# Determining Molecule Weight by Freezing Point Depression Method

Topics: Freezing-point depression, Melting point, Amount of substance Pages: 4 (975 words) Published: August 4, 2013
BASIC CHEMISTRY (UDBB 1114)
BACHELOR OF SCIENCE (BIOTECHNOLOGY)
SESSION : MAY 2013
DATE : 25 JUNE 2013
LECTERUR :
EXPERIMENT 4
DETERMINING MOLECULE WEIGHT BY FREEZING POINT DEPRESSION METHOD STUDENT NAME :
ID :
LAB PARTNERS|
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INTRODUCTION
According to Anne, n.d, the freezing point of a liquid is decreased by adding to another compound to it. This is known as freezing point depression. The pure solvent will have higher freezing point than the solution. Colligative property of matter can explain more about freezing point depression. Whereas, colligative properties depends on the number of particles present. Based on Hyperphysics, 2001, certain properties of the solute might effect the freezing point depression rather than concentration. By conducting this experiment, we can determine the molar mass of unknown substance. The relationship between the freezing point and concentration of solution is given by:

ΔT = Kf m = Kf x mass of material__________ Molecular Weight x kg solvent Where,
ΔT = freezing point depression
Kf = freezing point molar constant of solvent
m = molality concentration = mass of material / molecular weight x kg solvent
= number of moles of solute / kg solvent
METHODS
Part 1 : Determination of the Freezing Point of Naphthalene
1. A clean and dry test tube was weighted.
2. About 5 g naphthalene was added to the test tube and weighted again. 3. The naphthalene was melted by using a water bath.
4. The test tube with melted naphthalene was transferred from the water bath, dried and clamped it on a retort stand. 5. The liquid naphthalene was stirred continuously so that no temperature gradient develops and the temperature every 15 seconds for a few minutes was recorded and until no more change in the gradient for the temperature versus time graph was observed. 6. A graph of temperature versus...

References: Animations.physics.unsw.edu.au (2010). Freezing point depression and boiling point elevation: the effects of solutes and of pressure. [online] Retrieved from: http://www.animations.physics.unsw.edu.au/jw/freezing-point-depression-boiling-point-elevation.htm [Accessed: 29 Jun 2013].
Hyperphysics.phy-astr.gsu.edu (2001). Freezing Point Depression in Solutions. [online] Retrieved from: http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/meltpt.html [Accessed: 29 Jun 2013].