Molar Mass Determoned by Freezing Point Depression

Molar Mass Determoned by Freezing Point Depression

CHE-115-02 General Chemistry Lab II

Introduction

Colligative properties are properties of solutions that depend on the number of particles in a volume of solvent and not on the mass of the particles (Helmenstine, n.d.). It does not matter if how many particles in the solution, this property is based on what kind of chemical particle it is. One of the colligative properties is Freezing point pepression. Freezing point depression the phenomenon that occurs when the freezing point of a liquid (a solvent) is lowered by adding another compound to it, such that the solution has a lower freezing point than the pure solvent(Helmenstine, n.d.). There is another concept that is similar to freezing point depression is supercooling. Supercooling is often confused with freezing-point depression. Supercooling is the cooling of a liquid below its freezing point without it becoming solid. Freezing point depression is when a solution can be cooled below the freezing point of the corresponding pure liquid due to the presence of the solute; an example of this is the freezing point depression that occurs when sodium chloride is added to pure water (TechBlog, 2012).

When a solute is added to a solvent, the consequence is the freezing point of the solution depresses from the original freezing point of the solution. This is also shown as in the bellowing figure:

(source: Sparnotes, n.d.)

This experiment is to explore how freezing point depression system works by the equation:

                                                            TFP= -kf m

to determine the molar mass of an unknown substance out of the other two molar mass determination methods, boiling point elevation and osmotic pressure.

The solute plays a role like salt used in the winter to melt snow on roads. When salt is added to an icy road, it dissolves into water above the ice and keeps it from freezing even though the temperature of the road is lower than 0 degrees Celsius -- the freezing point of water. This happens because solvent molecules must order themselves into a lattice when freezing, and dissolved particles interfere with the process. In addition, heat is released when the salt dissolves, which melts some of the ice below, which then allows more salt to dissolve in a kind of chain reaction (Mullis, Media, n.d.).

Procedure

Part I Freezing Point of Lauric Acid

The LoggerPro is started and a water bath is set up with about 500ml of water in an 800 beaker by a heat plate, the water is heated to above 60℃. About 9g lauric acid is added to a dry Erlenmeyer flask which is weighted at the beginning, and the whole thing is weighed again. Then the flask is placed in the water bath to be heated till the lauric acid is melted. The flask is removed from the water, and the data collection is stared after lauric acid is cooled below 60℃. The data collection is stopped when the temperature stays constant for more than 2 minutes.

Part II Freezing Point of the Solution of Unknown in Lauric Acid

About 0.9g of unknown is weighted and added to the lauric acid. The flask is warmed up till the solution is melted. The data from last run is saved and another run of data collection is started for the new solution, and stopped when the data stays constant for 2 minutes.

Data

Part I

Mass of 125ml flask

97.95g

Mass of flask and lauric acid

106.805g

Constant temperature for Part I

43.19℃

Part II

Mass of unknown

0.917g

Constant temperature

41.31℃

Calculation

Molality

m===0.482mol/Kg

Moles unknown

Moles=mmass of solution=0.482mol/Kg(0.917g+106.805g-97.95g)Kg/g

                                                                         =0.004710mol

Molecular weight

Molar Mass===194.692g/mol

Result

Method of Freezing Point Determination: From the graph in lab the freezing point of lauric acid is 43.67 and the freezing point of the solution is 41.84.

Result of calculated Molar Mass of the unknown substance is 194.692g/mol

Discussion

There might be a slightly difference between the experimental determined molar mass and expected known molar mass. Since the expected know mass is not recorded, the percentage error cannot be calculated. The error might occur when measuring the data that when stirring the lauric acid and solution using the probe. Also, in the second run there might be some lauric acid stick on the wall of the flask and change the molality of the solution. Besides, the constant temperature measured on the graph is the freezing point of lauric acid and freezing point of the solution which are 43.19 and 41.31. However, after the linear fit, it shows a different constant since the temperature has a fluctuate when lauric acid and the solution are being measured. When next time this experiment is performed, the liquid should be stirred more gently and avoid letting liquid sticking on the wall of the flask.

Conclusion

This experiment is to determine the molar mass by the method of freeing point depression. From the differences between expected known molar mass and experimental molar mass, more accurate way to do the experiment is learned by analyzing where the error might occur.

Reference:

  1. Helmenstine, Anne M. “Colligative Properties Definition”. http://chemistry.about.com/od/chemistryglossary/a/colligativeprop.htm
  2. Helmenstine, Anne M. “Freezing Point Depression Definition”. http://chemistry.about.com/od/chemistryglossary/a/freezingpointde.htm
  3. “5 Awesome Examples of Supercooling Captured on Video”. http://www.techeblog.com/index.php/tech-gadget/5-awesome-examples-of-supercooling-captured-on-video
  4. “COLLIGATIVE PROPERTIES OF SOLUTIONS”. http://www.sparknotes.com/chemistry/solutions/colligative/section1.rhtml
  5. Mullis,Robert and Media, Demand. “Examples of Colligative Property”. http://everydaylife.globalpost.com/examples-colligative-property-37134.html
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