Experiment 16 Freezing Point Depression Chemistry 1220 136 September 16th 2013 Raffeal Bennett Purpose The purpose of this experiment was to use laboratory techniques to find the freezing point of a solution Also to determine the molecular weight of an unknown compound after measuring the freezing point Procedure Refer to pages 30 32 of General Chemistry 1220 Laboratory Manual by The Ohio State University Department of Chemistry 2013 2014 Calculations Please see attached calculations Discussion By comparing the freezing point of a pure substance of cyclohexane with the freezing point of a cyclohexane solution the molar weight can be determined The way this is determined is by taking the freezing point of the pure substance and then subtracting the freezing point of the solution this leads to the value of the freezing point depression Then after obtaining freezing point depression also known as Delta H molality can be calculated which is used to find the number of moles of the sample in the solution Finally using the mass of solute in the solution and dividing it by then number of moles yields the molecular weight In order to get average weights and constant values this experiment was done three different times using three different amounts of solute 20 01 mL of cyclohexane sample was frozen and used to compare to the freezing point of a solution with 29 g 19 g and 09 g of an unknown solute The reason the freezing point decreased is because more unknown sample kept being added to the solution The reason this happened is because when liquid freezes it becomes a solid which means the molecules of the substance line up in neat arrays By placing an impurity into the pure substance the molecules had a tougher time aligning therefore causing the freezing point to decrease There were a few factors of the experiment that had the potential to cause error Error in measurement could account of a minute amount The equipment used was a digital thermometer and an analytical balance along with a buret The uncertainties of these measuring instruments are 0 1 degrees Celsius 0001 grams and 0 01 mL respectively Also impurities in the instruments being used could have altered the pure cyclohexane readings Moreover another error that could have altered measurements was adding in the solute all of it needed to be inside the solution however some of it stuck to the wall of the tube which could have made the freezing point higher than it should have been Conclusion By using experimental techniques the freezing point of pure cyclohexane was able to be determined Then by determining the freezing point of the cyclohexane with different amounts of unknown solute added the freezing point depression molality and number of moles were calculated Finally using this information the molecular weight of the unknown substance was successfully established