Freezing Point Depression Lab Report

ABSTRACT The purpose of this lab was to determine the molar mass of biphenyl by using the freezing point depression method. This was done by taking biphenyl as the solute and dissolving it in a pure solvent, cyclohexane, to measure the freezing point depression. The average molar mass was 143.25g/mol and the average percent error was 5.79%. INTRODUCTION In this lab, the task is to find the molar mass of the solute between cyclohexane and biphenyl. In this lab, we noticed how when the solute was dissolved into the solvent, the freezing temperature of the solution is lowered in proportion to the number of moles of the solute that were added. The freezing point depression is a colligative property that depends on the ration between the solute and the solvent particles and not on the nature of the substance. This equation is depicted as: Delta T=Kf*m. Delta T is the freezing point depression, Kf is the freezing point depression constant (in this case it’s 20.2 C/m), and m is the molality of the solution. In this lab the first thing we need to do it determine the delta T. This can be done by using the following equation: Delta T= T1(pure liquid) - T2 (mixed liquid). When you gather that information you can begin to solve for the molality of the solution using Delta t= Kf*m, but rearrange so that you solve for molality instead (m=Delta T/Kf). When molality is found, the number of moles can be found by taking the number of grams and converting to kilograms so that the units in molality (mol/kg) cancel out leaving moles of biphenyl. The last thing to do after this is to calculate mass by subtracting the grams of biphenyl and cyclohexane and multiplying by the number of moles to get the molar mass (g/mol). DATA Below Table: The masses and freezing points of cyclohexane and cyclohexane biphenyl solution for trials one and two. (Table 3) Mass of pure cyclohexane (g) Freezing point of pure cyclohexane (⁰C) Mass of cyclohexane-bip henyl solution (g) Freezing point of cyclohexane-bip henyl solution (⁰C) 2.294 2.379 6.61 7.08 2.356 2.441 3.42 2.98 Trial 1 2 Below Graph: Graph Displaying the Time (mins) Vs. Temperature (C) in Order to Find the Molecular Weight (g/mol). Trial 1 w/o red Trial 1 w/ blue Trial 2 w/o green Trial 2 w/ orange RESULTS Trial 1 Step 1- Finding Delta T (freezing point of pure cyclohexane): Delta T of pure cyclohexane: 6.61C-3.42C= 3.19 C Step 2- Finding molality: Kf of cyclohexane: 20.2 C/m m=DeltaT/Kf m=3.19 C/20.2 C/m m=0.1579 Step 3- Finding moles of biphenyl solute (mol): (mol/kg*kg = mol) 0.1579mol/kg * 0.002356kg= 0.0003721mol Step 4- Molar Mass of biphenyl solute: 2.356g-2.294g=0.062g 0.062g/0.0003721mol=166.62g/mol Step 5- Calcultating percent error: |(154.1-166.62)| / (154.1) = 0.0812*100= 8.125% Trial 2 Step 1- finding Delta T(freezing point of pure cyclohexane): Delta T of pure cyclohexane: 7.08 C - 2.98C= 4.1 C Step 2- Finding molality: Kf of cyclohexane: 20.2 C/mol m=DeltaT/Kf m= 4.1 C/ 20.2 C/m m= 0.203 Step 3- Finding moles of biphenyl solute (mol): (mol/kg*kg = mol) 0.203 mol/kg * 0.002441kg= 0.0004955mol Step 4- Molar Mass of biphenyl solute: 2.441g-2.379g= 0.062g 0.062g/0.0004955mol= 125.13g/mol Step 5- Calcultating percent error: |(154.1-125.13)| / (154.1) =0.188*100= 18.80% Figure Below: Average data values containing the average molar mass and average percent error. (averages in the data table were found by adding up both trials and dividing by 2) DISCUSSION: After completing the experiment it was found that the molar mass for biphenyl was 143.25g/mol. This is lower than the accepted value of biphenyl, which is 154.21 g/mol, but still relatively close. The percent error was 5.79% which is within the accepted percent error of 20% that was discussed in class. When looking at the video it is observed that the experiment is conducted in an open environment with no lid to keep the reaction inside the beaker, this could result in outside factors affecting our results like the vapor pressure. In cyclohexane, the vapor pressure is 96.9mmHg at 25-degrees C, and in this experiment, the temperature was at right about 25-degrees C so it is very possible that with the higher vapor pressure some of the material evaporated into the open atmosphere. Since the vapor is going into the surrounding environment it is very possible that the person conducting this experiment can get irritation in the eyes or respiratory tract and lead to drowsiness and dizziness. Some suggestions for improvement would be to either get safer alternatives, wear a mask throughout the experiment, or close the top with a lid so that the vapor stays inside the beaker. It is expected that when the student accidentally loses some of the solid biphenyl during the transfer of the test tube, this would lower the measured molecular weight. It was not important to measure the temperature beyond the point of when the solvent equaled the temperature of the ice bath since the goals were to get the solvent to become a solid and to find the initial freezing point.