Freezing point depression lab Essay

August 24, 2017 General Studies

Using Freezing-Point Depression to Find Molecular Weight

In this lab. the intent was to utilize the freeze point depression method to find the molecular weight of acetylsalicylic acid. This was done by finding the freeze of t-butanol and that of a t-butanol and aspirin solution ; so happening the molal concentration of the solution. and moles of acetylsalicylic acid. In the consequences of the experiment. the molar mass was found to be 192. 2 g/mol. which differed from the established value of 180. 2 g/mol by 6. 7 % mistake. Introduction:

Freezing point depression defined as a perpendicular medium in mention to boiling point lift. The general definition is the consequence of take downing the stop deading point of a substance due to an increased sum of solute added to the dissolver. This consequence is known as colligative belongings. The equation that shows this relationship is:

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?T= Kf * milliequivalent. 1
The end of this experiment was to find the molar mass of acetylsalicylic acid utilizing the freeze point depression method. First. the freezing temperature of the pure dissolver and t-butanol were determined. Then by adding a known mass of acetylsalicylic acid to a known mass of t-butanol. the lowering of the freezing temperature of the solution was determined. By mensurating the freeze point depression. ?T. and the mass of acetylsalicylic acid. the molar mass of the aspirin solute was determined.

Equations used in this experiment:
The different in stop deading temperatures between t-butanol and mixture of t-butanol and acetylsalicylic acid:
?T= T1- T2Eq. 2
Where T1: temperature of t-butanol
T2: temperature of t-butanol and acetylsalicylic acid

The molar mass of acetylsalicylic acid:
Moles= mass/ molar massEq. 3
Table 1. Mass of all the measurings ( g )

Trial 1Trial 2
Mass of trial tubing ( g )
18. 3218. 32
Mass of trial tube+ t-butanol ( g )
20. 54820. 409
Mass of aspirin+ solvent+ trial tubing ( g ) 20. 504 20. 670


Table 2. Freezing temperature of t-butanol and aspirin-butanol solution ( °C )

Trial 1Trial 2
Freezing point of pure-butanol ( °C ) 17. 549. 850
Freezing point of pure-butanol-aspirin solution ( °C )
8. 7875. 829

The experimental molar mass of acetylsalicylic acid for test 1: 748 g/mol The experimental molar mass of acetylsalicylic acid for test 2: 192. 2 g/mol

% Error for test 1: ( 748- 180. 2 ) /180. 2 * 100 % = 315 %

% Error for test 2: ( 192. 2- 180. 2 ) /180. 2 *100 % = 6. 7 %


The add-on of aspirin solute to the t-butanol solution caused the vapor force per unit area to diminish. This lessening in vapor force per unit area and addition in solute nowadays caused the freeze point to became lower when assorted thin the solution. The intent of this lab was to find the molecular mass of acetylsalicylic acid by mensurating the freeze point depression. This was done by responding t-butanol with acetylsalicylic acid as a solute. A pure dissolver of t-butanol’s freeze point was foremost determined by runing and stop deading it and so the solute was added and the freezing point dropped. This difference was the freezing point depression of the solute. From this freezing point alteration. the molecular mass of the solute was calculated. The first test with 0. 200 g of the solute produced a molar mass of 748 g/mol and the 2nd test with 0. 195 g of the solute produced a molar mass of 192. 2 g/mol.

The molar mass of the first test was manner off from the recognized value. while the molar mass of the 2nd test was rather close to the accepted value. These consequences were confirmed when a per centum mistake of 315 % in for the first test and 6. 7 % in the 2nd test. Mistakes that contributed to the per centum mistake could include drosss in the acetylsalicylic acid and t-butanol mixture. blending the two in the chief beaker normally used to heat the solutions. and the coexistence of solid and liquid signifiers of the solutions. Adding aspirin to t-butanol in the aspirin-butanol mixture. the kinetic energy of the solution was inhibited evident of a lower vapour force per unit area and diminishing freezing/melting point. One other mistake could be that non all of the acetylsalicylic acid was transferred from the burdening boat into the trial tubing or holding the aspirin stuck on the walls of the trial tubing.


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