/.960 kg = 1.07 mol/kg or 1.07 m

  6. What is freezing point depression? Freezing point depression ( Tf): change in the freezing point of a solution It is how far the freezing point of a solution will lower (depress) due to the addition of solute particles http://calculator.swiftutors.com/freezing-point-depression-calculator.html Where: m = molality of the solution (calculated on previous slide) Kf = freezing point constant for the solvent for water = 1.86oC/m i = number of particles the solvent will dissociate -for ionic compounds, it is equal to the number of ions -for covalent compounds it is equal to ONE

  7. For our salt solution? http://calculator.swiftutors.com/freezing-point-depression-calculator.html NaCl is ionic and will dissociate into 2 ions (Na+ and Cl -) so i = 2 Kf = freezing point constant for the solvent for water = 1.86oC/m m = 1.07 m Tf= 2 x 1.86oC/m x 1.07 m = 3.98 oC Which means that by adding 60 g of salt to the 960 g of ice, the temperature of the solution now should be depressed 3.98 oC below normal freezing point (which is 0oC) new solution may chill down to 3.98oC

  8. So what does that mean for us? Ice cream we can depress the freezing point down to freeze the milk/sugar solution in the bag and enjoy

  9. A few final wordsand looking further Freezing point depression is also used to determine brine solutions, rock salt, quick melt (CaCl2) on roads/walk in winter to melt the ice It really doesn t melt the ice, but rather depresses the freezing point of the water (ice) Reason why these solutes don t work in very cold temperatures Won t depress the freezing point that far down Extensions: Could investigate effectiveness of various ionic compounds for melting ice Could investigate environmental impacts of various de-icers Could investigate other bioproducts to act as deicers

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