paraphrasing chemistry report

Abstract

In this lab the experimental 
molar mass of biphenyl was determined by calculating the molality 
of cyclohexane solvent. The freezing point depression method is used to find the freezing temperature of the pure solvent. The final molar mass of biphenyl is 148.6 and the percent error was calculated to be 3.6%.

Introduction

The purpose of this lab was to use the freezing point depression method to determine the molar mass of biphenyl. Dissolving a solute in a solvent can cause the freezing temperature to decrease according to the number of moles in the solute. Therefore, this property depends on the ratio of solute to solvent in a mixture. It is called the freezing-point depression, which is a colligative property. The following equation describes the property.

is the change of temperature in the freezing point depression of the solution. The temperature constant is for t- cyclohexane which is the solvent that used in this experiment. The molality is the ratio of the number of moles in the solute over the solvent in kg. The freezing temperature of pure cyclohexane was determined by placing 3mL of the substance in a test-tube that Then it was placed in 250 mL beaker that filled with ice and water. A temperature probe was in the test-tube to monitor the temperature change, which is then plotted. In order to calculate the molar mass of biphenyl, approximately 0.1g were measured and added to the cyclohexane. Likewise, the solution was then placed in the ice and water and data was collected for about 10 minutes. This process gave the freezing temperature of the solution. This was indicated in the graph due the gradual linear decrease of the temperature. Using the freezing temperature of pure cyclohexane and the solution the change of temperature can be calculated. Therefore, the molality of cyclohexane can be calculated with the equation below:

The weight of the solvent can be calculated by subtracting the mass of the solution, the mass of the test-tube and the mass of the biphenyl. By manipulating the equation above the number of moles of biphenyl can be determined. Finally to find the molar mass of biphenyl the following equation was used.

The relationship above gave the experimental molar mass of biphenyl. The value calculated was then compared to the actual value, which is 154.2

Discussion

The molar mass of biphenyl was determined to be 148.6(table 3). The average freezing point of pure cyclohexane was found 7.5 C shown on figure 1and 2. The graph illustrates the freezing point since the temperature becomes steady at approximately 7.5 . Moreover, table 1 shows the mass measurements used to conduct the experiment. The entire mass of the solution was important to measure in order to find the mass of the cyclohexane. Since, the solution consists of biphenyl and cyclohexane, the mass of cyclohexane was determined to be 2.25 g. The number of moles biphenyl was calculated to be moles that was then divided by the actual mass used. The accepted molar mass of biphenyl is 154.2 and the experimental value is 148.6 which result in 3.6% error.

The percent of error is reasonable and acceptable since several sources of error may have occurred. When transferring the biphenyl to the test-tube some particles of biphenyl stayed on the weighing boat. This will slightly alter the value of the molar mass. Moreover, stirring the solution with the copper stirrer can change the freezing point depression, since the temperature will constantly increase and decrease.

When more biphenyl was added to the solution, the freezing point depression decrease. This shows as the solute increases the freezing temperature decrease. Therefore, illustrates the colligative property, which depends on the number of molecules present not the type of molecules used in the solution.

Losing some of the solid biphenyl during transfer to the test tube will result in effect the measured molecular weight because it is calculated by dividing mass by number of moles. The mass of biphenyl used to calculate the molecular weight will be less than the mass that actually used in the equation because it was lost during the transfer, which mean the molecular weight will be less.