McMullen_exp12_graded - Experiment 12 – Colligative...

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Unformatted text preview: Experiment 12 – Colligative Properites Spring 2010 Name Chelsea McMullen Lab Section 428 Point Summary (See Blackboard for detailed grading rubric) Superior Excellent Satisfactory Fair Poor Omitted Introduction •Purpose of Report 4 •Goals of Experiment 3 Materials and Methods 4 Results and Discussion •Description of data 4 •Data Tables 5 •Data Table Titles 4 •Graphs 5 •Figure Captions 4 •Sample Calculations 4 •Systematic Error 6 •Random Error 6 •Discussion of discrepancies 6 Other 4 Lab Technique 15 TOTAL POINTS 74 TA Comments/Suggestions: No reference to the lab manual C HEMISTRY 102L R EPORT T EMPLATE EXPT. Colligative Properties 12 12 Introduction The purpose of this report is to identify the molar mass of an unknown substance in a solution using the difference of calculated freezing points in the pure solvent, cyclohexane, and the solution containing the unknown substance . The objective of the experiment was to obtain the average calculated freezing point for the pure solvent, cyclohexane as well as the solution with the unknown substance by submerging them into an ice bath and using a temperature sensor to create cooling and warming curves to find the inflection point, which corresponds to the freezing point . Materials and Methods An ice bath was prepared by placing 35 g of solid NaCl in a 400-mL beaker and adding 100-200mL of water to dissolve the solid. A 600-mL beaker was filled 2/3 with ice and the salt-water mixture was added to it. The ice bath was stirred and placed on a stir plate with a stir bar. A temperature sensor and a one-hole rubber stopper were obtained. Vacuum grease was placed on the tip of the temperature sensor and it was inserted into the hole of the rubber stopper. About 25-mL of cyclohexane was transferred to a 125-mL Erlenmeyer flask from a buret in the hood. The flask was sealed using the rubber stopper and temperature sensor, making sure the temperature sensor was immersed in the cyslohexane. The Erlenmeyer flask was immersed in the ice bath and the cyclohexane was left to cool to about 3 o C. The flask was raised out of the ice bath and the cyclohexane was allowed to warm to 12-15 o C. The flask was immersed and raised three more times for a total of four trials. The inflection point was found on the warming curve by determining where the positive concavity turned into negative concavity. An unknown substance was weighed to 0.7 to 0.8 g and the mass was recorded, as well as the number of the unknown substance. The unknown substance was added to the cyclohexane in the flask, and gently swirled. A new ice bath was made by filling a 600-mL beaker about 2/3 full with ice and 50 g of solid NaOH was added on top of the ice. A stir bar was used to mix the mixture. The flask containing the solution was immersed into the new ice bath and the solution was allowed to cool to about 1 o C. The flask was raised out of the ice bath and the solution was allowed to warm to about 12- 15 o C. At around 8 o C, the flask was warmed using the student’s hands to speed the warming process. C, the flask was warmed using the student’s hands to speed the warming process....
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This note was uploaded on 04/20/2010 for the course CHEM 102L taught by Professor N/a during the Fall '07 term at UNC.

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McMullen_exp12_graded - Experiment 12 – Colligative...

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