|
Chillin' Out with Freezing Point Depression
The Science of Making Ice Cream
data
| hook
| main
| background & resources
| student
Background
The freezing point of water depends on the number of ions disssolved in the liquid. When ions such as Na+ and Cl- are added, the vapor pressure of the water is lowered. Freezing occurs when the vapor pressure of water is equal to the vapor pressure of the surrounding air. If the vapor pressure of water is lowered, the freezing point is lowered.
It is possible to predict the freezing point depression in degrees C using the following formula:
T = Kf(m)(i)
Where Kf is the molal frezing point depression constant for NaCl.
Kf= 1.86 deg C/mol
m= molality (moles/Kg)
i= number of parcticles per formula unit (2 for Na+ and Cl-)
Rearranging this equation, one can calculate the theoretical molality of a solution given a measured freezing point depression.
Theoretical molality (m) = T/Kf(2)
The actual molality of the solution can be measured experimentally by measuring the mass of NaCl in a dehydrated sample of the solution.
Percent error between the experimental and theoretical values can then be calculated using the formula:
%Error=(Experimental value- Theoretical value)/Theoretical valuex100%
Resources and Reference Materials
Imogene Bennet
Chapparal High School
NAU Chemistry Teachers Workshop
"It's Getting Colder(Freezing Point Depression)"
J. MacQuade, G. Monks, D. Rickard, D. Walsh, I,Wilkins.
Woodrow Wilson Leadership Program in Chemistry
www.woodrow.org
Return to top of page
Back to: TEA Activities Page
data
| hook
| main
| background & resources
| student
|
